AI Trajectory

null

{
  "EnableConfigs": [
    "FUTEX_PRIVATE_HASH"
  ],
  "FocusSymbols": [
    "futex_hash_prctl",
    "futex_hash_allocate_default",
    "futex_hash"
  ],
  "KMSANReasoning": "The patch addresses a concurrency issue in futex hash allocation and pivoting by replacing a mutex with an RCU read lock, adding memory barriers, and using READ_ONCE/WRITE_ONCE macros for the `hash_new` pointer. It does not introduce any new data structures, allocations, or user-space copies that could lead to uninitialized memory usage or info-leaks. Any potential bugs introduced by these changes would be related to data races, use-after-free, or locking issues, which are better detected by KCSAN, KASAN, or LOCKDEP.",
  "NeedsKMSAN": false,
  "Reasoning": "The patch modifies the synchronization logic in the futex core for private hashes, changing a mutex to RCU and adding memory barriers. This is a functional change in core kernel logic that should be fuzzed.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit 417797e9ff1c1b2e89f54f976fda3007ee37c3af\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Thu Aug 13 05:40:36 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/kernel/futex/core.c b/kernel/futex/core.c\nindex 128c5752f225c..e84be541059ab 100644\n--- a/kernel/futex/core.c\n+++ b/kernel/futex/core.c\n@@ -202,7 +202,7 @@ static bool __futex_pivot_hash(struct mm_struct *mm, struct futex_private_hash *\n \tfph = rcu_dereference_protected(mmph-\u003ehash, lockdep_is_held(\u0026mmph-\u003elock));\n \tif (fph) {\n \t\tif (!futex_ref_is_dead(fph)) {\n-\t\t\tmmph-\u003ehash_new = new;\n+\t\t\tWRITE_ONCE(mmph-\u003ehash_new, new);\n \t\t\treturn false;\n \t\t}\n \n@@ -224,7 +224,7 @@ static void futex_pivot_hash(struct mm_struct *mm)\n \n \t\tfph = mm-\u003efutex.phash.hash_new;\n \t\tif (fph) {\n-\t\t\tmm-\u003efutex.phash.hash_new = NULL;\n+\t\t\tWRITE_ONCE(mm-\u003efutex.phash.hash_new, NULL);\n \t\t\t__futex_pivot_hash(mm, fph);\n \t\t}\n \t}\n@@ -1786,12 +1786,18 @@ static bool futex_pivot_pending(struct mm_struct *mm)\n \tstruct futex_mm_phash *mmph = \u0026mm-\u003efutex.phash;\n \tstruct futex_private_hash *fph;\n \n-\tguard(mutex)(\u0026mmph-\u003elock);\n+\tguard(rcu)();\n \n-\tif (!mmph-\u003ehash_new)\n+\tfph = rcu_dereference(mmph-\u003ehash);\n+\t/*\n+\t * Ensure that if we see the new hash, we will also see the cleared\n+\t * hash_new pointer. Pairs with rcu_assign_pointer() in\n+\t * __futex_pivot_hash().\n+\t */\n+\tsmp_rmb();\n+\tif (!READ_ONCE(mmph-\u003ehash_new))\n \t\treturn true;\n \n-\tfph = rcu_dereference_raw(mmph-\u003ehash);\n \treturn futex_ref_is_dead(fph);\n }\n \n@@ -1879,7 +1885,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)\n \t\tcur = rcu_dereference_protected(mm-\u003efutex.phash.hash,\n \t\t\t\t\t\tlockdep_is_held(\u0026mm-\u003efutex.phash.lock));\n \t\tnew = mm-\u003efutex.phash.hash_new;\n-\t\tmm-\u003efutex.phash.hash_new = NULL;\n+\t\tWRITE_ONCE(mm-\u003efutex.phash.hash_new, NULL);\n \n \t\tif (fph) {\n \t\t\tif (cur \u0026\u0026 !cur-\u003ehash_mask) {\n@@ -1889,7 +1895,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)\n \t\t\t\t * the second one returns here.\n \t\t\t\t */\n \t\t\t\tfree = fph;\n-\t\t\t\tmm-\u003efutex.phash.hash_new = new;\n+\t\t\t\tWRITE_ONCE(mm-\u003efutex.phash.hash_new, new);\n \t\t\t\treturn -EBUSY;\n \t\t\t}\n \t\t\tif (cur \u0026\u0026 !new) {\n"
}

best-expensive

null

{
  "EnableConfigs": [
    "FUTEX_PRIVATE_HASH"
  ],
  "FocusSymbols": [
    "futex_hash_prctl",
    "futex_hash_allocate_default",
    "futex_hash"
  ],
  "Reasoning": "The patch modifies the synchronization logic in the futex core for private hashes, changing a mutex to RCU and adding memory barriers. This is a functional change in core kernel logic that should be fuzzed.",
  "WorthFuzzing": true
}

You are an expert Linux kernel maintainer.
Your job is to review a provided patch series and determine
if it makes functional changes to the kernel that should be fuzzed.

IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in
your workspace. Do NOT rely on your internal knowledge of the kernel. You must actively
use your code access tools to examine the actual source code and confirm any assumptions.

Return WorthFuzzing=false if the patch only contains:
- Modifications to Documentation/, Kconfig files, or code comments.
- Purely decorative changes, such as logging (e.g., pr_err, printk) or tracepoints.
- Changes to numeric constants or macros that do not functionally alter execution flow.
- Code paths that are impossible to reach in virtualized environments like GCE or QEMU,
  even when utilizing software-emulated hardware (e.g., usb gadget, mac80211_hwsim).
- Code in vendor-specific PCIe switch, SmartNIC, or GPU drivers (e.g., mlxsw, pds_core, qed,
  ionic, amdgpu) that require physical PCIe hardware cards not emulated in standard QEMU.
- Driver .remove, .shutdown, or pci_unregister_driver teardown callbacks (e.g., igb_remove)
  that are executed only during PCI hot-unplug or sysfs driver unbind operations.

If it modifies reachable core kernel logic, drivers, or architectures, use your code search
tools to verify the code can be executed, then return WorthFuzzing=true.

When returning WorthFuzzing=true, you MUST ALSO:
1. Extract any specific kernel functions that should be heavily fuzzed into FocusSymbols.
   Avoid listing generic hot-path functions to prevent skewed test distributions.
   Prefer non-static, non-inlined API entrypoint functions over internal static helper functions
   (which are inlined by the compiler and do not have distinct symbol addresses).
2. Identify any specific CONFIG_ options required to properly test this new/modified feature.
   Go and look into the Kconfig files and check for ifdefs around the code, do not make assumptions.
   Also check "depends on" lines in Kconfig to include any non-standard parent subsystem configs
   needed for Kbuild to compile the code statically into vmlinux. List them in the EnableConfigs
   output array, and DO NOT add a '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.

For your convenience, here is the diff of the changes:
commit 417797e9ff1c1b2e89f54f976fda3007ee37c3af
Author: syz-cluster <triage@syzkaller.com>
Date:   Thu Aug 13 05:40:36 2026 +0000

    syz-cluster: applied patch under review

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 128c5752f225c..e84be541059ab 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -202,7 +202,7 @@ static bool __futex_pivot_hash(struct mm_struct *mm, struct futex_private_hash *
 	fph = rcu_dereference_protected(mmph->hash, lockdep_is_held(&mmph->lock));
 	if (fph) {
 		if (!futex_ref_is_dead(fph)) {
-			mmph->hash_new = new;
+			WRITE_ONCE(mmph->hash_new, new);
 			return false;
 		}
 
@@ -224,7 +224,7 @@ static void futex_pivot_hash(struct mm_struct *mm)
 
 		fph = mm->futex.phash.hash_new;
 		if (fph) {
-			mm->futex.phash.hash_new = NULL;
+			WRITE_ONCE(mm->futex.phash.hash_new, NULL);
 			__futex_pivot_hash(mm, fph);
 		}
 	}
@@ -1786,12 +1786,18 @@ static bool futex_pivot_pending(struct mm_struct *mm)
 	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;
 
-	guard(mutex)(&mmph->lock);
+	guard(rcu)();
 
-	if (!mmph->hash_new)
+	fph = rcu_dereference(mmph->hash);
+	/*
+	 * Ensure that if we see the new hash, we will also see the cleared
+	 * hash_new pointer. Pairs with rcu_assign_pointer() in
+	 * __futex_pivot_hash().
+	 */
+	smp_rmb();
+	if (!READ_ONCE(mmph->hash_new))
 		return true;
 
-	fph = rcu_dereference_raw(mmph->hash);
 	return futex_ref_is_dead(fph);
 }
 
@@ -1879,7 +1885,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		cur = rcu_dereference_protected(mm->futex.phash.hash,
 						lockdep_is_held(&mm->futex.phash.lock));
 		new = mm->futex.phash.hash_new;
-		mm->futex.phash.hash_new = NULL;
+		WRITE_ONCE(mm->futex.phash.hash_new, NULL);
 
 		if (fph) {
 			if (cur && !cur->hash_mask) {
@@ -1889,7 +1895,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 				 * the second one returns here.
 				 */
 				free = fph;
-				mm->futex.phash.hash_new = new;
+				WRITE_ONCE(mm->futex.phash.hash_new, new);
 				return -EBUSY;
 			}
 			if (cur && !new) {

gemini-3.1-pro-preview

null

null

							input: 2664
							output: 34
							thoughts: 233
						

**My Initial Thoughts on This Futex Patch**

Okay, so I've got a patch here that's poking around in `kernel/futex/core.c`, specifically with the private futex hashes. Right off the bat, I see a few key changes. The code's switched to using `WRITE_ONCE` instead of direct assignment for `mmph->hash_new` and similarly in the other instances. This immediately screams memory ordering concerns – we're dealing with concurrent access, and ensuring the new pointer value is visible correctly across different CPUs is crucial. We've got a swap from mutex to RCU (`guard(rcu)()` instead of `guard(mutex)(&mmph->lock);`) in `futex_pivot_pending`. And `READ_ONCE` has been added. These are functional changes to the futex core logic, directly impacting how private futex hashes are managed, specifically in the lock structure. The shift from a mutex to RCU is a significant performance optimization and indicates a design change that should be properly stress tested. This is a clear indicator of changes to the locking and memory barrier behavior, as the patch introduces `smp_rmb()`.

My gut tells me this needs some serious fuzzing. The interplay of concurrency, memory barriers, and RCU is a recipe for subtle bugs that can be tricky to expose. I'm going to dig into `futex_pivot_pending` and `futex_hash_allocate` in that file.

Before I go further, I should verify the futex configuration. `CONFIG_FUTEX` is usually enabled, but it's a good habit to check. Gotta ensure futexes are enabled on this particular build to avoid a wasted effort.

{
  "Expression": "futex_pivot_pending",
  "PathPrefix": "kernel/futex/core.c"
}

{
  "Output": "kernel/futex/core.c=1773=void futex_hash_free(struct mm_struct *mm)\n--\nkernel/futex/core.c-1783-\nkernel/futex/core.c:1784:static bool futex_pivot_pending(struct mm_struct *mm)\nkernel/futex/core.c-1785-{\n--\nkernel/futex/core.c=1828=static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)\n--\nkernel/futex/core.c-1877-again:\nkernel/futex/core.c:1878:\t\twait_var_event(mm, futex_pivot_pending(mm));\nkernel/futex/core.c-1879-\t}\n"
}

gemini-3.1-pro-preview

null

null

							input: 3508
							output: 31
							thoughts: 24
						

Here's my take on the thought process, as if it were my own:

**Prioritizing the Investigation**

Okay, so I need to figure out which functions are going to be key here. Let's see... what's drawing my attention? Aha! `futex_hash_allocate`. That's an interesting one, and it's being *called* from somewhere, which is often a good place to start digging deeper. It suggests a potential point of resource contention or allocation that we should focus on first.

{
  "Expression": "futex_hash_allocate",
  "PathPrefix": "kernel/futex/"
}

{
  "Output": "kernel/futex/core.c=1680=static void futex_ref_drop(struct futex_private_hash *fph)\n--\nkernel/futex/core.c-1700-\t *\t\t\t\t  rcu_assign_pointer(\u0026mm-\u003efutex.phash.hash, new);\nkernel/futex/core.c:1701:\t *\t\t\t\tfutex_hash_allocate()\nkernel/futex/core.c-1702-\t *\t\t\t\t  futex_ref_drop()\n--\nkernel/futex/core.c=1804=static bool futex_hash_less(struct futex_private_hash *a,\n--\nkernel/futex/core.c-1827-\nkernel/futex/core.c:1828:static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)\nkernel/futex/core.c-1829-{\n--\nkernel/futex/core.c-1937-\nkernel/futex/core.c:1938:int futex_hash_allocate_default(void)\nkernel/futex/core.c-1939-{\n--\nkernel/futex/core.c-1967-\nkernel/futex/core.c:1968:\treturn futex_hash_allocate(buckets, 0);\nkernel/futex/core.c-1969-}\n--\nkernel/futex/core.c=1971=static int futex_hash_get_slots(void)\n--\nkernel/futex/core.c-1981-#else  /* CONFIG_FUTEX_PRIVATE_HASH */\nkernel/futex/core.c:1982:static inline int futex_hash_allocate(unsigned int hslots, unsigned int flags) { return -EINVAL; }\nkernel/futex/core.c-1983-static inline int futex_hash_get_slots(void) { return 0; }\n--\nkernel/futex/core.c=2024=int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)\n--\nkernel/futex/core.c-2032-\t\t\treturn -EINVAL;\nkernel/futex/core.c:2033:\t\tret = futex_hash_allocate(arg3, flags);\nkernel/futex/core.c-2034-\t\tbreak;\n"
}

gemini-3.1-pro-preview

null

null

							input: 4283
							output: 27
							thoughts: 45
						

Here's my summary, as though I'm thinking through this process:

**Diving into Futex Hashing**

Okay, so I'm looking at this futex hashing implementation, and I think `futex_hash_prctl` looks like a solid starting point. It's often the case that these kinds of control path functions are good entry points for understanding the system's overall behavior. Also, let's include `futex_hash_allocate_default`. Now I need to see where `futex_hash_prctl` is being called to fully appreciate how it's integrated with the rest of the futex subsystem. Knowing its callers will illuminate its purpose within the larger context. Gotta map the control flow.

{
  "Expression": "futex_hash_prctl",
  "PathPrefix": ""
}

{
  "Output": "include/linux/futex.h=77=long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,\ninclude/linux/futex.h-78-\t      u32 __user *uaddr2, u32 val2, u32 val3);\ninclude/linux/futex.h:79:int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4);\ninclude/linux/futex.h-80-\n--\ninclude/linux/futex.h=94=static inline long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,\n--\ninclude/linux/futex.h-98-}\ninclude/linux/futex.h:99:static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)\ninclude/linux/futex.h-100-{\n--\nkernel/futex/core.c=2017=void futex_mm_init(struct mm_struct *mm)\n--\nkernel/futex/core.c-2023-\nkernel/futex/core.c:2024:int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)\nkernel/futex/core.c-2025-{\n--\nkernel/sys.c=2534=SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,\n--\nkernel/sys.c-2884-\tcase PR_FUTEX_HASH:\nkernel/sys.c:2885:\t\terror = futex_hash_prctl(arg2, arg3, arg4);\nkernel/sys.c-2886-\t\tbreak;\n"
}

gemini-3.1-pro-preview

null

null

							input: 4982
							output: 28
							thoughts: 50
						

Okay, here's my interpretation of that thought process, assuming I'm deep in the weeds of kernel debugging:

**Diving into the Futex Hash Kernel Path**

Alright, so `futex_hash_prctl` is clearly the entry point for handling `PR_FUTEX_HASH` calls. That's good, that's what I expected. It's a syscall entry, so the kernel is directly invoking this function. Now, the natural next step is to trace where that call is going. I need to understand what this `futex_hash_allocate_default` function is doing. Let's see if that provides any clues.

{
  "Expression": "futex_hash_allocate_default",
  "PathPrefix": ""
}

{
  "Output": "include/linux/futex.h=79=int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4);\n--\ninclude/linux/futex.h-81-#ifdef CONFIG_FUTEX_PRIVATE_HASH\ninclude/linux/futex.h:82:int futex_hash_allocate_default(void);\ninclude/linux/futex.h-83-void futex_hash_free(struct mm_struct *mm);\ninclude/linux/futex.h-84-#else  /* CONFIG_FUTEX_PRIVATE_HASH */\ninclude/linux/futex.h:85:static inline int futex_hash_allocate_default(void) { return 0; }\ninclude/linux/futex.h-86-static inline int futex_hash_free(struct mm_struct *mm) { return 0; }\n--\ninclude/linux/futex.h=99=static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)\n--\ninclude/linux/futex.h-102-}\ninclude/linux/futex.h:103:static inline int futex_hash_allocate_default(void) { return 0; }\ninclude/linux/futex.h-104-static inline int futex_hash_free(struct mm_struct *mm) { return 0; }\n--\nkernel/fork.c=1969=static void rv_task_fork(struct task_struct *p)\n--\nkernel/fork.c-1976-\nkernel/fork.c:1977:static bool need_futex_hash_allocate_default(u64 clone_flags)\nkernel/fork.c-1978-{\n--\nkernel/fork.c=1994=__latent_entropy struct task_struct *copy_process(\n--\nkernel/fork.c-2410-\nkernel/fork.c:2411:\tif (need_futex_hash_allocate_default(clone_flags)) {\nkernel/fork.c:2412:\t\tretval = futex_hash_allocate_default();\nkernel/fork.c-2413-\t\tif (retval)\n--\nkernel/futex/core.c=1828=static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)\n--\nkernel/futex/core.c-1937-\nkernel/futex/core.c:1938:int futex_hash_allocate_default(void)\nkernel/futex/core.c-1939-{\n"
}

gemini-3.1-pro-preview

null

null

							input: 5846
							output: 27
							thoughts: 59
						

Here's my summary, as though I'm thinking through this process:

**Diving into Futex Initialization During Process Creation**

Okay, so I'm tracing through this futex code, and it looks like `futex_hash_allocate_default` is getting called during `copy_process`. That's interesting, makes sense for handling futexes in new processes. The context seems to be set up through a fork or clone call. Now, the relevant configuration here appears to be `CONFIG_FUTEX_PRIVATE_HASH`. I need to understand that. Let me quickly jump into the Kconfig files and verify the definition and purpose of `CONFIG_FUTEX_PRIVATE_HASH`. I want to see what that particular config does, if anything specific to `copy_process` and futex initialization. Let me see where it's set and what the dependencies are. I'm especially keen on how this relates to the overall futex implementation.

{
  "Expression": "config FUTEX_PRIVATE_HASH",
  "PathPrefix": ""
}

{
  "Output": "init/Kconfig=1844=config FUTEX_PI\n--\ninit/Kconfig-1848-\ninit/Kconfig:1849:config FUTEX_PRIVATE_HASH\ninit/Kconfig-1850-\tbool\n"
}

gemini-3.1-pro-preview

null

null

							input: 6207
							output: 33
							thoughts: 18
						

Here's my attempt at summarizing the thought process, tailored for an expert audience:

**Diving into the Kernel Configuration Landscape**

Okay, so I'm thinking I need to understand something more about how this particular feature is configured. I'm going to jump into the heart of the matter, which for kernel configuration means `init/Kconfig`. I know this is where the core configuration options are defined. My gut feeling is that the relevant information is likely somewhere around line 1849, maybe where they start defining a new feature. Let's start there.

{
  "File": "init/Kconfig",
  "FirstLine": 1840,
  "LineCount": 20
}

{
  "Contents": "1840:\t\t  Disabling this option will cause the kernel to be built without\n1841:\t\t  support for \"fast userspace mutexes\".  The resulting kernel may not\n1842:\t\t  run glibc-based applications correctly.\n1843:\t\n1844:\tconfig FUTEX_PI\n1845:\t\tbool\n1846:\t\tdepends on FUTEX \u0026\u0026 RT_MUTEXES\n1847:\t\tdefault y\n1848:\t\n1849:\tconfig FUTEX_PRIVATE_HASH\n1850:\t\tbool\n1851:\t\tdepends on FUTEX \u0026\u0026 !BASE_SMALL \u0026\u0026 MMU\n1852:\t\tdefault y\n1853:\t\n1854:\tconfig FUTEX_MPOL\n1855:\t\tbool\n1856:\t\tdepends on FUTEX \u0026\u0026 NUMA\n1857:\t\tdefault y\n1858:\t\n1859:\tconfig HAVE_FUTEX_ROBUST_UNLOCK\n"
}

gemini-3.1-pro-preview

null

null

							input: 6636
							output: 34
							thoughts: 151
						

**Analyzing the FUTEX Hashing Configuration**

Okay, so looking at this FUTEX hashing implementation, the core enabler seems to be `FUTEX`, which makes sense. And, as expected, `FUTEX_PRIVATE_HASH` is enabled by default in this configuration. The critical functions to understand here are going to be `futex_hash_prctl` and `futex_hash_allocate_default`. I'll want to dig into those to see how the hashing actually works and when it's leveraged.

Wait, looking closer at the declaration, it seems like `futex_pivot_pending` is defined as `static`. Okay, and `futex_hash_allocate` is also static, so it's only visible within the compilation unit, which simplifies the scope of my analysis somewhat. That means I don't need to consider external callers. But `futex_hash_prctl` and `futex_hash_allocate_default` are *not* static, so I'll want to trace their use more generally. I'll need to figure out what functions outside the compilation unit that they support.

Finally, I should check `futex_pivot_hash` and `__futex_pivot_hash` to make sure I haven't missed anything obvious.

{
  "Expression": "futex_pivot_hash",
  "PathPrefix": "kernel/futex/core.c"
}

{
  "Output": "kernel/futex/core.c=161=static void futex_rehash_private(struct futex_private_hash *old,\n--\nkernel/futex/core.c-194-\nkernel/futex/core.c:195:static bool __futex_pivot_hash(struct mm_struct *mm, struct futex_private_hash *new)\nkernel/futex/core.c-196-{\n--\nkernel/futex/core.c-219-\nkernel/futex/core.c:220:static void futex_pivot_hash(struct mm_struct *mm)\nkernel/futex/core.c-221-{\n--\nkernel/futex/core.c-227-\t\t\tWRITE_ONCE(mm-\u003efutex.phash.hash_new, NULL);\nkernel/futex/core.c:228:\t\t\t__futex_pivot_hash(mm, fph);\nkernel/futex/core.c-229-\t\t}\n--\nkernel/futex/core.c=233=struct futex_private_hash *futex_private_hash(struct mm_struct *mm)\n--\nkernel/futex/core.c-255-\t}\nkernel/futex/core.c:256:\tfutex_pivot_hash(mm);\nkernel/futex/core.c-257-\tgoto again;\n--\nkernel/futex/core.c=260=struct futex_bucket_ref futex_hash(union futex_key *key)\n--\nkernel/futex/core.c-271-\t}\nkernel/futex/core.c:272:\tfutex_pivot_hash(key-\u003eprivate.mm);\nkernel/futex/core.c-273-\tgoto again;\n--\nkernel/futex/core.c=1680=static void futex_ref_drop(struct futex_private_hash *fph)\n--\nkernel/futex/core.c-1696-\t *\nkernel/futex/core.c:1697:\t * futex_hash()\t\t\t__futex_pivot_hash()\nkernel/futex/core.c-1698-\t *   guard(rcu);\t\t  guard(mm-\u003efutex.phash.lock);\n--\nkernel/futex/core.c=1784=static bool futex_pivot_pending(struct mm_struct *mm)\n--\nkernel/futex/core.c-1794-\t * hash_new pointer. Pairs with rcu_assign_pointer() in\nkernel/futex/core.c:1795:\t * __futex_pivot_hash().\nkernel/futex/core.c-1796-\t */\n--\nkernel/futex/core.c=1828=static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)\n--\nkernel/futex/core.c-1930-\t\t\t */\nkernel/futex/core.c:1931:\t\t\tif (!__futex_pivot_hash(mm, new) \u0026\u0026 custom)\nkernel/futex/core.c-1932-\t\t\t\tgoto again;\n"
}

gemini-3.1-pro-preview

null

null

							input: 7892
							output: 39
							thoughts: 97
						

**Analyzing Futex Hashing**

Okay, so I'm honing in on the futex hashing mechanisms. `futex_hash_prctl` and `futex_hash_allocate_default` seem like promising entry points, key focus areas for investigation. I should also consider `futex_hash` and `futex_private_hash` as potential hashing entry points. But wait, is `futex_hash` static? No, it's not. That’s good to know, that means it's exported and visible to other parts of the kernel. I should definitely check out the implementation of `futex_hash` in `kernel/futex/core.c` to see how it works. That's the best next step to fully understand the hashing process.

{
  "Expression": "struct futex_bucket_ref futex_hash",
  "PathPrefix": "kernel/futex/core.c"
}

{
  "Output": "kernel/futex/core.c=233=struct futex_private_hash *futex_private_hash(struct mm_struct *mm)\n--\nkernel/futex/core.c-259-\nkernel/futex/core.c:260:struct futex_bucket_ref futex_hash(union futex_key *key)\nkernel/futex/core.c-261-{\n--\nkernel/futex/core.c-277-\nkernel/futex/core.c:278:struct futex_bucket_ref futex_hash(union futex_key *key)\nkernel/futex/core.c-279-{\n"
}

gemini-3.1-pro-preview

null

null

							input: 8371
							output: 33
							thoughts: 59
						

Here's my take on those thoughts:

**Double Definition Suspect: futex_hash**

Okay, hold on a second. I'm seeing `futex_hash` defined twice, and that immediately sets off alarm bells. That usually means there's some conditional compilation involved – probably something tied to a feature flag, a configuration option. My gut says it's likely being controlled by `#ifdef CONFIG_FUTEX_PRIVATE_HASH`. I need to figure out where the *real* definition is and what's going on with the duplicate declarations. So, the first logical step is to check the header files. I'll need to look at both `include/linux/futex.h` and `kernel/futex/futex.h` to see which one defines the primary version of `futex_hash`, and what the `#ifdef` statement does. I need to be sure.

{
  "Expression": "futex_hash",
  "PathPrefix": "kernel/futex/futex.h"
}

{
  "Output": "kernel/futex/futex.h=137=static inline bool futex_key_is_private(union futex_key *key)\n--\nkernel/futex/futex.h-150- */\nkernel/futex/futex.h:151:struct futex_hash_bucket {\nkernel/futex/futex.h-152-\tatomic_t waiters;\n--\nkernel/futex/futex.h=241=struct futex_bucket_ref {\nkernel/futex/futex.h:242:\tstruct futex_hash_bucket *hb;\nkernel/futex/futex.h-243-\tstruct futex_private_hash *fph;\n--\nkernel/futex/futex.h=252=static inline void futex_private_hash_put(struct futex_private_hash *fph) { }\n--\nkernel/futex/futex.h-254-\nkernel/futex/futex.h:255:extern struct futex_bucket_ref futex_hash(union futex_key *key);\nkernel/futex/futex.h-256-\nkernel/futex/futex.h=257=DEFINE_CLASS(hbr, struct futex_bucket_ref,\nkernel/futex/futex.h-258-\t     if (_T.fph) futex_private_hash_put(_T.fph),\nkernel/futex/futex.h:259:\t     futex_hash(key), union futex_key *key);\nkernel/futex/futex.h-260-\n--\nkernel/futex/futex.h=287=extern int fault_in_user_writeable(u32 __user *uaddr);\nkernel/futex/futex.h:288:extern struct futex_q *futex_top_waiter(struct futex_hash_bucket *hb, union futex_key *key);\nkernel/futex/futex.h-289-\n--\nkernel/futex/futex.h=308=extern void __futex_unqueue(struct futex_q *q);\nkernel/futex/futex.h:309:extern void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,\nkernel/futex/futex.h-310-\t\t\t\tstruct task_struct *task);\nkernel/futex/futex.h=311=extern int futex_unqueue(struct futex_q *q);\n--\nkernel/futex/futex.h-313-/**\nkernel/futex/futex.h:314: * futex_queue() - Enqueue the futex_q on the futex_hash_bucket\nkernel/futex/futex.h-315- * @q:\tThe futex_q to enqueue\n--\nkernel/futex/futex.h-327- */\nkernel/futex/futex.h:328:static inline void futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,\nkernel/futex/futex.h-329-\t\t\t       struct task_struct *task)\n--\nkernel/futex/futex.h=340=extern void wait_for_owner_exiting(int ret, struct task_struct *exiting);\n--\nkernel/futex/futex.h-344- */\nkernel/futex/futex.h:345:static inline void futex_hb_waiters_inc(struct futex_hash_bucket *hb)\nkernel/futex/futex.h-346-{\n--\nkernel/futex/futex.h-359- */\nkernel/futex/futex.h:360:static inline void futex_hb_waiters_dec(struct futex_hash_bucket *hb)\nkernel/futex/futex.h-361-{\n--\nkernel/futex/futex.h-366-\nkernel/futex/futex.h:367:static inline int futex_hb_waiters_pending(struct futex_hash_bucket *hb)\nkernel/futex/futex.h-368-{\n--\nkernel/futex/futex.h-379-\nkernel/futex/futex.h:380:extern void futex_q_lock(struct futex_q *q, struct futex_hash_bucket *hb)\nkernel/futex/futex.h-381-\t__acquires(\u0026hb-\u003elock)\n--\nkernel/futex/futex.h-383-\nkernel/futex/futex.h:384:extern void futex_q_unlock(struct futex_hash_bucket *hb)\nkernel/futex/futex.h-385-\t__releases(\u0026hb-\u003elock);\nkernel/futex/futex.h-386-\nkernel/futex/futex.h:387:extern int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,\nkernel/futex/futex.h-388-\t\t\t\tunion futex_key *key,\n--\nkernel/futex/futex.h=402=static inline void\nkernel/futex/futex.h:403:double_lock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)\nkernel/futex/futex.h-404-\t__acquires(\u0026hb1-\u003elock)\n--\nkernel/futex/futex.h=416=static inline void\nkernel/futex/futex.h:417:double_unlock_hb(struct futex_hash_bucket *hb1, struct futex_hash_bucket *hb2)\nkernel/futex/futex.h-418-\t__releases(\u0026hb1-\u003elock)\n"
}

gemini-3.1-pro-preview

null

null

							input: 10128
							output: 109
							thoughts: 271
						

**My Fuzzing Strategy: Futex Private Hash**

Okay, let's solidify this fuzzing plan. The core of my attention needs to be on the `futex_hash` system, specifically the changes related to private hashes. The primary symbols I'm targeting are `futex_hash_prctl`, `futex_hash_allocate_default`, and of course, `futex_hash` itself.

The reasoning here is solid: we're fundamentally altering the synchronization logic within the futex core. We're moving from a mutex-based approach to an RCU (Read-Copy-Update) strategy and introducing memory barriers. That level of core logic modification *screams* for fuzzing. It's a prime target for uncovering subtle concurrency issues or other potential bugs.

To enable this, the configuration I need to be sure is enabled is `FUTEX_PRIVATE_HASH`. I checked; that's the key. Of course, `FUTEX` is also needed, but `FUTEX_PRIVATE_HASH` depends on it, so it's a sufficient indicator to ensure the feature is there.

Let's call the results and get it going!

{
  "EnableConfigs": [
    "FUTEX_PRIVATE_HASH"
  ],
  "FocusSymbols": [
    "futex_hash_prctl",
    "futex_hash_allocate_default",
    "futex_hash"
  ],
  "Reasoning": "The patch modifies the synchronization logic in the futex core for private hashes, changing a mutex to RCU and adding memory barriers. This is a functional change in core kernel logic that should be fuzzed.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "FUTEX_PRIVATE_HASH"
  ],
  "FocusSymbols": [
    "futex_hash_prctl",
    "futex_hash_allocate_default",
    "futex_hash"
  ],
  "Reasoning": "The patch modifies the synchronization logic in the futex core for private hashes, changing a mutex to RCU and adding memory barriers. This is a functional change in core kernel logic that should be fuzzed.",
  "WorthFuzzing": true
}

{
  "WorthFuzzing": true
}

null

best-expensive

null

{
  "KMSANReasoning": "The patch addresses a concurrency issue in futex hash allocation and pivoting by replacing a mutex with an RCU read lock, adding memory barriers, and using READ_ONCE/WRITE_ONCE macros for the `hash_new` pointer. It does not introduce any new data structures, allocations, or user-space copies that could lead to uninitialized memory usage or info-leaks. Any potential bugs introduced by these changes would be related to data races, use-after-free, or locking issues, which are better detected by KCSAN, KASAN, or LOCKDEP.",
  "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.

For your convenience, here is the diff of the changes:
commit 417797e9ff1c1b2e89f54f976fda3007ee37c3af
Author: syz-cluster <triage@syzkaller.com>
Date:   Thu Aug 13 05:40:36 2026 +0000

    syz-cluster: applied patch under review

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 128c5752f225c..e84be541059ab 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -202,7 +202,7 @@ static bool __futex_pivot_hash(struct mm_struct *mm, struct futex_private_hash *
 	fph = rcu_dereference_protected(mmph->hash, lockdep_is_held(&mmph->lock));
 	if (fph) {
 		if (!futex_ref_is_dead(fph)) {
-			mmph->hash_new = new;
+			WRITE_ONCE(mmph->hash_new, new);
 			return false;
 		}
 
@@ -224,7 +224,7 @@ static void futex_pivot_hash(struct mm_struct *mm)
 
 		fph = mm->futex.phash.hash_new;
 		if (fph) {
-			mm->futex.phash.hash_new = NULL;
+			WRITE_ONCE(mm->futex.phash.hash_new, NULL);
 			__futex_pivot_hash(mm, fph);
 		}
 	}
@@ -1786,12 +1786,18 @@ static bool futex_pivot_pending(struct mm_struct *mm)
 	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;
 
-	guard(mutex)(&mmph->lock);
+	guard(rcu)();
 
-	if (!mmph->hash_new)
+	fph = rcu_dereference(mmph->hash);
+	/*
+	 * Ensure that if we see the new hash, we will also see the cleared
+	 * hash_new pointer. Pairs with rcu_assign_pointer() in
+	 * __futex_pivot_hash().
+	 */
+	smp_rmb();
+	if (!READ_ONCE(mmph->hash_new))
 		return true;
 
-	fph = rcu_dereference_raw(mmph->hash);
 	return futex_ref_is_dead(fph);
 }
 
@@ -1879,7 +1885,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		cur = rcu_dereference_protected(mm->futex.phash.hash,
 						lockdep_is_held(&mm->futex.phash.lock));
 		new = mm->futex.phash.hash_new;
-		mm->futex.phash.hash_new = NULL;
+		WRITE_ONCE(mm->futex.phash.hash_new, NULL);
 
 		if (fph) {
 			if (cur && !cur->hash_mask) {
@@ -1889,7 +1895,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 				 * the second one returns here.
 				 */
 				free = fph;
-				mm->futex.phash.hash_new = new;
+				WRITE_ONCE(mm->futex.phash.hash_new, new);
 				return -EBUSY;
 			}
 			if (cur && !new) {

gemini-3.1-pro-preview

null

null

							input: 2661
							output: 146
							thoughts: 567
						

**My Analysis of the Futex Concurrency Patch**

Okay, so this patch is digging into `kernel/futex/core.c`, specifically around how the `mmph->hash_new` field is accessed. My initial thought is that this is a concurrency fix – changing the locking mechanism around `futex_pivot_pending` and how that `hash_new` flag is handled. They're moving from a mutex to RCU and adding memory barriers with `READ_ONCE` and `WRITE_ONCE`. Pretty standard stuff for this kind of optimization.

My immediate question, as an expert in this area, is always "are we introducing any uninitialized memory risks with this change?" The answer is likely no, but I need to be sure. This change is all about *synchronization* and handling potential data races. KMSAN isn't the right tool here. We'd be looking at KCSAN or KASAN to catch use-after-free conditions, or LOCKDEP to flag any locking issues, but not KMSAN.

Let's break down the KMSAN reasoning. The patch's core is about concurrency in futex hash allocation and pivoting. It's not creating or allocating any new structures. It's swapping a mutex for RCU read lock and memory barriers, and using READ_ONCE/WRITE_ONCE on the `hash_new` variable. There aren't any allocations happening and no data copying to or from user space, which would be the only way to expose uninitialized memory. This doesn't seem to be touching any memory that could be uninitialized.

Therefore, KMSAN is not needed here. We can formulate a response with that in mind:

NeedsKMSAN: false
KMSANReasoning: The patch addresses a concurrency issue in futex hash allocation and pivoting by replacing a mutex with RCU read lock, memory barriers, and READ_ONCE/WRITE_ONCE macros. It does not introduce any new data structures, allocations, or user-space copies that could lead to uninitialized memory usage or info-leaks. Any potential bugs introduced by these changes would be related to data races, use-after-free, or locking issues, which are better detected by KCSAN, KASAN, or LOCKDEP.

{
  "KMSANReasoning": "The patch addresses a concurrency issue in futex hash allocation and pivoting by replacing a mutex with an RCU read lock, adding memory barriers, and using READ_ONCE/WRITE_ONCE macros for the `hash_new` pointer. It does not introduce any new data structures, allocations, or user-space copies that could lead to uninitialized memory usage or info-leaks. Any potential bugs introduced by these changes would be related to data races, use-after-free, or locking issues, which are better detected by KCSAN, KASAN, or LOCKDEP.",
  "NeedsKMSAN": false
}

{
  "KMSANReasoning": "The patch addresses a concurrency issue in futex hash allocation and pivoting by replacing a mutex with an RCU read lock, adding memory barriers, and using READ_ONCE/WRITE_ONCE macros for the `hash_new` pointer. It does not introduce any new data structures, allocations, or user-space copies that could lead to uninitialized memory usage or info-leaks. Any potential bugs introduced by these changes would be related to data races, use-after-free, or locking issues, which are better detected by KCSAN, KASAN, or LOCKDEP.",
  "NeedsKMSAN": false
}