AI Trajectory

null

{
  "EnableConfigs": [
    "MEMFD_CREATE",
    "SHMEM",
    "HUGETLBFS"
  ],
  "FocusSymbols": [
    "memfd_add_seals",
    "memfd_fcntl",
    "shmem_fallocate",
    "hugetlbfs_punch_hole",
    "hugetlbfs_fallocate"
  ],
  "KMSANReasoning": "The patch adds a new file seal, F_SEAL_PUNCH_HOLE (0x0040), which restricts hole punching (via fallocate or MADV_REMOVE) on memfd-backed hugetlbfs and tmpfs files. The changes are confined to defining the flag in the UAPI header, allowing it in F_ALL_SEALS, and checking the inode's seal mask to reject hole-punching operations with -EPERM.\n\nNo new memory allocations, uninitialized variables, or user-space copies (info-leaks) are introduced. The seal state is stored in existing, properly initialized fields within inode structures (e.g., hugetlbfs_inode_info and shmem_inode_info). Because the changes only introduce permission checks on existing operations and do not create any uninitialized memory read risks, running a dedicated KMSAN fuzzing session is not warranted. Any memory safety concerns in file operations would be handled by standard KASAN.",
  "NeedsKMSAN": false,
  "Reasoning": "The patch introduces a new memfd sealing flag, F_SEAL_PUNCH_HOLE, allowing processes to restrict hole-punching operations (via fallocate with FALLOC_FL_PUNCH_HOLE and madvise with MADV_REMOVE) on memfd-backed shmem and hugetlbfs files without preventing general writes or file resizing. These changes modify core memory management and virtual filesystem logic reachable by unprivileged userspace in virtualized environments. Fuzzing is warranted to ensure the new seal correctly interacts with page cache eviction, hole punching, madvise, and existing file sealing invariants without race conditions or permission bypasses.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit ded47992c4767064c1f0bab8de80211fec7d9554\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Tue Oct 27 02:46:07 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c\nindex ab1e4dec3f77b8..6e1e2474cc13a0 100644\n--- a/fs/hugetlbfs/inode.c\n+++ b/fs/hugetlbfs/inode.c\n@@ -660,7 +660,8 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)\n \tinode_lock(inode);\n \n \t/* protected by i_rwsem */\n-\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {\n+\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\n+\t\t\t   F_SEAL_PUNCH_HOLE)) {\n \t\tinode_unlock(inode);\n \t\treturn -EPERM;\n \t}\ndiff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h\nindex e43e3de3e9ee44..f9dd929adaf402 100644\n--- a/include/uapi/linux/fcntl.h\n+++ b/include/uapi/linux/fcntl.h\n@@ -50,6 +50,7 @@\n #define F_SEAL_WRITE\t0x0008\t/* prevent writes */\n #define F_SEAL_FUTURE_WRITE\t0x0010  /* prevent future writes while mapped */\n #define F_SEAL_EXEC\t0x0020  /* prevent chmod modifying exec bits */\n+#define F_SEAL_PUNCH_HOLE\t0x0040  /* prevent punching holes */\n /* (1U \u003c\u003c 31) is reserved for signed error codes */\n \n /*\ndiff --git a/mm/memfd.c b/mm/memfd.c\nindex 0f6fff004f5e4c..3f0ef72fb30120 100644\n--- a/mm/memfd.c\n+++ b/mm/memfd.c\n@@ -227,7 +227,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)\n \t\t     F_SEAL_SHRINK | \\\n \t\t     F_SEAL_GROW | \\\n \t\t     F_SEAL_WRITE | \\\n-\t\t     F_SEAL_FUTURE_WRITE)\n+\t\t     F_SEAL_FUTURE_WRITE | \\\n+\t\t     F_SEAL_PUNCH_HOLE)\n \n int memfd_add_seals(struct file *file, unsigned int seals)\n {\n@@ -253,6 +254,7 @@ int memfd_add_seals(struct file *file, unsigned int seals)\n \t *   SEAL_GROW: Prevent the file from growing\n \t *   SEAL_WRITE: Prevent write access to the file\n \t *   SEAL_EXEC: Prevent modification of the exec bits in the file mode\n+\t *   SEAL_PUNCH_HOLE: Prevent punching holes into the file\n \t *\n \t * As we don't require any trust relationship between two parties, we\n \t * must prevent seals from being removed. Therefore, sealing a file\ndiff --git a/mm/shmem.c b/mm/shmem.c\nindex fd0953d55fd642..e79a298d6292e7 100644\n--- a/mm/shmem.c\n+++ b/mm/shmem.c\n@@ -3853,7 +3853,8 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,\n \t\tDECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);\n \n \t\t/* protected by i_rwsem */\n-\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {\n+\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\n+\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\n \t\t\terror = -EPERM;\n \t\t\tgoto out;\n \t\t}\ndiff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c\nindex cdab3a8376244a..095c5eb255af1b 100644\n--- a/tools/testing/selftests/memfd/memfd_test.c\n+++ b/tools/testing/selftests/memfd/memfd_test.c\n@@ -35,6 +35,10 @@\n \n #define F_SEAL_EXEC\t0x0020\n \n+#ifndef F_SEAL_PUNCH_HOLE\n+#define F_SEAL_PUNCH_HOLE\t0x0040\n+#endif\n+\n #define F_WX_SEALS (F_SEAL_SHRINK | \\\n \t\t    F_SEAL_GROW | \\\n \t\t    F_SEAL_WRITE | \\\n@@ -455,7 +459,7 @@ static void mfd_assert_fork_private_write(int fd)\n \tmunmap(p, mfd_def_size);\n }\n \n-static void mfd_assert_write(int fd)\n+static void __mfd_assert_write(int fd)\n {\n \tssize_t l;\n \tvoid *p;\n@@ -523,6 +527,11 @@ static void mfd_assert_write(int fd)\n \n \t*(char *)p = 0;\n \tmunmap(p, mfd_def_size);\n+}\n+\n+static void mfd_assert_punch_hole(int fd)\n+{\n+\tint r;\n \n \t/* verify PUNCH_HOLE works */\n \tr = fallocate(fd,\n@@ -535,6 +544,49 @@ static void mfd_assert_write(int fd)\n \t}\n }\n \n+static void mfd_fail_punch_hole(int fd)\n+{\n+\tvoid *p;\n+\tint r;\n+\n+\t/* verify PUNCH_HOLE fails */\n+\tr = fallocate(fd,\n+\t\t      FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,\n+\t\t      0,\n+\t\t      mfd_def_size);\n+\tif (r \u003e= 0 || errno != EPERM) {\n+\t\tprintf(\"expected EPERM on fallocate(PUNCH_HOLE), but got %d: %m\\n\",\n+\t\t       r);\n+\t\tabort();\n+\t}\n+\n+\t/* verify MADV_REMOVE fails, it punches a hole as well */\n+\tp = mmap(NULL,\n+\t\t mfd_def_size,\n+\t\t PROT_READ | PROT_WRITE,\n+\t\t MAP_SHARED,\n+\t\t fd,\n+\t\t 0);\n+\tif (p == MAP_FAILED) {\n+\t\tprintf(\"mmap() failed: %m\\n\");\n+\t\tabort();\n+\t}\n+\n+\tr = madvise(p, mfd_def_size, MADV_REMOVE);\n+\tif (r \u003e= 0 || errno != EPERM) {\n+\t\tprintf(\"expected EPERM on madvise(MADV_REMOVE), but got %d: %m\\n\",\n+\t\t       r);\n+\t\tabort();\n+\t}\n+\tmunmap(p, mfd_def_size);\n+}\n+\n+static void mfd_assert_write(int fd)\n+{\n+\t__mfd_assert_write(fd);\n+\tmfd_assert_punch_hole(fd);\n+}\n+\n static void mfd_fail_write(int fd)\n {\n \tssize_t l;\n@@ -1117,6 +1169,61 @@ static void test_seal_resize(void)\n \tclose(fd);\n }\n \n+/*\n+ * Test SEAL_PUNCH_HOLE\n+ * Test whether SEAL_PUNCH_HOLE actually prevents punching holes, but still\n+ * allows writing and resizing\n+ */\n+static void test_seal_punch_hole(void)\n+{\n+\tint fd;\n+\n+\tprintf(\"%s SEAL-PUNCH-HOLE\\n\", memfd_str);\n+\n+\tfd = mfd_assert_new(\"kern_memfd_seal_punch_hole\",\n+\t\t\t    mfd_def_size,\n+\t\t\t    MFD_CLOEXEC | MFD_ALLOW_SEALING);\n+\tmfd_assert_has_seals(fd, 0);\n+\tmfd_assert_add_seals(fd, F_SEAL_PUNCH_HOLE);\n+\tmfd_assert_has_seals(fd, F_SEAL_PUNCH_HOLE);\n+\n+\tmfd_assert_read(fd);\n+\t__mfd_assert_write(fd);\n+\tmfd_fail_punch_hole(fd);\n+\tmfd_assert_shrink(fd);\n+\tmfd_assert_grow(fd);\n+\tmfd_assert_grow_write(fd);\n+\n+\tclose(fd);\n+}\n+\n+/*\n+ * Test SEAL_PUNCH_HOLE | SEAL_SHRINK\n+ * Test whether SEAL_PUNCH_HOLE | SEAL_SHRINK prevents removing pages from\n+ * the file\n+ */\n+static void test_seal_punch_hole_shrink(void)\n+{\n+\tint fd;\n+\n+\tprintf(\"%s SEAL-PUNCH-HOLE-SHRINK\\n\", memfd_str);\n+\n+\tfd = mfd_assert_new(\"kern_memfd_seal_punch_hole_shrink\",\n+\t\t\t    mfd_def_size,\n+\t\t\t    MFD_CLOEXEC | MFD_ALLOW_SEALING);\n+\tmfd_assert_has_seals(fd, 0);\n+\tmfd_assert_add_seals(fd, F_SEAL_PUNCH_HOLE | F_SEAL_SHRINK);\n+\tmfd_assert_has_seals(fd, F_SEAL_PUNCH_HOLE | F_SEAL_SHRINK);\n+\n+\tmfd_assert_read(fd);\n+\t__mfd_assert_write(fd);\n+\tmfd_fail_punch_hole(fd);\n+\tmfd_fail_shrink(fd);\n+\tmfd_assert_grow(fd);\n+\n+\tclose(fd);\n+}\n+\n /*\n  * Test SEAL_EXEC\n  * Test fd is created with exec and allow sealing.\n@@ -1736,6 +1843,8 @@ int main(int argc, char **argv)\n \ttest_seal_shrink();\n \ttest_seal_grow();\n \ttest_seal_resize();\n+\ttest_seal_punch_hole();\n+\ttest_seal_punch_hole_shrink();\n \n \tif (pid_ns_supported()) {\n \t\ttest_sysctl_simple();\n"
}

core

null

{
  "EnableConfigs": [
    "MEMFD_CREATE",
    "SHMEM",
    "HUGETLBFS"
  ],
  "FocusSymbols": [
    "memfd_add_seals",
    "memfd_fcntl",
    "shmem_fallocate",
    "hugetlbfs_punch_hole",
    "hugetlbfs_fallocate"
  ],
  "Reasoning": "The patch introduces a new memfd sealing flag, F_SEAL_PUNCH_HOLE, allowing processes to restrict hole-punching operations (via fallocate with FALLOC_FL_PUNCH_HOLE and madvise with MADV_REMOVE) on memfd-backed shmem and hugetlbfs files without preventing general writes or file resizing. These changes modify core memory management and virtual filesystem logic reachable by unprivileged userspace in virtualized environments. Fuzzing is warranted to ensure the new seal correctly interacts with page cache eviction, hole punching, madvise, and existing file sealing invariants without race conditions or permission bypasses.",
  "WorthFuzzing": true
}

You are an expert Linux kernel maintainer and security engineer.
Your job is to review a provided patch series and evaluate whether it warrants fuzzing with syzkaller.

IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in
your workspace. Do NOT rely on internal assumptions. You must actively use your code access
tools to inspect the actual source code, callers, and surrounding context.

================================================================================
1. CORE TRIAGE PHILOSOPHY
================================================================================
The goal of patch fuzzing is to discover crashes, regressions, exposed latent bugs,
and newly triggered assertions introduced by the patch series.

- REACHABILITY IS THE PRIMARY GATE:
  Fuzzing can only discover bugs in code that can actually execute in standard virtualized
  environments (GCE or QEMU, utilizing software-emulated devices like USB gadgets, netdev, tun/tap).
  If the modified code is structurally unreachable (see Section 2), it MUST NOT be fuzzed,
  regardless of whether it adds assertions or complex logic.

- DO NOT BLINDLY TRUST "NO FUNCTIONAL CHANGE" (NFCI) OR "REFACTORING" CLAIMS:
  Patch authors routinely label changes as "cleanups", "refactorings", or state
  "No functional change intended". Do NOT take these claims at face value.
  Code refactorings that rearrange logic, introduce helper functions, or alter state management
  in core subsystems frequently introduce subtle semantic shifts or uncover latent kernel bugs.
  If reachable executable code is modified or refactored, it MUST be fuzzed.

- NEW OR MODIFIED ASSERTIONS IN REACHABLE CODE MUST BE FUZZED:
  When a patch introduces or modifies runtime checks or assertions (e.g., WARN_ON*, VM_WARN_ON*,
  BUG_ON*, lockdep_assert*) in reachable code paths, it enforces new or stricter invariants.
  Even if the author believes the invariant always holds, fuzzing is essential to verify whether
  an unusual sequence of operations can violate it.

================================================================================
2. WHEN TO RETURN WorthFuzzing=false (NEGATIVE CRITERIA)
================================================================================
Return WorthFuzzing=false ONLY IF all modified code falls strictly into one or more of these categories:

- Non-kernel and non-executable changes:
  * Modifications to Documentation/, comments, or spelling fixes.
  * User-space directories, self-tests, samples, or scripts (e.g., tools/, samples/, scripts/, usr/)
    that do not affect the compiled kernel image (vmlinux) or kernel modules.
  * Purely decorative logging (e.g., message strings in pr_err, printk, dev_info) or tracepoints
    that do not alter control flow or data structures.
  * Build system or Kconfig changes that do not alter compiled C logic.
- Structurally unreachable hardware:
  * Vendor-specific PCIe switches, SmartNICs, or GPU drivers (e.g., mlxsw, pds_core, qed,
    ionic, amdgpu) requiring physical ASIC/PCIe cards not emulated in standard QEMU.
- Unreachable execution paths:
  * Driver teardown callbacks (.remove, .shutdown, pci_unregister_driver) executed only during
    physical PCI hot-unplug or manual sysfs driver unbinding.
  * Code paths exclusive to architectures other than the target architecture.

================================================================================
3. WHEN TO RETURN WorthFuzzing=true (POSITIVE CRITERIA)
================================================================================
Return WorthFuzzing=true whenever the patch touches reachable executable code, including:
- Core Subsystems:
  * Any logic modifications in memory management (mm/), synchronization/locking (kernel/locking/),
    BPF, scheduler, core networking, VFS, or syscall handling.
- Refactorings and Code Cleanups:
  * Any restructuring of reachable data structures, helper abstractions, or algorithm flows.
- Runtime Assertions and Defensive Checks:
  * Any introduction or alteration of assertions (WARN_ON*, VM_WARN_ON*, BUG_ON*, etc.) in reachable paths.
- Reachable Drivers and Protocols:
  * Drivers accessible via virtual buses (virtio, USB gadget, loopback, netlink, binder, sockets, etc.).

================================================================================
4. EXTRACTING FocusSymbols (PREVENTING DILUTION)
================================================================================
When WorthFuzzing=true, you must extract specific kernel functions into FocusSymbols to guide the fuzzer:

- AVOID UBIQUITOUS LIFECYCLE HOT-PATHS:
  Do NOT list generic, ubiquitous functions called by almost every program in the corpus
  (including, but not limited to: general memory allocators and deallocators, page fault
  and trap handlers, or core synchronization primitives; this is not an exhaustive list).
  Listing ubiquitous functions causes the fuzzer to classify thousands of unrelated tests as "focused",
  which severely dilutes fuzzing effort away from the actual changes.

- TARGET SPECIFIC FEATURE LOGIC AND ENTRYPOINTS:
  List functions that specifically implement the logic being added or altered, or direct API entrypoints
  for the subsystem feature under review.

- HANDLING STATIC INLINE FUNCTIONS IN HEADERS (.h):
  Compiler-inlined static functions (such as static inlines in mm/*.h or include/linux/*.h) lack
  distinct symbol addresses in vmlinux and cannot be targeted directly by symbol coverage filters.
  If the changes are primarily in static inline helpers, identify non-static, feature-specific caller
  functions in .c files that exercise them (avoiding ubiquitous lifecycle wrappers).

================================================================================
5. IDENTIFYING EnableConfigs
================================================================================
Identify any specific CONFIG_ options required to properly compile and reach the modified code:
- Inspect Kconfig files and #ifdef guards; do not make assumptions.
- Check "depends on" lines in Kconfig to include any non-standard parent subsystem configs needed.
- Strip any 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4").
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

Target architecture: amd64

For your convenience, here is the diff of the changes:
commit ded47992c4767064c1f0bab8de80211fec7d9554
Author: syz-cluster <triage@syzkaller.com>
Date:   Tue Oct 27 02:46:07 2026 +0000

    syz-cluster: applied patch under review

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index ab1e4dec3f77b8..6e1e2474cc13a0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -660,7 +660,8 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 	inode_lock(inode);
 
 	/* protected by i_rwsem */
-	if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
+	if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |
+			   F_SEAL_PUNCH_HOLE)) {
 		inode_unlock(inode);
 		return -EPERM;
 	}
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index e43e3de3e9ee44..f9dd929adaf402 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -50,6 +50,7 @@
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
 #define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
 #define F_SEAL_EXEC	0x0020  /* prevent chmod modifying exec bits */
+#define F_SEAL_PUNCH_HOLE	0x0040  /* prevent punching holes */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/memfd.c b/mm/memfd.c
index 0f6fff004f5e4c..3f0ef72fb30120 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -227,7 +227,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
 		     F_SEAL_SHRINK | \
 		     F_SEAL_GROW | \
 		     F_SEAL_WRITE | \
-		     F_SEAL_FUTURE_WRITE)
+		     F_SEAL_FUTURE_WRITE | \
+		     F_SEAL_PUNCH_HOLE)
 
 int memfd_add_seals(struct file *file, unsigned int seals)
 {
@@ -253,6 +254,7 @@ int memfd_add_seals(struct file *file, unsigned int seals)
 	 *   SEAL_GROW: Prevent the file from growing
 	 *   SEAL_WRITE: Prevent write access to the file
 	 *   SEAL_EXEC: Prevent modification of the exec bits in the file mode
+	 *   SEAL_PUNCH_HOLE: Prevent punching holes into the file
 	 *
 	 * As we don't require any trust relationship between two parties, we
 	 * must prevent seals from being removed. Therefore, sealing a file
diff --git a/mm/shmem.c b/mm/shmem.c
index fd0953d55fd642..e79a298d6292e7 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3853,7 +3853,8 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_rwsem */
-		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |
+				   F_SEAL_PUNCH_HOLE)) {
 			error = -EPERM;
 			goto out;
 		}
diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index cdab3a8376244a..095c5eb255af1b 100644
--- a/tools/testing/selftests/memfd/memfd_test.c
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -35,6 +35,10 @@
 
 #define F_SEAL_EXEC	0x0020
 
+#ifndef F_SEAL_PUNCH_HOLE
+#define F_SEAL_PUNCH_HOLE	0x0040
+#endif
+
 #define F_WX_SEALS (F_SEAL_SHRINK | \
 		    F_SEAL_GROW | \
 		    F_SEAL_WRITE | \
@@ -455,7 +459,7 @@ static void mfd_assert_fork_private_write(int fd)
 	munmap(p, mfd_def_size);
 }
 
-static void mfd_assert_write(int fd)
+static void __mfd_assert_write(int fd)
 {
 	ssize_t l;
 	void *p;
@@ -523,6 +527,11 @@ static void mfd_assert_write(int fd)
 
 	*(char *)p = 0;
 	munmap(p, mfd_def_size);
+}
+
+static void mfd_assert_punch_hole(int fd)
+{
+	int r;
 
 	/* verify PUNCH_HOLE works */
 	r = fallocate(fd,
@@ -535,6 +544,49 @@ static void mfd_assert_write(int fd)
 	}
 }
 
+static void mfd_fail_punch_hole(int fd)
+{
+	void *p;
+	int r;
+
+	/* verify PUNCH_HOLE fails */
+	r = fallocate(fd,
+		      FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+		      0,
+		      mfd_def_size);
+	if (r >= 0 || errno != EPERM) {
+		printf("expected EPERM on fallocate(PUNCH_HOLE), but got %d: %m\n",
+		       r);
+		abort();
+	}
+
+	/* verify MADV_REMOVE fails, it punches a hole as well */
+	p = mmap(NULL,
+		 mfd_def_size,
+		 PROT_READ | PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	r = madvise(p, mfd_def_size, MADV_REMOVE);
+	if (r >= 0 || errno != EPERM) {
+		printf("expected EPERM on madvise(MADV_REMOVE), but got %d: %m\n",
+		       r);
+		abort();
+	}
+	munmap(p, mfd_def_size);
+}
+
+static void mfd_assert_write(int fd)
+{
+	__mfd_assert_write(fd);
+	mfd_assert_punch_hole(fd);
+}
+
 static void mfd_fail_write(int fd)
 {
 	ssize_t l;
@@ -1117,6 +1169,61 @@ static void test_seal_resize(void)
 	close(fd);
 }
 
+/*
+ * Test SEAL_PUNCH_HOLE
+ * Test whether SEAL_PUNCH_HOLE actually prevents punching holes, but still
+ * allows writing and resizing
+ */
+static void test_seal_punch_hole(void)
+{
+	int fd;
+
+	printf("%s SEAL-PUNCH-HOLE\n", memfd_str);
+
+	fd = mfd_assert_new("kern_memfd_seal_punch_hole",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_PUNCH_HOLE);
+	mfd_assert_has_seals(fd, F_SEAL_PUNCH_HOLE);
+
+	mfd_assert_read(fd);
+	__mfd_assert_write(fd);
+	mfd_fail_punch_hole(fd);
+	mfd_assert_shrink(fd);
+	mfd_assert_grow(fd);
+	mfd_assert_grow_write(fd);
+
+	close(fd);
+}
+
+/*
+ * Test SEAL_PUNCH_HOLE | SEAL_SHRINK
+ * Test whether SEAL_PUNCH_HOLE | SEAL_SHRINK prevents removing pages from
+ * the file
+ */
+static void test_seal_punch_hole_shrink(void)
+{
+	int fd;
+
+	printf("%s SEAL-PUNCH-HOLE-SHRINK\n", memfd_str);
+
+	fd = mfd_assert_new("kern_memfd_seal_punch_hole_shrink",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_PUNCH_HOLE | F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd, F_SEAL_PUNCH_HOLE | F_SEAL_SHRINK);
+
+	mfd_assert_read(fd);
+	__mfd_assert_write(fd);
+	mfd_fail_punch_hole(fd);
+	mfd_fail_shrink(fd);
+	mfd_assert_grow(fd);
+
+	close(fd);
+}
+
 /*
  * Test SEAL_EXEC
  * Test fd is created with exec and allow sealing.
@@ -1736,6 +1843,8 @@ int main(int argc, char **argv)
 	test_seal_shrink();
 	test_seal_grow();
 	test_seal_resize();
+	test_seal_punch_hole();
+	test_seal_punch_hole_shrink();
 
 	if (pid_ns_supported()) {
 		test_sysctl_simple();

gemini-3.8-flash

null

null

							input: 5567
							output: 34
							thoughts: 542
						

**Analyzing the F_SEAL_PUNCH_HOLE Patch**

Okay, let's break this down. A new seal flag, `F_SEAL_PUNCH_HOLE`, is being introduced. I see it defined in `include/uapi/linux/fcntl.h` – that's the obvious starting point. The patch itself touches a few key areas, and that's what makes this interesting.

First, I see modifications in `fs/hugetlbfs/inode.c`, specifically within the `hugetlbfs_punch_hole` function. The code now checks for this new seal flag in conjunction with `F_SEAL_WRITE` and `F_SEAL_FUTURE_WRITE`. Makes sense – preventing hole punching after certain seals are applied.

Then, the memfd side – which is where it gets more relevant. `mm/memfd.c` has been updated to include `F_SEAL_PUNCH_HOLE` in the allowed seals within `memfd_add_seals`. This is important, as it directly impacts memfd's behavior. We've got another modification in `mm/shmem.c` to handle the same concerns as with hugetlbfs. It seems the same restrictions are applied here as well. The selftests in `tools/testing/selftests/memfd/memfd_test.c` are also being updated.

Essentially, this brings hole punching capabilities (via `fallocate(FALLOC_FL_PUNCH_HOLE)` and `madvise(MADV_REMOVE)`) under the seal framework for memfd and, by extension, shmem and hugetlbfs. This is core functionality, and, importantly, accessible to unprivileged userspace. This makes it a great target for closer inspection.

Now, the crucial question: Is this worth fuzzing? Let's check the criteria outlined in Section 3. Well, memfd, shmem, and hugetlbfs are definitely core memory management and VFS subsystems. And yes, this patch modifies *reachable executable code* within `mm/memfd.c`, `mm/shmem.c`, and `fs/hugetlbfs/inode.c`. So, *WorthFuzzing* is indeed TRUE. That's a clear signal.

Next, I need to get into the code itself. I'm going to need to examine these modified functions and their callers in detail. I'll need to figure out what configs are necessary to reach the code and what the exact interaction between all these seals actually is. Time to fire up `grepper` or `read-file` and dive deeper. This could be interesting.

{
  "File": "mm/memfd.c",
  "FirstLine": 220,
  "LineCount": 45
}

{
  "Contents": " 220:\t#endif\n 221:\t\n 222:\t\treturn NULL;\n 223:\t}\n 224:\t\n 225:\t#define F_ALL_SEALS (F_SEAL_SEAL | \\\n 226:\t\t\t     F_SEAL_EXEC | \\\n 227:\t\t\t     F_SEAL_SHRINK | \\\n 228:\t\t\t     F_SEAL_GROW | \\\n 229:\t\t\t     F_SEAL_WRITE | \\\n 230:\t\t\t     F_SEAL_FUTURE_WRITE | \\\n 231:\t\t\t     F_SEAL_PUNCH_HOLE)\n 232:\t\n 233:\tint memfd_add_seals(struct file *file, unsigned int seals)\n 234:\t{\n 235:\t\tstruct inode *inode = file_inode(file);\n 236:\t\tunsigned int *file_seals;\n 237:\t\tint error;\n 238:\t\n 239:\t\t/*\n 240:\t\t * SEALING\n 241:\t\t * Sealing allows multiple parties to share a tmpfs or hugetlbfs file\n 242:\t\t * but restrict access to a specific subset of file operations. Seals\n 243:\t\t * can only be added, but never removed. This way, mutually untrusted\n 244:\t\t * parties can share common memory regions with a well-defined policy.\n 245:\t\t * A malicious peer can thus never perform unwanted operations on a\n 246:\t\t * shared object.\n 247:\t\t *\n 248:\t\t * Seals are only supported on special tmpfs or hugetlbfs files and\n 249:\t\t * always affect the whole underlying inode. Once a seal is set, it\n 250:\t\t * may prevent some kinds of access to the file. Currently, the\n 251:\t\t * following seals are defined:\n 252:\t\t *   SEAL_SEAL: Prevent further seals from being set on this file\n 253:\t\t *   SEAL_SHRINK: Prevent the file from shrinking\n 254:\t\t *   SEAL_GROW: Prevent the file from growing\n 255:\t\t *   SEAL_WRITE: Prevent write access to the file\n 256:\t\t *   SEAL_EXEC: Prevent modification of the exec bits in the file mode\n 257:\t\t *   SEAL_PUNCH_HOLE: Prevent punching holes into the file\n 258:\t\t *\n 259:\t\t * As we don't require any trust relationship between two parties, we\n 260:\t\t * must prevent seals from being removed. Therefore, sealing a file\n 261:\t\t * only adds a given set of seals to the file, it never touches\n 262:\t\t * existing seals. Furthermore, the \"setting seals\"-operation can be\n 263:\t\t * sealed itself, which basically prevents any further seal from being\n 264:\t\t * added.\n"
}

gemini-3.8-flash

null

null

							input: 7466
							output: 42
							thoughts: 35
						

Okay, let's dive into this. Here's how I'd approach this task, as if these were my internal thoughts:

**Investigating Memory Management Configurations**

Alright, I need to get a clear picture of how these memory-related features are configured. My first instinct is to double-check the relevant configuration settings, specifically focusing on `memfd`, `hugetlbfs`, and `shmem`. I want to make sure I understand the current state of those and what potential configuration options are in play.

Once I've verified the configuration, I'll drill down further, it seems. To get a handle on the codebase, I'll then zero in on the `Makefile`s within the `mm/` directory and the `fs/hugetlbfs/` directory. That's where the core build instructions and potential dependencies for memory management features are likely defined. Understanding the build process is key to understanding the integration and dependencies between these subsystems.

{
  "Expression": "memfd|hugetlbfs",
  "PathPrefixes": [
    "mm/Makefile",
    "fs/hugetlbfs/Makefile"
  ]
}

{
  "Output": "fs/hugetlbfs/Makefile-5-\nfs/hugetlbfs/Makefile:6:obj-$(CONFIG_HUGETLBFS) += hugetlbfs.o\nfs/hugetlbfs/Makefile-7-\nfs/hugetlbfs/Makefile:8:hugetlbfs-objs := inode.o\n--\nmm/Makefile=103=obj-$(CONFIG_PAGE_COUNTER) += page_counter.o\nmm/Makefile:104:obj-$(CONFIG_LIVEUPDATE_MEMFD) += memfd_luo.o\nmm/Makefile-105-obj-$(CONFIG_MEMCG_V1) += memcontrol-v1.o\n--\nmm/Makefile=141=obj-$(CONFIG_HMM_MIRROR) += hmm.o\nmm/Makefile:142:obj-$(CONFIG_MEMFD_CREATE) += memfd.o\nmm/Makefile-143-obj-$(CONFIG_MAPPING_DIRTY_HELPERS) += mapping_dirty_helpers.o\n"
}

gemini-3.8-flash

null

null

							input: 7996
							output: 37
							thoughts: 41
						

Okay, here's my take on those thoughts, framed as an expert's internal monologue:

**Kconfig Dependency Deep Dive**

Alright, time to get granular. I need to quickly assess the implications of a change, and that means tracing the Kconfig dependencies. My immediate concern is ensuring that the configuration options are set up correctly for memory-backed file descriptors. Specifically, I need to check the status of `CONFIG_MEMFD_CREATE`, `CONFIG_HUGETLBFS`, and `CONFIG_SHMEM`. My intuition suggests there's a relationship there; maybe I'm looking to create a memory-backed file descriptor, but the configuration relies on the other components.

So, first things first, let's drill down and see how `MEMFD_CREATE` is defined in Kconfig. What are its dependencies? What are the implications of enabling or disabling it? I need to understand if it’s directly linked to any of the other options I listed. And are there any subtle conflicts or unexpected interactions? The devil, as always, is in the Kconfig details, and the goal is to make sure everything lines up. Let's see what's what.

{
  "Expression": "config MEMFD_CREATE",
  "PathPrefixes": [
    "init/Kconfig",
    "mm/Kconfig"
  ]
}

{
  "Output": "mm/Kconfig=1333=config KMAP_LOCAL_NON_LINEAR_PTE_ARRAY\n--\nmm/Kconfig-1335-\nmm/Kconfig:1336:config MEMFD_CREATE\nmm/Kconfig-1337-\tbool \"Enable memfd_create() system call\" if EXPERT\n"
}

gemini-3.8-flash

null

null

							input: 8412
							output: 33
							thoughts: 18
						

Here's a summary of my thought process, as requested:

**Diving into `mm/Kconfig`**

Okay, let's get down to brass tacks. I'm going to zero in on the `mm/Kconfig` file, specifically around line 1336. I need to understand something related to memory management configuration, and this file is the definitive source for how kernel options are set in that area. Locating a specific line is usually the best place to start when you are digging through this. It will help me narrow down the specific code. By examining the context of that line and the surrounding code, I should be able to quickly grasp the relevant configuration options, dependencies, and any conditional logic at play. Time to get to work.

{
  "File": "mm/Kconfig",
  "FirstLine": 1335,
  "LineCount": 20
}

{
  "Contents": "1335:\t\n1336:\tconfig MEMFD_CREATE\n1337:\t\tbool \"Enable memfd_create() system call\" if EXPERT\n1338:\t\n1339:\tconfig SECRETMEM\n1340:\t\tdefault y\n1341:\t\tbool \"Enable memfd_secret() system call\" if EXPERT\n1342:\t\tdepends on ARCH_HAS_SET_DIRECT_MAP\n1343:\t\thelp\n1344:\t\t  Enable the memfd_secret() system call with the ability to create\n1345:\t\t  memory areas visible only in the context of the owning process and\n1346:\t\t  not mapped to other processes and other kernel page tables.\n1347:\t\n1348:\tconfig ANON_VMA_NAME\n1349:\t\tbool \"Anonymous VMA name support\"\n1350:\t\tdepends on PROC_FS \u0026\u0026 ADVISE_SYSCALLS \u0026\u0026 MMU\n1351:\t\n1352:\t\thelp\n1353:\t\t  Allow naming anonymous virtual memory areas.\n1354:\t\n"
}

gemini-3.8-flash

null

null

							input: 8910
							output: 50
							thoughts: 19
						

Okay, here's my take on those thoughts, tailored for an expert audience:

**Examining Shared Memory and Huge Pages**

Alright, let's dive in. My immediate thought is to check the configuration options related to shared memory and huge pages, specifically `CONFIG_SHMEM` and `CONFIG_HUGETLBFS`. I need to ensure these are properly configured for the current context. This isn't just a casual glance; I'm thinking about the implications. Are these features enabled and built in, or perhaps as modules? What version of the kernel am I working with? Any interaction between these parameters and the hardware I am dealing with?
Are there any specific scenarios or performance bottlenecks I'm troubleshooting that would lean me toward double-checking these settings? This level of detail will drive me to see if these are set optimally for my target workload or use case.
I will also quickly think about the interplay between shared memory segments and huge pages, and how their interactions might affect application performance.
I'm mentally running through the various potential pitfalls and optimization strategies related to memory management, considering the specific architecture and system environment at hand. It's a quick, efficient scan of the landscape to see if there are any obvious low-hanging fruit or red flags that need attention. It's about proactive assessment and setting a baseline for further investigation if needed.

{
  "Expression": "config SHMEM$|config HUGETLBFS$",
  "PathPrefixes": [
    "init/Kconfig",
    "mm/Kconfig",
    "fs/Kconfig"
  ]
}

{
  "Output": "fs/Kconfig=247=config ARCH_SUPPORTS_HUGETLBFS\n--\nfs/Kconfig-249-\nfs/Kconfig:250:menuconfig HUGETLBFS\nfs/Kconfig-251-\tbool \"HugeTLB file system support\"\n--\ninit/Kconfig=1930=config EVENTFD\n--\ninit/Kconfig-1938-\ninit/Kconfig:1939:config SHMEM\ninit/Kconfig-1940-\tbool \"Use full shmem filesystem\" if EXPERT\n"
}

gemini-3.8-flash

null

null

							input: 9407
							output: 38
							thoughts: 55
						

Okay, let's dive into this. Here's how I'd approach this task, walking through the logic:

**Examining `hugetlbfs_punch_hole`**

Alright, I need to get a handle on what's happening with that `hugetlbfs_punch_hole` function. The first thing I'm doing is navigating directly to the source code, specifically `fs/hugetlbfs/inode.c`. That's where it lives, as I recall. Now, let's zero in on the relevant section. I'll need to understand the function's purpose within the larger context of the hugetlbfs filesystem. The code around line 660, in that file, will likely give me that context. I'll be looking for clues about how it interacts with other functions, data structures and other components of the filesystem implementation.

{
  "File": "fs/hugetlbfs/inode.c",
  "FirstLine": 650,
  "LineCount": 35
}

{
  "Contents": " 650:\t\tstruct hstate *h = hstate_inode(inode);\n 651:\t\tloff_t hpage_size = huge_page_size(h);\n 652:\t\tloff_t hole_start, hole_end;\n 653:\t\n 654:\t\t/*\n 655:\t\t * hole_start and hole_end indicate the full pages within the hole.\n 656:\t\t */\n 657:\t\thole_start = round_up(offset, hpage_size);\n 658:\t\thole_end = round_down(offset + len, hpage_size);\n 659:\t\n 660:\t\tinode_lock(inode);\n 661:\t\n 662:\t\t/* protected by i_rwsem */\n 663:\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\n 664:\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\n 665:\t\t\tinode_unlock(inode);\n 666:\t\t\treturn -EPERM;\n 667:\t\t}\n 668:\t\n 669:\t\ti_mmap_lock_write(mapping);\n 670:\t\n 671:\t\t/* If range starts before first full page, zero partial page. */\n 672:\t\tif (offset \u003c hole_start)\n 673:\t\t\thugetlbfs_zero_partial_page(h, mapping,\n 674:\t\t\t\t\toffset, min(offset + len, hole_start));\n 675:\t\n 676:\t\t/* Unmap users of full pages in the hole. */\n 677:\t\tif (hole_end \u003e hole_start) {\n 678:\t\t\tif (mapping_mapped(mapping))\n 679:\t\t\t\thugetlb_vmdelete_list(mapping,\n 680:\t\t\t\t\t\t      hole_start \u003e\u003e PAGE_SHIFT,\n 681:\t\t\t\t\t\t      hole_end \u003e\u003e PAGE_SHIFT, 0);\n 682:\t\t}\n 683:\t\n 684:\t\t/* If range extends beyond last full page, zero partial page. */\n"
}

gemini-3.8-flash

null

null

							input: 10238
							output: 36
							thoughts: 31
						

Okay, let's dive into this. Here's how I'd approach this task, as if these were my internal thoughts:

**Investigating `hugetlbfs_punch_hole` Callers**

Alright, I need to understand the call flow of `hugetlbfs_punch_hole`. This is a critical function, so I need to know *exactly* where it's being used within the system. My initial instinct is to use the standard tools for static analysis - I'll need to locate all instances of its calls. I need a comprehensive overview of how this function is integrated within the kernel. The goal is to build a complete picture of its usage patterns. I'll need to know the calling context in each case.

{
  "Expression": "hugetlbfs_punch_hole",
  "PathPrefixes": [
    "fs/hugetlbfs/"
  ]
}

{
  "Output": "fs/hugetlbfs/inode.c=623=static void hugetlbfs_zero_partial_page(struct hstate *h,\n--\nfs/hugetlbfs/inode.c-645-\nfs/hugetlbfs/inode.c:646:static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)\nfs/hugetlbfs/inode.c-647-{\n--\nfs/hugetlbfs/inode.c=700=static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,\n--\nfs/hugetlbfs/inode.c-718-\tif (mode \u0026 FALLOC_FL_PUNCH_HOLE) {\nfs/hugetlbfs/inode.c:719:\t\terror = hugetlbfs_punch_hole(inode, offset, len);\nfs/hugetlbfs/inode.c-720-\t\tgoto out_nolock;\n"
}

gemini-3.8-flash

null

null

							input: 10721
							output: 35
							thoughts: 50
						

Here's my thought process, as it stands:

**Digging into Fallocate Implementations**

Okay, so `hugetlbfs_fallocate` ultimately calls `hugetlbfs_punch_hole`. That's straightforward, but it begs the question of how analogous functionality behaves in other memory systems. Specifically, I'm curious about the shared memory implementation. I need to see what `shmem_fallocate` does. I'm going to check `mm/shmem.c` around line 3853 – that seems like a good place to start my investigation into shared memory. Let's see how things are handled there. I expect a similar approach, but the details are likely to be interesting given the differing characteristics of shared memory versus hugetlbfs.

{
  "File": "mm/shmem.c",
  "FirstLine": 3845,
  "LineCount": 35
}

{
  "Contents": "3845:\t\t\terror = -EPERM;\n3846:\t\t\tgoto out;\n3847:\t\t}\n3848:\t\n3849:\t\tif (mode \u0026 FALLOC_FL_PUNCH_HOLE) {\n3850:\t\t\tstruct address_space *mapping = file-\u003ef_mapping;\n3851:\t\t\tloff_t unmap_start = round_up(offset, PAGE_SIZE);\n3852:\t\t\tloff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;\n3853:\t\t\tDECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);\n3854:\t\n3855:\t\t\t/* protected by i_rwsem */\n3856:\t\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\n3857:\t\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\n3858:\t\t\t\terror = -EPERM;\n3859:\t\t\t\tgoto out;\n3860:\t\t\t}\n3861:\t\n3862:\t\t\tshmem_falloc.waitq = \u0026shmem_falloc_waitq;\n3863:\t\t\tshmem_falloc.start = (u64)unmap_start \u003e\u003e PAGE_SHIFT;\n3864:\t\t\tshmem_falloc.next = (unmap_end + 1) \u003e\u003e PAGE_SHIFT;\n3865:\t\t\tspin_lock(\u0026inode-\u003ei_lock);\n3866:\t\t\tWRITE_ONCE(inode-\u003ei_private, \u0026shmem_falloc);\n3867:\t\t\tspin_unlock(\u0026inode-\u003ei_lock);\n3868:\t\n3869:\t\t\tif ((u64)unmap_end \u003e (u64)unmap_start)\n3870:\t\t\t\tunmap_mapping_range(mapping, unmap_start,\n3871:\t\t\t\t\t\t    1 + unmap_end - unmap_start, 0);\n3872:\t\t\tshmem_truncate_range(inode, offset, offset + len - 1);\n3873:\t\t\t/* No need to unmap again: hole-punching leaves COWed pages */\n3874:\t\n3875:\t\t\tspin_lock(\u0026inode-\u003ei_lock);\n3876:\t\t\tWRITE_ONCE(inode-\u003ei_private, NULL);\n3877:\t\t\twake_up_all(\u0026shmem_falloc_waitq);\n3878:\t\t\tWARN_ON_ONCE(!list_empty(\u0026shmem_falloc_waitq.head));\n3879:\t\t\tspin_unlock(\u0026inode-\u003ei_lock);\n"
}

gemini-3.8-flash

null

null

							input: 11645
							output: 19
							thoughts: 32
						

Okay, here's my take on those thoughts, tailored for an expert audience:

**Investigating Potential Data Integrity Bottlenecks**

Hmm, a gut feeling tells me there might be another area where data integrity checks or processing related to seals or hole punching are taking place besides the one I'm immediately focused on. This kind of redundancy or unexpected handling can often lead to tricky edge cases, performance issues, or even subtle bugs down the line, and I need to make sure I'm across all relevant code.

So, instead of just assuming the current location is the only place we manage such things, I should take a systematic approach. The most straightforward initial step is a targeted search. I'll launch a comprehensive search across the entire codebase for any instances of `F_SEAL_`. The `F_SEAL_` prefix is often used to signal a function, constant, or class related to seal processing in our team's code, so this should be the right place to start. This way, I'm hoping to quickly identify *every* interaction related to sealing or hole punching and confirm my initial assumptions or, more importantly, discover any unexpected dependencies or parallel processing that need to be accounted for.

{
  "Expression": "F_SEAL_"
}

{
  "Output": "\nFull output is too long, showing 200 out of 445 lines.\nUse more precise expression or PathPrefixes if possible.\n\nNumber of matching lines per file (17 files in total):\nDocumentation/userspace-api/mfd_noexec.rst:2\nDocumentation/userspace-api/mseal.rst:1\narch/um/os-Linux/skas/process.c:1\ndrivers/dma-buf/udmabuf.c:2\nfs/hugetlbfs/inode.c:6\ninclude/linux/kho/abi/memfd.h:6\ninclude/linux/shmem_fs.h:1\ninclude/uapi/linux/fcntl.h:7\nmm/memfd.c:15\nmm/shmem.c:11\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:6\ntools/testing/selftests/bpf/prog_tests/dmabuf_iter.c:1\ntools/testing/selftests/drivers/dma-buf/udmabuf.c:2\ntools/testing/selftests/drivers/net/hw/ncdevmem.c:1\ntools/testing/selftests/memfd/fuse_test.c:4\ntools/testing/selftests/memfd/memfd_test.c:90\ntools/testing/selftests/mm/guard-regions.c:1\n\nDocumentation/userspace-api/mfd_noexec.rst=36=User API\n--\nDocumentation/userspace-api/mfd_noexec.rst-41-\tWhen MFD_NOEXEC_SEAL bit is set in the ``flags``, memfd is created\nDocumentation/userspace-api/mfd_noexec.rst:42:\twith NX. F_SEAL_EXEC is set and the memfd can't be modified to\nDocumentation/userspace-api/mfd_noexec.rst-43-\tadd X later. MFD_ALLOW_SEALING is also implied.\n--\nDocumentation/userspace-api/mfd_noexec.rst=49=Note:\nDocumentation/userspace-api/mfd_noexec.rst-50-\t``MFD_NOEXEC_SEAL`` implies ``MFD_ALLOW_SEALING``. In case that\nDocumentation/userspace-api/mfd_noexec.rst:51:\tan app doesn't want sealing, it can add F_SEAL_SEAL after creation.\nDocumentation/userspace-api/mfd_noexec.rst-52-\n--\nDocumentation/userspace-api/mseal.rst=28=mseal syscall signature\n--\nDocumentation/userspace-api/mseal.rst-74-      have munseal, this is consistent with other sealing feature, e.g.\nDocumentation/userspace-api/mseal.rst:75:      F_SEAL_SEAL for file.\nDocumentation/userspace-api/mseal.rst-76-\n--\narch/um/os-Linux/skas/process.c=364=static int __init init_stub_exe_fd(void)\n--\narch/um/os-Linux/skas/process.c-405-\t\tfcntl(stub_exe_fd, F_ADD_SEALS,\narch/um/os-Linux/skas/process.c:406:\t\t      F_SEAL_WRITE | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_SEAL);\narch/um/os-Linux/skas/process.c-407-\t} else {\n--\ndrivers/dma-buf/udmabuf.c=258=static const struct dma_buf_ops udmabuf_ops = {\n--\ndrivers/dma-buf/udmabuf.c-268-\ndrivers/dma-buf/udmabuf.c:269:#define SEALS_WANTED (F_SEAL_SHRINK)\ndrivers/dma-buf/udmabuf.c:270:#define SEALS_DENIED (F_SEAL_WRITE|F_SEAL_FUTURE_WRITE)\ndrivers/dma-buf/udmabuf.c-271-\n--\nfs/hugetlbfs/inode.c=646=static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)\n--\nfs/hugetlbfs/inode.c-662-\t/* protected by i_rwsem */\nfs/hugetlbfs/inode.c:663:\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\nfs/hugetlbfs/inode.c:664:\t\t\t   F_SEAL_PUNCH_HOLE)) {\nfs/hugetlbfs/inode.c-665-\t\tinode_unlock(inode);\n--\nfs/hugetlbfs/inode.c=700=static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,\n--\nfs/hugetlbfs/inode.c-737-\nfs/hugetlbfs/inode.c:738:\tif ((info-\u003eseals \u0026 F_SEAL_GROW) \u0026\u0026 offset + len \u003e inode-\u003ei_size) {\nfs/hugetlbfs/inode.c-739-\t\terror = -EPERM;\n--\nfs/hugetlbfs/inode.c=831=static int hugetlbfs_setattr(struct mnt_idmap *idmap,\n--\nfs/hugetlbfs/inode.c-852-\t\t/* protected by i_rwsem */\nfs/hugetlbfs/inode.c:853:\t\tif ((newsize \u003c oldsize \u0026\u0026 (info-\u003eseals \u0026 F_SEAL_SHRINK)) ||\nfs/hugetlbfs/inode.c:854:\t\t    (newsize \u003e oldsize \u0026\u0026 (info-\u003eseals \u0026 F_SEAL_GROW)))\nfs/hugetlbfs/inode.c-855-\t\t\treturn -EPERM;\n--\nfs/hugetlbfs/inode.c=894=static struct inode *hugetlbfs_get_inode(struct super_block *sb,\n--\nfs/hugetlbfs/inode.c-922-\t\tinfo-\u003eresv_map = resv_map;\nfs/hugetlbfs/inode.c:923:\t\tinfo-\u003eseals = F_SEAL_SEAL;\nfs/hugetlbfs/inode.c-924-\t\thugetlbfs_pmd_sharing_init(inode);\n--\ninclude/linux/kho/abi/memfd.h=53=struct memfd_luo_folio_ser {\n--\ninclude/linux/kho/abi/memfd.h-62- */\ninclude/linux/kho/abi/memfd.h:63:#define MEMFD_LUO_ALL_SEALS (F_SEAL_SEAL | \\\ninclude/linux/kho/abi/memfd.h:64:\t\t\t     F_SEAL_SHRINK | \\\ninclude/linux/kho/abi/memfd.h:65:\t\t\t     F_SEAL_GROW | \\\ninclude/linux/kho/abi/memfd.h:66:\t\t\t     F_SEAL_WRITE | \\\ninclude/linux/kho/abi/memfd.h:67:\t\t\t     F_SEAL_FUTURE_WRITE | \\\ninclude/linux/kho/abi/memfd.h:68:\t\t\t     F_SEAL_EXEC)\ninclude/linux/kho/abi/memfd.h-69-\n--\ninclude/linux/shmem_fs.h-29- *\ninclude/linux/shmem_fs.h:30: * In some ways similar to F_SEAL_GROW | F_SEAL_SHRINK, but can be removed and\ninclude/linux/shmem_fs.h-31- * isn't directly visible to userspace.\n--\ninclude/uapi/linux/fcntl.h-46- */\ninclude/uapi/linux/fcntl.h:47:#define F_SEAL_SEAL\t0x0001\t/* prevent further seals from being set */\ninclude/uapi/linux/fcntl.h:48:#define F_SEAL_SHRINK\t0x0002\t/* prevent file from shrinking */\ninclude/uapi/linux/fcntl.h:49:#define F_SEAL_GROW\t0x0004\t/* prevent file from growing */\ninclude/uapi/linux/fcntl.h:50:#define F_SEAL_WRITE\t0x0008\t/* prevent writes */\ninclude/uapi/linux/fcntl.h:51:#define F_SEAL_FUTURE_WRITE\t0x0010  /* prevent future writes while mapped */\ninclude/uapi/linux/fcntl.h:52:#define F_SEAL_EXEC\t0x0020  /* prevent chmod modifying exec bits */\ninclude/uapi/linux/fcntl.h:53:#define F_SEAL_PUNCH_HOLE\t0x0040  /* prevent punching holes */\ninclude/uapi/linux/fcntl.h-54-/* (1U \u003c\u003c 31) is reserved for signed error codes */\n--\nmm/memfd.c=212=static unsigned int *memfd_file_seals_ptr(struct file *file)\n--\nmm/memfd.c-224-\nmm/memfd.c:225:#define F_ALL_SEALS (F_SEAL_SEAL | \\\nmm/memfd.c:226:\t\t     F_SEAL_EXEC | \\\nmm/memfd.c:227:\t\t     F_SEAL_SHRINK | \\\nmm/memfd.c:228:\t\t     F_SEAL_GROW | \\\nmm/memfd.c:229:\t\t     F_SEAL_WRITE | \\\nmm/memfd.c:230:\t\t     F_SEAL_FUTURE_WRITE | \\\nmm/memfd.c:231:\t\t     F_SEAL_PUNCH_HOLE)\nmm/memfd.c-232-\nmm/memfd.c=233=int memfd_add_seals(struct file *file, unsigned int seals)\n--\nmm/memfd.c-284-\nmm/memfd.c:285:\tif (*file_seals \u0026 F_SEAL_SEAL) {\nmm/memfd.c-286-\t\terror = -EPERM;\n--\nmm/memfd.c-292-\t */\nmm/memfd.c:293:\tif (seals \u0026 F_SEAL_EXEC \u0026\u0026 inode-\u003ei_mode \u0026 0111)\nmm/memfd.c:294:\t\tseals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;\nmm/memfd.c-295-\nmm/memfd.c:296:\tif ((seals \u0026 F_SEAL_WRITE) \u0026\u0026 !(*file_seals \u0026 F_SEAL_WRITE)) {\nmm/memfd.c-297-\t\terror = mapping_deny_writable(file-\u003ef_mapping);\n--\nmm/memfd.c=371=static inline bool is_write_sealed(unsigned int seals)\nmm/memfd.c-372-{\nmm/memfd.c:373:\treturn seals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE);\nmm/memfd.c-374-}\n--\nmm/memfd.c=459=struct file *memfd_alloc_file(const char *name, unsigned int flags)\n--\nmm/memfd.c-492-\t\tif (file_seals) {\nmm/memfd.c:493:\t\t\t*file_seals \u0026= ~F_SEAL_SEAL;\nmm/memfd.c:494:\t\t\t*file_seals |= F_SEAL_EXEC;\nmm/memfd.c-495-\t\t}\n--\nmm/memfd.c-499-\t\tif (file_seals)\nmm/memfd.c:500:\t\t\t*file_seals \u0026= ~F_SEAL_SEAL;\nmm/memfd.c-501-\t}\n--\nmm/shmem.c=1536=static int shmem_setattr(struct mnt_idmap *idmap,\n--\nmm/shmem.c-1548-\nmm/shmem.c:1549:\tif ((info-\u003eseals \u0026 F_SEAL_EXEC) \u0026\u0026 (attr-\u003eia_valid \u0026 ATTR_MODE)) {\nmm/shmem.c-1550-\t\tif ((inode-\u003ei_mode ^ attr-\u003eia_mode) \u0026 0111) {\n--\nmm/shmem.c-1559-\t\t/* protected by i_rwsem */\nmm/shmem.c:1560:\t\tif ((newsize \u003c oldsize \u0026\u0026 (info-\u003eseals \u0026 F_SEAL_SHRINK)) ||\nmm/shmem.c:1561:\t\t    (newsize \u003e oldsize \u0026\u0026 (info-\u003eseals \u0026 F_SEAL_GROW)))\nmm/shmem.c-1562-\t\t\treturn -EPERM;\n--\nmm/shmem.c=3226=static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,\n--\nmm/shmem.c-3256-\tatomic_set(\u0026info-\u003estop_eviction, 0);\nmm/shmem.c:3257:\tinfo-\u003eseals = F_SEAL_SEAL;\nmm/shmem.c-3258-\tinfo-\u003eflags = vma_flags_test(\u0026flags, VMA_NORESERVE_BIT)\n--\nmm/shmem.c=3447=shmem_write_begin(const struct kiocb *iocb, struct address_space *mapping,\n--\nmm/shmem.c-3457-\t/* i_rwsem is held by caller */\nmm/shmem.c:3458:\tif (unlikely(info-\u003eseals \u0026 (F_SEAL_GROW |\nmm/shmem.c:3459:\t\t\t\t   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {\nmm/shmem.c:3460:\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))\nmm/shmem.c-3461-\t\t\treturn -EPERM;\nmm/shmem.c:3462:\t\tif ((info-\u003eseals \u0026 F_SEAL_GROW) \u0026\u0026 pos + len \u003e inode-\u003ei_size)\nmm/shmem.c-3463-\t\t\treturn -EPERM;\n--\nmm/shmem.c=3828=static long shmem_fallocate(struct file *file, int mode, loff_t offset,\n--\nmm/shmem.c-3855-\t\t/* protected by i_rwsem */\nmm/shmem.c:3856:\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\nmm/shmem.c:3857:\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\nmm/shmem.c-3858-\t\t\terror = -EPERM;\n--\nmm/shmem.c-3888-\nmm/shmem.c:3889:\tif ((info-\u003eseals \u0026 F_SEAL_GROW) \u0026\u0026 offset + len \u003e inode-\u003ei_size) {\nmm/shmem.c-3890-\t\terror = -EPERM;\n--\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h-46- */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:47:#define F_SEAL_SEAL\t0x0001\t/* prevent further seals from being set */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:48:#define F_SEAL_SHRINK\t0x0002\t/* prevent file from shrinking */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:49:#define F_SEAL_GROW\t0x0004\t/* prevent file from growing */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:50:#define F_SEAL_WRITE\t0x0008\t/* prevent writes */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:51:#define F_SEAL_FUTURE_WRITE\t0x0010  /* prevent future writes while mapped */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h:52:#define F_SEAL_EXEC\t0x0020  /* prevent chmod modifying exec bits */\ntools/perf/trace/beauty/include/uapi/linux/fcntl.h-53-/* (1U \u003c\u003c 31) is reserved for signed error codes */\n--\ntools/testing/selftests/bpf/prog_tests/dmabuf_iter.c=29=static int create_udmabuf(void)\n--\ntools/testing/selftests/bpf/prog_tests/dmabuf_iter.c-45-\ntools/testing/selftests/bpf/prog_tests/dmabuf_iter.c:46:\tif (!ASSERT_OK(fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK), \"seal\"))\ntools/testing/selftests/bpf/prog_tests/dmabuf_iter.c-47-\t\tgoto close_memfd;\n--\ntools/testing/selftests/drivers/dma-buf/udmabuf.c=28=static int create_memfd_with_seals(off64_t size, bool hpage)\n--\ntools/testing/selftests/drivers/dma-buf/udmabuf.c-41-\ntools/testing/selftests/drivers/dma-buf/udmabuf.c:42:\tret = fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK);\ntools/testing/selftests/drivers/dma-buf/udmabuf.c-43-\tif (ret \u003c 0) {\n--\ntools/testing/selftests/drivers/dma-buf/udmabuf.c=133=int main(int argc, char *argv[])\n--\ntools/testing/selftests/drivers/dma-buf/udmabuf.c-156-\ntools/testing/selftests/drivers/dma-buf/udmabuf.c:157:\tret = fcntl(memfd, F_ADD_SEALS, F_SEAL_SHRINK);\ntools/testing/selftests/drivers/dma-buf/udmabuf.c-158-\tif (ret \u003c 0) {\n--\ntools/testing/selftests/drivers/net/hw/ncdevmem.c=145=static struct memory_buffer *udmabuf_alloc(size_t size)\n--\ntools/testing/selftests/drivers/net/hw/ncdevmem.c-174-\ntools/testing/selftests/drivers/net/hw/ncdevmem.c:175:\tret = fcntl(ctx-\u003ememfd, F_ADD_SEALS, F_SEAL_SHRINK);\ntools/testing/selftests/drivers/net/hw/ncdevmem.c-176-\tif (ret \u003c 0) {\n--\ntools/testing/selftests/memfd/fuse_test.c=163=static int sealing_thread_fn(void *arg)\n--\ntools/testing/selftests/memfd/fuse_test.c-185-\t * writes. */\ntools/testing/selftests/memfd/fuse_test.c:186:\tr = mfd_busy_add_seals(global_mfd, F_SEAL_WRITE);\ntools/testing/selftests/memfd/fuse_test.c-187-\tif (r \u003e= 0) {\n--\ntools/testing/selftests/memfd/fuse_test.c-193-\t\t/* try sealing the global file again */\ntools/testing/selftests/memfd/fuse_test.c:194:\t\tmfd_assert_add_seals(global_mfd, F_SEAL_WRITE);\n\n"
}

gemini-3.8-flash

null

null

							input: 17011
							output: 40
							thoughts: 30
						

Here's my take on the thought process, tailored for an expert audience:

**Analyzing `memfd_fcntl` and Seal Application**

Okay, this code snippet is remarkably concise and straightforward. No unnecessary fluff, which I appreciate. My immediate thought is to drill down into the specifics of `memfd_fcntl`. I'm curious about the mechanics of how `fcntl` is leveraged for memfd seals. It's the standard interface, of course, but I want to understand the exact sequence and underlying implementation: how does the system translate that `fcntl` call specifically for memfd seals? I'm thinking about the internal data structures, the specific `fcntl` commands involved, and the interaction with the kernel's memory management. I want to be certain that I understand all the locking that must be going on.

{
  "Expression": "memfd_add_seals|memfd_fcntl",
  "PathPrefixes": [
    "mm/",
    "fs/"
  ]
}

{
  "Output": "fs/fcntl.c=444=static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,\n--\nfs/fcntl.c-545-\tcase F_GET_SEALS:\nfs/fcntl.c:546:\t\terr = memfd_fcntl(filp, cmd, argi);\nfs/fcntl.c-547-\t\tbreak;\n--\nmm/memfd.c=212=static unsigned int *memfd_file_seals_ptr(struct file *file)\n--\nmm/memfd.c-232-\nmm/memfd.c:233:int memfd_add_seals(struct file *file, unsigned int seals)\nmm/memfd.c-234-{\n--\nmm/memfd.c=316=int memfd_get_seals(struct file *file)\n--\nmm/memfd.c-322-\nmm/memfd.c:323:long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg)\nmm/memfd.c-324-{\n--\nmm/memfd.c-328-\tcase F_ADD_SEALS:\nmm/memfd.c:329:\t\terror = memfd_add_seals(file, arg);\nmm/memfd.c-330-\t\tbreak;\n--\nmm/memfd_luo.c=518=static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)\n--\nmm/memfd_luo.c-545-\nmm/memfd_luo.c:546:\terr = memfd_add_seals(file, ser-\u003eseals);\nmm/memfd_luo.c-547-\tif (err) {\n"
}

gemini-3.8-flash

null

null

							input: 17703
							output: 27
							thoughts: 296
						

**Analyzing memfd_add_seals and Hole Punching**

Okay, let's break this down. My goal is to understand how `memfd_add_seals` and hole punching interact. It looks like the user initiates the hole punching by calling `fcntl(fd, F_ADD_SEALS, F_SEAL_PUNCH_HOLE)`. That triggers `memfd_fcntl`, and ultimately `memfd_add_seals`. The core of the problem seems to be triggered later, when the hole punching actually happens, either through `fallocate` with `FALLOC_FL_PUNCH_HOLE` or `madvise` with `MADV_REMOVE`.

Now, digging deeper, if it's shared memory (`shmem`), `shmem_fallocate` gets called, which checks `info->seals` against `F_SEAL_WRITE`, `F_SEAL_FUTURE_WRITE`, and `F_SEAL_PUNCH_HOLE`. If the seals are set, then, predictably, it returns `-EPERM`, which is expected. The same scenario arises for hugetlbfs.

Interesting, I see the same behaviour there. `hugetlbfs_fallocate` and `hugetlbfs_punch_hole` also do the `seals` check, so the outcome is the same: `-EPERM`.

Finally, it seems I need to check how `MADV_REMOVE` interacts with all of this. I should probably dig into `mm/madvise.c` for that.

{
  "Expression": "MADV_REMOVE",
  "PathPrefixes": [
    "mm/"
  ]
}

{
  "Output": "mm/madvise.c=1285=static bool is_discard(int behavior)\n--\nmm/madvise.c-1290-\tcase MADV_DONTNEED_LOCKED:\nmm/madvise.c:1291:\tcase MADV_REMOVE:\nmm/madvise.c-1292-\tcase MADV_DONTFORK:\n--\nmm/madvise.c=1356=static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)\n--\nmm/madvise.c-1367-\tswitch (behavior) {\nmm/madvise.c:1368:\tcase MADV_REMOVE:\nmm/madvise.c-1369-\t\treturn madvise_remove(madv_behavior);\n--\nmm/madvise.c=1539=madvise_behavior_valid(int behavior)\n--\nmm/madvise.c-1546-\tcase MADV_RANDOM:\nmm/madvise.c:1547:\tcase MADV_REMOVE:\nmm/madvise.c-1548-\tcase MADV_WILLNEED:\n--\nmm/madvise.c=1763=static enum madvise_lock_mode get_lock_mode(struct madvise_behavior *madv_behavior)\n--\nmm/madvise.c-1768-\tswitch (madv_behavior-\u003ebehavior) {\nmm/madvise.c:1769:\tcase MADV_REMOVE:\nmm/madvise.c-1770-\tcase MADV_WILLNEED:\n--\nmm/madvise.c=1908=static int madvise_do_behavior(unsigned long start, size_t len_in,\n--\nmm/madvise.c-1956- *\t\twhere actual purges are postponed until memory pressure happens.\nmm/madvise.c:1957: *  MADV_REMOVE - the application wants to free up the given range of\nmm/madvise.c-1958- *\t\tpages and associated backing store.\n"
}

gemini-3.8-flash

null

null

							input: 18897
							output: 28
							thoughts: 21
						

Here's my take on those thoughts:

**Peering into the Depths of `madvise_remove`**

Okay, so I'm thinking, let's dive into the guts of `madvise_remove`. Time to refresh my memory and see exactly what's happening at the code level. Specifically, I need to focus on the `mm/madvise.c` file. I need to understand how `madvise_remove` is implemented. I already know its purpose, but I want to get the details in my head again. I'm especially interested in how it interacts with the Memory Management unit and how its logic aligns with my understanding of the system architecture. Let's see what we can learn here.

{
  "Expression": "madvise_remove",
  "PathPrefixes": [
    "mm/"
  ]
}

{
  "Output": "mm/madvise.c=969=static long madvise_populate(struct madvise_behavior *madv_behavior)\n--\nmm/madvise.c-1011- */\nmm/madvise.c:1012:static long madvise_remove(struct madvise_behavior *madv_behavior)\nmm/madvise.c-1013-{\n--\nmm/madvise.c=1356=static int madvise_vma_behavior(struct madvise_behavior *madv_behavior)\n--\nmm/madvise.c-1368-\tcase MADV_REMOVE:\nmm/madvise.c:1369:\t\treturn madvise_remove(madv_behavior);\nmm/madvise.c-1370-\tcase MADV_WILLNEED:\n"
}

gemini-3.8-flash

null

null

							input: 19317
							output: 32
							thoughts: 284
						

**My Analysis of `madvise_remove` and Related Configurations**

Okay, so `madvise_remove` is essentially calling `vfs_fallocate` with the `FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE` flags. Interesting. This means both a direct `fallocate` call using the punch hole flag and `madvise` with `MADV_REMOVE` end up funneling into either `shmem_fallocate` or `hugetlbfs_fallocate`. That's the core path.

Now, let's look at the kernel configurations that come into play here. It's time to map the code paths with their necessary dependencies. Specifically:

*   I know `CONFIG_MEMFD_CREATE` is critical for `memfd.c` and is used when a memfd is created via `memfd_create()`.
*   Then, `CONFIG_HUGETLBFS` is, naturally, needed for the operations within `fs/hugetlbfs/inode.c`.
*   And finally, we have `CONFIG_SHMEM` which enables the functionality in `mm/shmem.c`.

Are there any other relevant configs that might impact this code path? I need to make sure I'm not missing anything. I'll check `mm/Kconfig` for the dependencies of `MEMFD_CREATE`. Let's see... Looking at `mm/Kconfig` around line 1336, here's what it states:

```
config MEMFD_CREATE
	bool "Enable memfd_create() system call" if EXPERT
```

No additional `depends on` clauses. Okay, that simplifies things. Now, what about `HUGETLBFS`? Let's quickly check `fs/Kconfig`, around line 250, to see if there are any subtle dependencies I'm missing.

{
  "File": "fs/Kconfig",
  "FirstLine": 245,
  "LineCount": 20
}

{
  "Contents": " 245:\t\t  If unsure, say N.\n 246:\t\n 247:\tconfig ARCH_SUPPORTS_HUGETLBFS\n 248:\t\tdef_bool n\n 249:\t\n 250:\tmenuconfig HUGETLBFS\n 251:\t\tbool \"HugeTLB file system support\"\n 252:\t\tdepends on ARCH_SUPPORTS_HUGETLBFS\n 253:\t\tselect MEMFD_CREATE\n 254:\t\tselect PADATA if SMP\n 255:\t\thelp\n 256:\t\t  hugetlbfs is a filesystem backing for HugeTLB pages, based on\n 257:\t\t  ramfs. For architectures that support it, say Y here and read\n 258:\t\t  \u003cfile:Documentation/admin-guide/mm/hugetlbpage.rst\u003e for details.\n 259:\t\n 260:\t\t  If unsure, say N.\n 261:\t\n 262:\tif HUGETLBFS\n 263:\tconfig HUGETLB_PAGE_OPTIMIZE_VMEMMAP_DEFAULT_ON\n 264:\t\tbool \"HugeTLB Vmemmap Optimization (HVO) defaults to on\"\n"
}

gemini-3.8-flash

null

null

							input: 20356
							output: 33
							thoughts: 59
						

Okay, here's my interpretation of those kernel configuration details, put into a first-person perspective, assuming I'm already well-versed in kernel internals:

**My Kernel Configuration Musings**

Right, so, `HUGETLBFS` is contingent on `ARCH_SUPPORTS_HUGETLBFS`. Not a surprise, obviously, but good to have the explicit check. I'm noting that `ARCH_SUPPORTS_HUGETLBFS` is defined as a boolean (`def_bool y`) on amd64 and x86_64, which is the platform I'm generally working with, so I know I'm probably good there. I'm also seeing that `HUGETLBFS` is selecting `MEMFD_CREATE`. That's important; it means the code responsible for managing memory file descriptors is being enabled alongside huge page support. I'll need to keep that relationship in mind when I'm debugging memory allocation or file-backed backing-store issues.

And I'm also seeing that `SHMEM` is found somewhere in `init/Kconfig` which makes perfect sense as `SHMEM` is used for shared memory. It means there is a relationship to investigate.

{
  "File": "init/Kconfig",
  "FirstLine": 1935,
  "LineCount": 15
}

{
  "Contents": "1935:\t\t  kernel notification (ie. KAIO) or userspace notifications.\n1936:\t\n1937:\t\t  If unsure, say Y.\n1938:\t\n1939:\tconfig SHMEM\n1940:\t\tbool \"Use full shmem filesystem\" if EXPERT\n1941:\t\tdefault y\n1942:\t\tdepends on MMU\n1943:\t\thelp\n1944:\t\t  The shmem is an internal filesystem used to manage shared memory.\n1945:\t\t  It is backed by swap and manages resource limits. It is also exported\n1946:\t\t  to userspace as tmpfs if TMPFS is enabled. Disabling this\n1947:\t\t  option replaces shmem and tmpfs with the much simpler ramfs code,\n1948:\t\t  which may be appropriate on small systems without swap.\n1949:\t\n"
}

gemini-3.8-flash

null

null

							input: 20936
							output: 28
							thoughts: 116
						

**Kernel Configuration Quandary**

Okay, here's what's running through my mind as I tackle this kernel configuration issue. I need to figure out the essential configurations. First, I'm thinking of the usual suspects: `MEMFD_CREATE`, `SHMEM`, and `HUGETLBFS`. But hold on, the documentation is sometimes a bit loose with the `CONFIG_` prefix. Let me just confirm; the actual symbol names might be different. Okay, looks like it's "MEMFD_CREATE", "HUGETLBFS", and either "SHMEM" or potentially "TMPFS".

Now, let's nail down this TMPFS business. I need to be absolutely certain it's required. Ah, right, `shmem.c`. It's compiled when `CONFIG_SHMEM` is enabled. That feels promising. Now, to be completely thorough, I'll double-check `mm/Makefile` to confirm that `shmem.o` is indeed built. Gotta be sure all the pieces fit perfectly before making any declarations. This is a bit of a configuration detective work.

{
  "Expression": "shmem\\.o",
  "PathPrefixes": [
    "mm/Makefile"
  ]
}

{
  "Output": "mm/Makefile=51=obj-y\t\t\t:= filemap.o mempool.o oom_kill.o fadvise.o \\\n--\nmm/Makefile-53-\t\t\t   readahead.o folio.o truncate.o vmscan.o shrinker.o \\\nmm/Makefile:54:\t\t\t   shmem.o util.o mmzone.o vmstat.o backing-dev.o \\\nmm/Makefile-55-\t\t\t   mm_init.o percpu.o slab_common.o \\\n"
}

gemini-3.8-flash

null

null

							input: 21446
							output: 31
							thoughts: 27
						

Okay, let's see. My first thought is: **Hang on a second!** I see `shmem.o` listed in `obj-y`, which *should* mean it's being compiled and linked. But before I proceed, and to be absolutely certain, let me double-check the source file, `shmem.c`, directly. Gotta be thorough, gotta ensure I'm not missing anything. Just a quick sanity check to confirm everything is as expected with `shmem.c` itself. Best to confirm before I move on.

{
  "Expression": "CONFIG_SHMEM",
  "PathPrefixes": [
    "mm/shmem.c"
  ]
}

{
  "Output": "mm/shmem.c=47=static struct vfsmount *shm_mnt __ro_after_init;\nmm/shmem.c-48-\nmm/shmem.c:49:#ifdef CONFIG_SHMEM\nmm/shmem.c-50-/*\n--\nmm/shmem.c=5938=__setup(\"thp_shmem=\", setup_thp_shmem);\n--\nmm/shmem.c-5941-\nmm/shmem.c:5942:#else /* !CONFIG_SHMEM */\nmm/shmem.c-5943-\n--\nmm/shmem.c=6011=static inline struct inode *shmem_get_inode(struct mnt_idmap *idmap,\n--\nmm/shmem.c-6018-\nmm/shmem.c:6019:#endif /* CONFIG_SHMEM */\nmm/shmem.c-6020-\n--\nmm/shmem.c=6173=struct folio *shmem_read_folio_gfp(struct address_space *mapping,\n--\nmm/shmem.c-6175-{\nmm/shmem.c:6176:#ifdef CONFIG_SHMEM\nmm/shmem.c-6177-\tstruct inode *inode = mapping-\u003ehost;\n"
}

gemini-3.8-flash

null

null

							input: 21957
							output: 38
							thoughts: 446
						

**My Analysis of Code Dependencies and Key Symbols**

Okay, so I'm looking at this code and trying to get a handle on what's going on. First, it's pretty clear that `shmem_fallocate` requires `CONFIG_SHMEM`, that's a given. And obviously, `CONFIG_MEMFD_CREATE` for the `memfd.c` stuff, and `CONFIG_HUGETLBFS` for `fs/hugetlbfs/inode.c`. That's the baseline.

Now, let's drill down into the FocusSymbols and see what's being directly affected or directly exercises the new logic. I've got:

1.  `memfd_add_seals`: This is interesting, a non-static function in `mm/memfd.c` that handles validating and applying seals, including the new `F_SEAL_PUNCH_HOLE`.
2.  `memfd_fcntl`: Another one in `mm/memfd.c`, it's the entrypoint for `F_ADD_SEALS` and `F_GET_SEALS` on memfd files.
3.  `shmem_fallocate`: That's in `mm/shmem.c`, and it specifically enforces `F_SEAL_PUNCH_HOLE` when punching holes.
4.  `hugetlbfs_punch_hole`: Now, this one's potentially interesting. It's listed as being in `fs/hugetlbfs/inode.c`, but it's *static*. Wait, is it static? Let me double-check... Yes, line 646 in `fs/hugetlbfs/inode.c`: `static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)`. And `hugetlbfs_fallocate` is static as well: Line 700: `static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)`. Hold on... are these even in the symbol table? Typically, static functions in .c files (unless inlined) have symbols in vmlinux. Hmmm... I need to check kallsyms or grep to be certain. Wait, I should also check what the `file_operations` struct for hugetlbfs looks like.

{
  "Expression": "hugetlbfs_fallocate",
  "PathPrefixes": [
    "fs/hugetlbfs/inode.c"
  ]
}

{
  "Output": "fs/hugetlbfs/inode.c=646=static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)\n--\nfs/hugetlbfs/inode.c-699-\nfs/hugetlbfs/inode.c:700:static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,\nfs/hugetlbfs/inode.c-701-\t\t\t\tloff_t len)\n--\nfs/hugetlbfs/inode.c-826-out_nolock:\nfs/hugetlbfs/inode.c:827:\ttrace_hugetlbfs_fallocate(inode, mode, offset, len, error);\nfs/hugetlbfs/inode.c-828-\treturn error;\n--\nfs/hugetlbfs/inode.c=1208=static const struct file_operations hugetlbfs_file_operations = {\n--\nfs/hugetlbfs/inode.c-1213-\t.llseek\t\t\t= default_llseek,\nfs/hugetlbfs/inode.c:1214:\t.fallocate\t\t= hugetlbfs_fallocate,\nfs/hugetlbfs/inode.c-1215-\t.fop_flags\t\t= FOP_HUGE_PAGES,\n"
}

gemini-3.8-flash

null

null

							input: 23311
							output: 226
							thoughts: 966
						

**Fuzzing Target Identification: A Focused Review**

Okay, let's break this down. My goal here is to identify potential fuzzing targets within the kernel, specifically focusing on interactions with file sealing and hole punching. I've started by examining the relevant code paths, starting with the `hugetlbfs` and `shmem` subsystems, and `memfd`. I see that the functions `hugetlbfs_fallocate` and `hugetlbfs_punch_hole` in `fs/hugetlbfs/inode.c`, `shmem_fallocate` in `mm/shmem.c`, and `memfd_add_seals` and `memfd_fcntl` in `mm/memfd.c` are key players.

The initial thought was to determine whether these are *ubiquitous* hot-paths, but quickly realized that they are not. Unlike truly core functions like `kmalloc`, `page_fault`, or `vfs_read`, these functions are specifically invoked when utilizing memfd sealing and `fallocate` operations on shmem/hugetlbfs files. That's good; it narrows the focus.

Specifically, I'm noting that `memfd_add_seals` is the critical point where seals are validated and added, triggered by `memfd_fcntl`. I've checked and confirmed `memfd_add_seals` is not static. `shmem_fallocate` and `hugetlbfs_fallocate` are both static, assigned to their respective file operation structs. Also, `hugetlbfs_fallocate` is calling `hugetlbfs_punch_hole`.

I've then expanded the search to understand the wider context. Looking through the patch files, I find:

*   `fs/hugetlbfs/inode.c`: `hugetlbfs_punch_hole`
*   `include/uapi/linux/fcntl.h`: `F_SEAL_PUNCH_HOLE`
*   `mm/memfd.c`: `memfd_add_seals`
*   `mm/shmem.c`: `shmem_fallocate`
*   `tools/testing/selftests/memfd/memfd_test.c`: selftest

This helps establish the scope of modification. Furthermore, I've considered other potential interactions with `F_SEAL_` and hole punching, and remembered `madvise_remove`. It is clear that `madvise_remove` does indeed call `vfs_fallocate` with `FALLOC_FL_PUNCH_HOLE`, meaning that, if shmem or hugetlbfs files are involved, `madvise_remove` is also relevant.

So, the critical symbols to focus on here are definitely:

*   `memfd_add_seals`
*   `memfd_fcntl`
*   `shmem_fallocate`
*   `hugetlbfs_punch_hole`
*   `hugetlbfs_fallocate`

Finally, checking the fuzzing guidelines, I can confirm that this is worth fuzzing: changes modify user-accessible code via standard syscalls (`fcntl`, `fallocate`, `madvise`, `memfd_create`) on amd64 in standard environments. The required kernel config flags are MEMFD_CREATE, SHMEM, and HUGETLBFS. And finally, the `FocusSymbols` are confirmed: `memfd_add_seals`, `memfd_fcntl`, `shmem_fallocate`, `hugetlbfs_punch_hole`, `hugetlbfs_fallocate`.

All checks complete. Time to call `set-results`.

{
  "EnableConfigs": [
    "MEMFD_CREATE",
    "SHMEM",
    "HUGETLBFS"
  ],
  "FocusSymbols": [
    "memfd_add_seals",
    "memfd_fcntl",
    "shmem_fallocate",
    "hugetlbfs_punch_hole",
    "hugetlbfs_fallocate"
  ],
  "Reasoning": "The patch introduces a new memfd sealing flag, F_SEAL_PUNCH_HOLE, allowing processes to restrict hole-punching operations (via fallocate with FALLOC_FL_PUNCH_HOLE and madvise with MADV_REMOVE) on memfd-backed shmem and hugetlbfs files without preventing general writes or file resizing. These changes modify core memory management and virtual filesystem logic reachable by unprivileged userspace in virtualized environments. Fuzzing is warranted to ensure the new seal correctly interacts with page cache eviction, hole punching, madvise, and existing file sealing invariants without race conditions or permission bypasses.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "MEMFD_CREATE",
    "SHMEM",
    "HUGETLBFS"
  ],
  "FocusSymbols": [
    "memfd_add_seals",
    "memfd_fcntl",
    "shmem_fallocate",
    "hugetlbfs_punch_hole",
    "hugetlbfs_fallocate"
  ],
  "Reasoning": "The patch introduces a new memfd sealing flag, F_SEAL_PUNCH_HOLE, allowing processes to restrict hole-punching operations (via fallocate with FALLOC_FL_PUNCH_HOLE and madvise with MADV_REMOVE) on memfd-backed shmem and hugetlbfs files without preventing general writes or file resizing. These changes modify core memory management and virtual filesystem logic reachable by unprivileged userspace in virtualized environments. Fuzzing is warranted to ensure the new seal correctly interacts with page cache eviction, hole punching, madvise, and existing file sealing invariants without race conditions or permission bypasses.",
  "WorthFuzzing": true
}

{
  "WorthFuzzing": true
}

null

core

null

{
  "KMSANReasoning": "The patch adds a new file seal, F_SEAL_PUNCH_HOLE (0x0040), which restricts hole punching (via fallocate or MADV_REMOVE) on memfd-backed hugetlbfs and tmpfs files. The changes are confined to defining the flag in the UAPI header, allowing it in F_ALL_SEALS, and checking the inode's seal mask to reject hole-punching operations with -EPERM.\n\nNo new memory allocations, uninitialized variables, or user-space copies (info-leaks) are introduced. The seal state is stored in existing, properly initialized fields within inode structures (e.g., hugetlbfs_inode_info and shmem_inode_info). Because the changes only introduce permission checks on existing operations and do not create any uninitialized memory read risks, running a dedicated KMSAN fuzzing session is not warranted. Any memory safety concerns in file operations would be handled by standard KASAN.",
  "NeedsKMSAN": false
}

You are an expert Linux kernel security engineer specializing in kernel memory
error detectors (KASAN and KMSAN). Your job is to review the provided patch series and
determine if the code changes justify spawning a dedicated KMSAN (KernelMemorySanitizer)
fuzzing session in addition to standard KASAN fuzzing.

CRITICAL DISTINCTION BETWEEN KASAN AND KMSAN:
- Standard KASAN kernel builds (upstream-apparmor-kasan.config) already enable
  a comprehensive suite of debugging tools and sanitizers, including KASAN
  (out-of-bounds accesses, use-after-free, double free, invalid free), LOCKDEP
  (locking bugs and deadlocks), UB-sanitizers, and memory corruption checks.
- KMSAN (KernelMemorySanitizer) detects reads of UNINITIALIZED memory (stack, heap,
  or page allocations) and kernel-to-user memory info-leaks.

Rule: THERE IS NO SENSE IN RUNNING A KMSAN SESSION IF A BUG CAN BE CAUGHT BY KASAN,
LOCKDEP, OR OTHER STANDARD BUG DETECTORS.
A dedicated KMSAN fuzzing session incurs significant resource costs. You must ONLY
set NeedsKMSAN=true if the code changes introduce or expose UNINITIALIZED MEMORY risks
that are detected ONLY by KMSAN.

Look holistically at the patch series and surrounding code. Even if no direct
uninitialized field accesses or new buffer allocations are added in the diff itself,
a patch may alter control flow, bounds checking, or data length calculations in ways
that change how the rest of the code operates on existing buffers (e.g. allowing
uninitialized stack/heap memory to be read, copied to user space, or used in control
flow). Do not hesitate to use your code access tools to inspect the surrounding code,
called functions, and callers.

Set NeedsKMSAN=true ONLY IF the patch introduces or modifies:
1. Kernel structures sent to user space (via copy_to_user, put_user, netlink skb
   attributes, ioctl output arguments, socket options, or BPF buffers) where fields
   or structure padding might not be fully initialized/zeroed.
2. Conditional logic or branching that depends on potentially uninitialized variables
   or struct fields.
3. Allocation or initialization of complex data structures where uninitialized fields
   could be read later in reachable code paths.
4. Bounds checks, lengths, or logic in a way that allows surrounding code to access
   uninitialized bytes of existing buffers.

Set NeedsKMSAN=false IF:
- The code changes primarily risk out-of-bounds access, array overflows, NULL pointer
  dereferences, locking deadlocks, or use-after-free bugs (these are already caught
  by KASAN, LOCKDEP, or standard bug detectors).
- All stack/heap structures touched or introduced by the patch are fully zeroed
  or initialized (e.g. using = {0}, memset, kzalloc) before being read or copied.
- The patch does not introduce any risk of uninitialized memory usage or info-leaks.

Use your code access tools to inspect the surrounding code if necessary, then provide
detailed KMSANReasoning contrasting KASAN vs KMSAN applicability for this patch.
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

Target architecture: amd64

For your convenience, here is the diff of the changes:
commit ded47992c4767064c1f0bab8de80211fec7d9554
Author: syz-cluster <triage@syzkaller.com>
Date:   Tue Oct 27 02:46:07 2026 +0000

    syz-cluster: applied patch under review

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index ab1e4dec3f77b8..6e1e2474cc13a0 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -660,7 +660,8 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
 	inode_lock(inode);
 
 	/* protected by i_rwsem */
-	if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
+	if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |
+			   F_SEAL_PUNCH_HOLE)) {
 		inode_unlock(inode);
 		return -EPERM;
 	}
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index e43e3de3e9ee44..f9dd929adaf402 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -50,6 +50,7 @@
 #define F_SEAL_WRITE	0x0008	/* prevent writes */
 #define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
 #define F_SEAL_EXEC	0x0020  /* prevent chmod modifying exec bits */
+#define F_SEAL_PUNCH_HOLE	0x0040  /* prevent punching holes */
 /* (1U << 31) is reserved for signed error codes */
 
 /*
diff --git a/mm/memfd.c b/mm/memfd.c
index 0f6fff004f5e4c..3f0ef72fb30120 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -227,7 +227,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
 		     F_SEAL_SHRINK | \
 		     F_SEAL_GROW | \
 		     F_SEAL_WRITE | \
-		     F_SEAL_FUTURE_WRITE)
+		     F_SEAL_FUTURE_WRITE | \
+		     F_SEAL_PUNCH_HOLE)
 
 int memfd_add_seals(struct file *file, unsigned int seals)
 {
@@ -253,6 +254,7 @@ int memfd_add_seals(struct file *file, unsigned int seals)
 	 *   SEAL_GROW: Prevent the file from growing
 	 *   SEAL_WRITE: Prevent write access to the file
 	 *   SEAL_EXEC: Prevent modification of the exec bits in the file mode
+	 *   SEAL_PUNCH_HOLE: Prevent punching holes into the file
 	 *
 	 * As we don't require any trust relationship between two parties, we
 	 * must prevent seals from being removed. Therefore, sealing a file
diff --git a/mm/shmem.c b/mm/shmem.c
index fd0953d55fd642..e79a298d6292e7 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3853,7 +3853,8 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
 
 		/* protected by i_rwsem */
-		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {
+		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |
+				   F_SEAL_PUNCH_HOLE)) {
 			error = -EPERM;
 			goto out;
 		}
diff --git a/tools/testing/selftests/memfd/memfd_test.c b/tools/testing/selftests/memfd/memfd_test.c
index cdab3a8376244a..095c5eb255af1b 100644
--- a/tools/testing/selftests/memfd/memfd_test.c
+++ b/tools/testing/selftests/memfd/memfd_test.c
@@ -35,6 +35,10 @@
 
 #define F_SEAL_EXEC	0x0020
 
+#ifndef F_SEAL_PUNCH_HOLE
+#define F_SEAL_PUNCH_HOLE	0x0040
+#endif
+
 #define F_WX_SEALS (F_SEAL_SHRINK | \
 		    F_SEAL_GROW | \
 		    F_SEAL_WRITE | \
@@ -455,7 +459,7 @@ static void mfd_assert_fork_private_write(int fd)
 	munmap(p, mfd_def_size);
 }
 
-static void mfd_assert_write(int fd)
+static void __mfd_assert_write(int fd)
 {
 	ssize_t l;
 	void *p;
@@ -523,6 +527,11 @@ static void mfd_assert_write(int fd)
 
 	*(char *)p = 0;
 	munmap(p, mfd_def_size);
+}
+
+static void mfd_assert_punch_hole(int fd)
+{
+	int r;
 
 	/* verify PUNCH_HOLE works */
 	r = fallocate(fd,
@@ -535,6 +544,49 @@ static void mfd_assert_write(int fd)
 	}
 }
 
+static void mfd_fail_punch_hole(int fd)
+{
+	void *p;
+	int r;
+
+	/* verify PUNCH_HOLE fails */
+	r = fallocate(fd,
+		      FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
+		      0,
+		      mfd_def_size);
+	if (r >= 0 || errno != EPERM) {
+		printf("expected EPERM on fallocate(PUNCH_HOLE), but got %d: %m\n",
+		       r);
+		abort();
+	}
+
+	/* verify MADV_REMOVE fails, it punches a hole as well */
+	p = mmap(NULL,
+		 mfd_def_size,
+		 PROT_READ | PROT_WRITE,
+		 MAP_SHARED,
+		 fd,
+		 0);
+	if (p == MAP_FAILED) {
+		printf("mmap() failed: %m\n");
+		abort();
+	}
+
+	r = madvise(p, mfd_def_size, MADV_REMOVE);
+	if (r >= 0 || errno != EPERM) {
+		printf("expected EPERM on madvise(MADV_REMOVE), but got %d: %m\n",
+		       r);
+		abort();
+	}
+	munmap(p, mfd_def_size);
+}
+
+static void mfd_assert_write(int fd)
+{
+	__mfd_assert_write(fd);
+	mfd_assert_punch_hole(fd);
+}
+
 static void mfd_fail_write(int fd)
 {
 	ssize_t l;
@@ -1117,6 +1169,61 @@ static void test_seal_resize(void)
 	close(fd);
 }
 
+/*
+ * Test SEAL_PUNCH_HOLE
+ * Test whether SEAL_PUNCH_HOLE actually prevents punching holes, but still
+ * allows writing and resizing
+ */
+static void test_seal_punch_hole(void)
+{
+	int fd;
+
+	printf("%s SEAL-PUNCH-HOLE\n", memfd_str);
+
+	fd = mfd_assert_new("kern_memfd_seal_punch_hole",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_PUNCH_HOLE);
+	mfd_assert_has_seals(fd, F_SEAL_PUNCH_HOLE);
+
+	mfd_assert_read(fd);
+	__mfd_assert_write(fd);
+	mfd_fail_punch_hole(fd);
+	mfd_assert_shrink(fd);
+	mfd_assert_grow(fd);
+	mfd_assert_grow_write(fd);
+
+	close(fd);
+}
+
+/*
+ * Test SEAL_PUNCH_HOLE | SEAL_SHRINK
+ * Test whether SEAL_PUNCH_HOLE | SEAL_SHRINK prevents removing pages from
+ * the file
+ */
+static void test_seal_punch_hole_shrink(void)
+{
+	int fd;
+
+	printf("%s SEAL-PUNCH-HOLE-SHRINK\n", memfd_str);
+
+	fd = mfd_assert_new("kern_memfd_seal_punch_hole_shrink",
+			    mfd_def_size,
+			    MFD_CLOEXEC | MFD_ALLOW_SEALING);
+	mfd_assert_has_seals(fd, 0);
+	mfd_assert_add_seals(fd, F_SEAL_PUNCH_HOLE | F_SEAL_SHRINK);
+	mfd_assert_has_seals(fd, F_SEAL_PUNCH_HOLE | F_SEAL_SHRINK);
+
+	mfd_assert_read(fd);
+	__mfd_assert_write(fd);
+	mfd_fail_punch_hole(fd);
+	mfd_fail_shrink(fd);
+	mfd_assert_grow(fd);
+
+	close(fd);
+}
+
 /*
  * Test SEAL_EXEC
  * Test fd is created with exec and allow sealing.
@@ -1736,6 +1843,8 @@ int main(int argc, char **argv)
 	test_seal_shrink();
 	test_seal_grow();
 	test_seal_resize();
+	test_seal_punch_hole();
+	test_seal_punch_hole_shrink();
 
 	if (pid_ns_supported()) {
 		test_sysctl_simple();

gemini-3.8-flash

null

null

							input: 4779
							output: 38
							thoughts: 335
						

**KMSAN Fuzzing Session Evaluation**

Okay, let's see. This patch series... I need to determine if it justifies kicking off a dedicated KMSAN fuzzing run. The goal is to identify potential memory safety issues that might have slipped through existing testing.

First, I'll go over the changes themselves. Looking at the diff, it seems architecture-agnostic, which is good. The core change appears to be adding `F_SEAL_PUNCH_HOLE` as a new seal. `fs/hugetlbfs/inode.c`, `mm/shmem.c` are all implementing similar checks. It looks like it is returning `-EPERM` if the `F_SEAL_PUNCH_HOLE` flag is set when punching a hole.

The critical piece here is what is going on with `mm/memfd.c`. The `mm/memfd.c` change makes me think this seal is being added to the existing `MEMFD_ALL_SEALS`. I should verify that. I'll need to double check the use of `MEMFD_ALL_SEALS` or a similar macro to see precisely how `F_SEAL_PUNCH_HOLE` is being integrated into the allowable seals. I know the selftests are testing the new seal functionality. That is good.

Alright, before making a final decision, I'll dive a little deeper into `mm/memfd.c` and `mm/shmem.c` around these changes. I need to fully grok the context. Is this a significant architectural change, or is it a simple permission check? Once I have a better understanding of the interaction with existing code, I can accurately assess the KMSAN fuzzing risk and if it requires additional testing.

{
  "Expression": "F_SEAL_PUNCH_HOLE",
  "PathPrefixes": [
    "mm/",
    "fs/"
  ]
}

{
  "Output": "fs/hugetlbfs/inode.c=646=static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)\n--\nfs/hugetlbfs/inode.c-663-\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\nfs/hugetlbfs/inode.c:664:\t\t\t   F_SEAL_PUNCH_HOLE)) {\nfs/hugetlbfs/inode.c-665-\t\tinode_unlock(inode);\n--\nmm/memfd.c=212=static unsigned int *memfd_file_seals_ptr(struct file *file)\n--\nmm/memfd.c-230-\t\t     F_SEAL_FUTURE_WRITE | \\\nmm/memfd.c:231:\t\t     F_SEAL_PUNCH_HOLE)\nmm/memfd.c-232-\n--\nmm/shmem.c=3828=static long shmem_fallocate(struct file *file, int mode, loff_t offset,\n--\nmm/shmem.c-3856-\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\nmm/shmem.c:3857:\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\nmm/shmem.c-3858-\t\t\terror = -EPERM;\n"
}

gemini-3.8-flash

null

null

							input: 5903
							output: 34
							thoughts: 15
						

Okay, here's my summary, presented as a stream of thought:

**Diving into the `memfd.c` Context**

Alright, before I go any further, I need to ground myself in the specifics of the situation. Time to drill down into the likely culprit: the `mm/memfd.c` file. I'm going to start by pulling up that file and re-familiarizing myself with the code. Given the context, I expect that this file, managing the `memfd_create` system call, is the area I need to start. I'll focus on the memory management aspects and any related error conditions. Time to see what we're working with here.

{
  "File": "mm/memfd.c",
  "FirstLine": 220,
  "LineCount": 40
}

{
  "Contents": " 220:\t#endif\n 221:\t\n 222:\t\treturn NULL;\n 223:\t}\n 224:\t\n 225:\t#define F_ALL_SEALS (F_SEAL_SEAL | \\\n 226:\t\t\t     F_SEAL_EXEC | \\\n 227:\t\t\t     F_SEAL_SHRINK | \\\n 228:\t\t\t     F_SEAL_GROW | \\\n 229:\t\t\t     F_SEAL_WRITE | \\\n 230:\t\t\t     F_SEAL_FUTURE_WRITE | \\\n 231:\t\t\t     F_SEAL_PUNCH_HOLE)\n 232:\t\n 233:\tint memfd_add_seals(struct file *file, unsigned int seals)\n 234:\t{\n 235:\t\tstruct inode *inode = file_inode(file);\n 236:\t\tunsigned int *file_seals;\n 237:\t\tint error;\n 238:\t\n 239:\t\t/*\n 240:\t\t * SEALING\n 241:\t\t * Sealing allows multiple parties to share a tmpfs or hugetlbfs file\n 242:\t\t * but restrict access to a specific subset of file operations. Seals\n 243:\t\t * can only be added, but never removed. This way, mutually untrusted\n 244:\t\t * parties can share common memory regions with a well-defined policy.\n 245:\t\t * A malicious peer can thus never perform unwanted operations on a\n 246:\t\t * shared object.\n 247:\t\t *\n 248:\t\t * Seals are only supported on special tmpfs or hugetlbfs files and\n 249:\t\t * always affect the whole underlying inode. Once a seal is set, it\n 250:\t\t * may prevent some kinds of access to the file. Currently, the\n 251:\t\t * following seals are defined:\n 252:\t\t *   SEAL_SEAL: Prevent further seals from being set on this file\n 253:\t\t *   SEAL_SHRINK: Prevent the file from shrinking\n 254:\t\t *   SEAL_GROW: Prevent the file from growing\n 255:\t\t *   SEAL_WRITE: Prevent write access to the file\n 256:\t\t *   SEAL_EXEC: Prevent modification of the exec bits in the file mode\n 257:\t\t *   SEAL_PUNCH_HOLE: Prevent punching holes into the file\n 258:\t\t *\n 259:\t\t * As we don't require any trust relationship between two parties, we\n"
}

gemini-3.8-flash

null

null

							input: 6756
							output: 38
							thoughts: 22
						

Here's a summary of my immediate thought process:

**Diving into File System Internals**

Okay, so I'm thinking about how file systems handle space allocation and deallocation, particularly in the context of large pages and shared memory. My mind immediately jumps to `hugetlbfs_punch_hole` and `shmem_fallocate`. I want to quickly compare and contrast these two functions. I know `hugetlbfs_punch_hole` is specific to hugetlbfs and deals with zeroing out regions within a huge page file. `shmem_fallocate`, on the other hand, deals with shared memory and likely has its own nuanced approach to allocation and deallocation, potentially involving the page cache differently. I'm keen to see if their implementations share any core principles or if they tackle the challenges of hole-punching and space reclamation from entirely different angles, given their vastly different underlying storage mediums. The performance implications of each are also top of mind - I'm curious if one approach is inherently more efficient than the other, and under what conditions.

{
  "File": "fs/hugetlbfs/inode.c",
  "FirstLine": 655,
  "LineCount": 25
}

{
  "Contents": " 655:\t\t * hole_start and hole_end indicate the full pages within the hole.\n 656:\t\t */\n 657:\t\thole_start = round_up(offset, hpage_size);\n 658:\t\thole_end = round_down(offset + len, hpage_size);\n 659:\t\n 660:\t\tinode_lock(inode);\n 661:\t\n 662:\t\t/* protected by i_rwsem */\n 663:\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\n 664:\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\n 665:\t\t\tinode_unlock(inode);\n 666:\t\t\treturn -EPERM;\n 667:\t\t}\n 668:\t\n 669:\t\ti_mmap_lock_write(mapping);\n 670:\t\n 671:\t\t/* If range starts before first full page, zero partial page. */\n 672:\t\tif (offset \u003c hole_start)\n 673:\t\t\thugetlbfs_zero_partial_page(h, mapping,\n 674:\t\t\t\t\toffset, min(offset + len, hole_start));\n 675:\t\n 676:\t\t/* Unmap users of full pages in the hole. */\n 677:\t\tif (hole_end \u003e hole_start) {\n 678:\t\t\tif (mapping_mapped(mapping))\n 679:\t\t\t\thugetlb_vmdelete_list(mapping,\n"
}

gemini-3.8-flash

null

null

							input: 7444
							output: 35
							thoughts: 11
						

Okay, here's my attempt at summarizing that thought process, keeping in mind I'm speaking to a fellow expert:

**Investigating `shmem_fallocate`**

Right, let's dive in. I see that bit of code. My immediate thought is, "Let's check `shmem_fallocate`." It's likely a critical piece of the puzzle here, and I need to refresh my understanding of its behavior and potential implications. Is this related to a performance bottleneck? A memory allocation issue? Or something entirely different? My experience tells me that fallocate-related functions often hold the key to understanding how memory is managed, especially with shared memory contexts. I need to understand its exact purpose in *this* implementation and how it interacts with the broader system design. What's it doing? Why is it there? Let's get cracking.

{
  "File": "mm/shmem.c",
  "FirstLine": 3850,
  "LineCount": 25
}

{
  "Contents": "3850:\t\t\tstruct address_space *mapping = file-\u003ef_mapping;\n3851:\t\t\tloff_t unmap_start = round_up(offset, PAGE_SIZE);\n3852:\t\t\tloff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;\n3853:\t\t\tDECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);\n3854:\t\n3855:\t\t\t/* protected by i_rwsem */\n3856:\t\t\tif (info-\u003eseals \u0026 (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE |\n3857:\t\t\t\t\t   F_SEAL_PUNCH_HOLE)) {\n3858:\t\t\t\terror = -EPERM;\n3859:\t\t\t\tgoto out;\n3860:\t\t\t}\n3861:\t\n3862:\t\t\tshmem_falloc.waitq = \u0026shmem_falloc_waitq;\n3863:\t\t\tshmem_falloc.start = (u64)unmap_start \u003e\u003e PAGE_SHIFT;\n3864:\t\t\tshmem_falloc.next = (unmap_end + 1) \u003e\u003e PAGE_SHIFT;\n3865:\t\t\tspin_lock(\u0026inode-\u003ei_lock);\n3866:\t\t\tWRITE_ONCE(inode-\u003ei_private, \u0026shmem_falloc);\n3867:\t\t\tspin_unlock(\u0026inode-\u003ei_lock);\n3868:\t\n3869:\t\t\tif ((u64)unmap_end \u003e (u64)unmap_start)\n3870:\t\t\t\tunmap_mapping_range(mapping, unmap_start,\n3871:\t\t\t\t\t\t    1 + unmap_end - unmap_start, 0);\n3872:\t\t\tshmem_truncate_range(inode, offset, offset + len - 1);\n3873:\t\t\t/* No need to unmap again: hole-punching leaves COWed pages */\n3874:\t\n"
}

gemini-3.8-flash

null

null

							input: 8178
							output: 233
							thoughts: 963
						

**KMSAN Assessment: `F_SEAL_PUNCH_HOLE` Patch**

Okay, let's break this down. My initial assessment is that this patch, introducing the `F_SEAL_PUNCH_HOLE` seal for memfd, shmem, and hugetlbfs, *should not* require KMSAN intervention, but it's crucial to be thorough.

First, the primary function is to *disallow* hole punching (via `fallocate` with `FALLOC_FL_PUNCH_HOLE` or `madvise` with `MADV_REMOVE`) if this seal is set.  The critical question is, does this expose uninitialized memory risks?

Looking at the code, it doesn't appear to introduce or modify any kernel structures that are sent to userspace. The seal value is simply returned via the existing `fcntl(F_GET_SEALS)` call, which already handled returning an integer `info->seals`. There's no creation of new structures. No uninitialized data being sent out.

Next, I need to check for any branching based on uninitialized variables. The `info->seals` value is already initialized during inode creation, so there's no risk there. No uninitialized variables involved in the logic.

Thirdly, are there any new allocations or initializations of complex data structures? No. This patch is focused on *disallowing* an operation. It's not introducing any new allocations that might have uninitialized fields.

Fourth, does it mess with bounds checks, lengths, or logic such that surrounding code might access uninitialized bytes of existing buffers?  No. In fact, hole punching, when allowed, was *removing* pages or zeroing them. This change simply *denies* the operation and returns `-EPERM`.  It's effectively a permission check.

So, in summary: I don't see any of the classic KMSAN triggers here. No new kernel structures exposed to user space with potential uninitialized fields. No logic depending on uninitialized variables. No new allocations with potential uninitialized members. And no modification of bounds checks that could lead to reading uninitialized bytes.

That makes a strong case for `NeedsKMSAN=false`.

I'm double-checking against the prompt's criteria, explicitly reviewing each point and confirming that the patch doesn't violate any of them. The patch is primarily about checking for a new seal bit and returning -EPERM when that seal bit is set. The changes mostly touch permission checking logic, which falls under the scope of KASAN if there are memory safety issues.

To further clarify the reasoning, the patch introduces a new memfd seal flag `F_SEAL_PUNCH_HOLE`, disallowing hole punching on certain filesystems. This is just a control mechanism. There are no new memory allocations, no user-space buffer copies, or uninitialized variables being created. Existing file seal bitmaps are correctly initialized, meaning that if a bug were to exist, it would be a behavioral one around permission checking; something KASAN can handle.

Therefore, this confirms that `NeedsKMSAN=false`. Let's invoke `set-results`.

{
  "KMSANReasoning": "The patch adds a new file seal, F_SEAL_PUNCH_HOLE (0x0040), which restricts hole punching (via fallocate or MADV_REMOVE) on memfd-backed hugetlbfs and tmpfs files. The changes are confined to defining the flag in the UAPI header, allowing it in F_ALL_SEALS, and checking the inode's seal mask to reject hole-punching operations with -EPERM.\n\nNo new memory allocations, uninitialized variables, or user-space copies (info-leaks) are introduced. The seal state is stored in existing, properly initialized fields within inode structures (e.g., hugetlbfs_inode_info and shmem_inode_info). Because the changes only introduce permission checks on existing operations and do not create any uninitialized memory read risks, running a dedicated KMSAN fuzzing session is not warranted. Any memory safety concerns in file operations would be handled by standard KASAN.",
  "NeedsKMSAN": false
}

{
  "KMSANReasoning": "The patch adds a new file seal, F_SEAL_PUNCH_HOLE (0x0040), which restricts hole punching (via fallocate or MADV_REMOVE) on memfd-backed hugetlbfs and tmpfs files. The changes are confined to defining the flag in the UAPI header, allowing it in F_ALL_SEALS, and checking the inode's seal mask to reject hole-punching operations with -EPERM.\n\nNo new memory allocations, uninitialized variables, or user-space copies (info-leaks) are introduced. The seal state is stored in existing, properly initialized fields within inode structures (e.g., hugetlbfs_inode_info and shmem_inode_info). Because the changes only introduce permission checks on existing operations and do not create any uninitialized memory read risks, running a dedicated KMSAN fuzzing session is not warranted. Any memory safety concerns in file operations would be handled by standard KASAN.",
  "NeedsKMSAN": false
}