AI Trajectory

null

{
  "EnableConfigs": [
    "BINFMT_MISC"
  ],
  "FocusSymbols": [
    "put_binfmt_handler",
    "load_misc_binary",
    "bm_evict_inode",
    "bm_entry_write",
    "bm_register_write"
  ],
  "Reasoning": "The patch fixes a bug where `exe_file_allow_write_access` is not called before `filp_close` in `put_binfmt_handler`. This is a functional change that affects the binfmt_misc filesystem.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit 474ff6650abd8538fd00185f30de7c3c3baff908\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Wed Jul 22 02:22:58 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c\nindex 84349fcb93f13a..de50a7468b079d 100644\n--- a/fs/binfmt_misc.c\n+++ b/fs/binfmt_misc.c\n@@ -162,8 +162,10 @@ static Node *get_binfmt_handler(struct binfmt_misc *misc,\n static void put_binfmt_handler(Node *e)\n {\n \tif (refcount_dec_and_test(\u0026e-\u003eusers)) {\n-\t\tif (e-\u003eflags \u0026 MISC_FMT_OPEN_FILE)\n+\t\tif (e-\u003eflags \u0026 MISC_FMT_OPEN_FILE) {\n+\t\t\texe_file_allow_write_access(e-\u003einterp_file);\n \t\t\tfilp_close(e-\u003einterp_file, NULL);\n+\t\t}\n \t\tkfree(e);\n \t}\n }\n"
}

best-expensive

null

{
  "EnableConfigs": [
    "BINFMT_MISC"
  ],
  "FocusSymbols": [
    "put_binfmt_handler",
    "load_misc_binary",
    "bm_evict_inode",
    "bm_entry_write",
    "bm_register_write"
  ],
  "Reasoning": "The patch fixes a bug where `exe_file_allow_write_access` is not called before `filp_close` in `put_binfmt_handler`. This is a functional change that affects the binfmt_misc filesystem.",
  "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).

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.
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.
   Do not list too generic configs (we already have them enabled). Only list those that
   specifically cover the modified code. 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 474ff6650abd8538fd00185f30de7c3c3baff908
Author: syz-cluster <triage@syzkaller.com>
Date:   Wed Jul 22 02:22:58 2026 +0000

    syz-cluster: applied patch under review

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 84349fcb93f13a..de50a7468b079d 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -162,8 +162,10 @@ static Node *get_binfmt_handler(struct binfmt_misc *misc,
 static void put_binfmt_handler(Node *e)
 {
 	if (refcount_dec_and_test(&e->users)) {
-		if (e->flags & MISC_FMT_OPEN_FILE)
+		if (e->flags & MISC_FMT_OPEN_FILE) {
+			exe_file_allow_write_access(e->interp_file);
 			filp_close(e->interp_file, NULL);
+		}
 		kfree(e);
 	}
 }

gemini-3.1-pro-preview

null

null

							input: 1933
							output: 34
							thoughts: 164
						

**Analyzing the BinFmt_Misc Patch**

Okay, so I've got a patch here that's making a functional change to `fs/binfmt_misc.c`. It's adding a call to `exe_file_allow_write_access(e->interp_file)` right before `filp_close(e->interp_file, NULL)` in the `put_binfmt_handler` function. My immediate thought is, what's the purpose of this write access check before closing the interpreter file? And why here, specifically?

Let me quickly refresh my memory on `put_binfmt_handler`. I recall it being called when a binfmt_misc handler is removed, potentially during unmounting of the filesystem, or simply when a handler's reference count drops to zero. That's the context I need to keep in mind here. It's the cleanup path. Given that context, I need to understand what `exe_file_allow_write_access` is doing. This likely involves some kind of permission check or adjustment.

Time to dive into `fs/binfmt_misc.c` and confirm where exactly `put_binfmt_handler` is being called and get a clearer picture of that function's overall purpose. I need to understand the implications of this addition to the execution flow. This might reveal a potential security hardening, or maybe it's just addressing a subtle race condition related to file permissions. Let's see.

{
  "Expression": "put_binfmt_handler",
  "PathPrefix": "fs/binfmt_misc.c"
}

{
  "Output": "fs/binfmt_misc.c=141=static Node *get_binfmt_handler(struct binfmt_misc *misc,\n--\nfs/binfmt_misc.c-154-/**\nfs/binfmt_misc.c:155: * put_binfmt_handler - put binary handler node\nfs/binfmt_misc.c-156- * @e: node to put\n--\nfs/binfmt_misc.c-161- */\nfs/binfmt_misc.c:162:static void put_binfmt_handler(Node *e)\nfs/binfmt_misc.c-163-{\n--\nfs/binfmt_misc.c=205=static int load_misc_binary(struct linux_binprm *bprm)\n--\nfs/binfmt_misc.c-277-\t */\nfs/binfmt_misc.c:278:\tput_binfmt_handler(fmt);\nfs/binfmt_misc.c-279-\n--\nfs/binfmt_misc.c=662=static void bm_evict_inode(struct inode *inode)\n--\nfs/binfmt_misc.c-675-\t\twrite_unlock(\u0026misc-\u003eentries_lock);\nfs/binfmt_misc.c:676:\t\tput_binfmt_handler(e);\nfs/binfmt_misc.c-677-\t}\n"
}

gemini-3.1-pro-preview

null

null

							input: 2799
							output: 33
							thoughts: 41
						

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

**Analyzing `put_binfmt_handler` Usage**

Alright, so `put_binfmt_handler` is called within both `load_misc_binary` and `bm_evict_inode`. That immediately sets off a flag. Typically, I'd expect handler unregistration (which is what `put_binfmt_handler` does) to be more closely tied to the registration process, and eviction logic is a definite second location of note. Let's dig into `load_misc_binary` first. It's the more likely candidate for containing the core logic related to binary format loading and subsequent unloading. I need to understand how and when it's invoking `put_binfmt_handler` there. Is it simply on a clean exit path, or are there error cases where the handler might not be properly unregistered? And does this usage pattern in `load_misc_binary` differ fundamentally from its use in `bm_evict_inode`? Those are the questions I need to answer.

{
  "Expression": "load_misc_binary",
  "PathPrefix": "fs/binfmt_misc.c"
}

{
  "Output": "fs/binfmt_misc.c=50=typedef struct {\n--\nfs/binfmt_misc.c-60-\tstruct file *interp_file;\nfs/binfmt_misc.c:61:\trefcount_t users;\t\t/* sync removal with load_misc_binary() */\nfs/binfmt_misc.c-62-} Node;\n--\nfs/binfmt_misc.c=141=static Node *get_binfmt_handler(struct binfmt_misc *misc,\n--\nfs/binfmt_misc.c-157- *\nfs/binfmt_misc.c:158: * Free node syncing with load_misc_binary() and defer final free to\nfs/binfmt_misc.c:159: * load_misc_binary() in case it is using the binary type handler we were\nfs/binfmt_misc.c-160- * requested to remove.\n--\nfs/binfmt_misc.c=162=static void put_binfmt_handler(Node *e)\n--\nfs/binfmt_misc.c-175- *\nfs/binfmt_misc.c:176: * To be called in load_misc_binary() to load the relevant struct binfmt_misc.\nfs/binfmt_misc.c-177- * If a user namespace doesn't have its own binfmt_misc mount it can make use\n--\nfs/binfmt_misc.c=184=static struct binfmt_misc *load_binfmt_misc(void)\n--\nfs/binfmt_misc.c-204- */\nfs/binfmt_misc.c:205:static int load_misc_binary(struct linux_binprm *bprm)\nfs/binfmt_misc.c-206-{\n--\nfs/binfmt_misc.c-272-\t * If we actually put the node here all concurrent calls to\nfs/binfmt_misc.c:273:\t * load_misc_binary() will have finished. We also know\nfs/binfmt_misc.c-274-\t * that for the refcount to be zero someone must have concurently\n--\nfs/binfmt_misc.c=721=static ssize_t bm_entry_write(struct file *file, const char __user *buffer,\n--\nfs/binfmt_misc.c-746-\t\t * The lock is exclusive ensuring that the list can't be\nfs/binfmt_misc.c:747:\t\t * modified. Only load_misc_binary() can access but does so\nfs/binfmt_misc.c-748-\t\t * read-only. So we only need to take the write lock when we\n--\nfs/binfmt_misc.c=863=static ssize_t bm_status_write(struct file *file, const char __user *buffer,\n--\nfs/binfmt_misc.c-890-\t\t * The lock is exclusive ensuring that the list can't be\nfs/binfmt_misc.c:891:\t\t * modified. Only load_misc_binary() can access but does so\nfs/binfmt_misc.c-892-\t\t * read-only. So we only need to take the write lock when we\n--\nfs/binfmt_misc.c=929=static int bm_fill_super(struct super_block *sb, struct fs_context *fc)\n--\nfs/binfmt_misc.c-982-\t * -\u003eput_super() is called we know that the binary type list for this\nfs/binfmt_misc.c:983:\t * bintfmt_misc mount is empty making load_misc_binary() return\nfs/binfmt_misc.c-984-\t * -ENOEXEC independent of whether -\u003eenabled is true. Instead, if\n--\nfs/binfmt_misc.c=1018=static struct linux_binfmt misc_format = {\nfs/binfmt_misc.c-1019-\t.module = THIS_MODULE,\nfs/binfmt_misc.c:1020:\t.load_binary = load_misc_binary,\nfs/binfmt_misc.c-1021-};\n"
}

gemini-3.1-pro-preview

null

null

							input: 4122
							output: 29
							thoughts: 75
						

**Analyzing a Critical Bug Fix**

Okay, so this patch addresses a pretty important issue – specifically, that `exe_file_allow_write_access` wasn't being called *before* `filp_close` within the `put_binfmt_handler` function. This feels like it could create a significant vulnerability, potentially allowing a write access issue to slip through the cracks. It's definitely something that warrants rigorous fuzzing to ensure it's been correctly addressed and that there are no remaining edge cases. Given the nature of the fix, fuzzing feels like a critical next step.

I need to confirm the relevant configuration options. The config is listed as `BINFMT_MISC`. I'll go check `fs/Kconfig.binfmt` or a similar configuration file to get the correct context for the patch. I need to understand exactly what this `BINFMT_MISC` setting controls in order to effectively test this fix.

{
  "Expression": "config BINFMT_MISC",
  "PathPrefix": "fs/"
}

{
  "Output": "fs/Kconfig.binfmt=138=config BINFMT_ZFLAT\n--\nfs/Kconfig.binfmt-144-\nfs/Kconfig.binfmt:145:config BINFMT_MISC\nfs/Kconfig.binfmt-146-\ttristate \"Kernel support for MISC binaries\"\n"
}

gemini-3.1-pro-preview

null

null

							input: 4520
							output: 31
							thoughts: 66
						

**Analyzing the BinFmt_Misc Configuration and Related Functions**

Okay, so the kernel config is definitely set to `BINFMT_MISC`. That's the starting point. Now, I need to zero in on the key players within that context. I want to really understand the interactions, potential vulnerabilities, and overall design. My focus is going to be on `put_binfmt_handler`, `load_misc_binary`, `bm_evict_inode`, and `bm_entry_write`. These functions are central to how this binary format is handled, loaded, and managed at runtime. It's time to dig into the code and trace their flow. I especially want to understand how writes are handled. I need to check how `exe_file_allow_write_access` is being used within that context; it seems like a potential point for privilege escalation or unexpected behavior and needs to be thoroughly scrutinized.

{
  "Expression": "exe_file_allow_write_access",
  "PathPrefix": "fs/"
}

{
  "Output": "fs/binfmt_elf.c=832=static int load_elf_binary(struct linux_binprm *bprm)\n--\nfs/binfmt_elf.c-1272-\nfs/binfmt_elf.c:1273:\t\texe_file_allow_write_access(interpreter);\nfs/binfmt_elf.c-1274-\t\tfput(interpreter);\n--\nfs/binfmt_elf.c-1388-out_free_file:\nfs/binfmt_elf.c:1389:\texe_file_allow_write_access(interpreter);\nfs/binfmt_elf.c-1390-\tif (interpreter)\n--\nfs/binfmt_elf_fdpic.c=182=static int load_elf_fdpic_binary(struct linux_binprm *bprm)\n--\nfs/binfmt_elf_fdpic.c-396-\nfs/binfmt_elf_fdpic.c:397:\t\texe_file_allow_write_access(interpreter);\nfs/binfmt_elf_fdpic.c-398-\t\tfput(interpreter);\n--\nfs/binfmt_elf_fdpic.c-469-\tif (interpreter) {\nfs/binfmt_elf_fdpic.c:470:\t\texe_file_allow_write_access(interpreter);\nfs/binfmt_elf_fdpic.c-471-\t\tfput(interpreter);\n--\nfs/binfmt_misc.c=162=static void put_binfmt_handler(Node *e)\n--\nfs/binfmt_misc.c-165-\t\tif (e-\u003eflags \u0026 MISC_FMT_OPEN_FILE) {\nfs/binfmt_misc.c:166:\t\t\texe_file_allow_write_access(e-\u003einterp_file);\nfs/binfmt_misc.c-167-\t\t\tfilp_close(e-\u003einterp_file, NULL);\n--\nfs/binfmt_misc.c=801=static ssize_t bm_register_write(struct file *file, const char __user *buffer,\n--\nfs/binfmt_misc.c-835-\t\tif (f) {\nfs/binfmt_misc.c:836:\t\t\texe_file_allow_write_access(f);\nfs/binfmt_misc.c-837-\t\t\tfilp_close(f, NULL);\n--\nfs/exec.c=768=static struct file *do_open_execat(int fd, struct filename *name, int flags)\n--\nfs/exec.c-814- * As this is a wrapper for the internal do_open_execat(), callers\nfs/exec.c:815: * must call exe_file_allow_write_access() before fput() on release. Also see\nfs/exec.c-816- * do_close_execat().\n--\nfs/exec.c=1389=static void do_close_execat(struct file *file)\n--\nfs/exec.c-1392-\t\treturn;\nfs/exec.c:1393:\texe_file_allow_write_access(file);\nfs/exec.c-1394-\tfput(file);\n--\nfs/exec.c=1709=static int exec_binprm(struct linux_binprm *bprm)\n--\nfs/exec.c-1735-\nfs/exec.c:1736:\t\texe_file_allow_write_access(exec);\nfs/exec.c-1737-\t\tif (unlikely(bprm-\u003ehave_execfd)) {\n"
}

gemini-3.1-pro-preview

null

null

							input: 5719
							output: 131
							thoughts: 109
						

**Analyzing a BinFmt Security Flaw**

Okay, so I'm looking at this patch and my initial thought is, "Ah, finally, someone caught this." It looks like there's a bug in how `exe_file_allow_write_access` is handled within `put_binfmt_handler`. Specifically, it seems like we weren't calling that critical function *before* `filp_close` in the code, which could lead to some nasty security implications. This is clearly a functional change – it impacts how the system handles binary formats, so it warrants careful scrutiny.

My focus is narrowing in on the `BINFMT_MISC` configuration. I need to dive into `put_binfmt_handler` and trace its interaction with the other key functions involved: `load_misc_binary`, `bm_evict_inode`, `bm_entry_write`, and `bm_register_write`. Those are the prime suspects, and I need to figure out exactly how the lack of the `exe_file_allow_write_access` call impacts the interaction of these functions, to determine the exact nature of the problem, and its potential impact.

Let's call this `set-results`.

{
  "EnableConfigs": [
    "BINFMT_MISC"
  ],
  "FocusSymbols": [
    "put_binfmt_handler",
    "load_misc_binary",
    "bm_evict_inode",
    "bm_entry_write",
    "bm_register_write"
  ],
  "Reasoning": "The patch fixes a bug where `exe_file_allow_write_access` is not called before `filp_close` in `put_binfmt_handler`. This is a functional change that affects the binfmt_misc filesystem.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "BINFMT_MISC"
  ],
  "FocusSymbols": [
    "put_binfmt_handler",
    "load_misc_binary",
    "bm_evict_inode",
    "bm_entry_write",
    "bm_register_write"
  ],
  "Reasoning": "The patch fixes a bug where `exe_file_allow_write_access` is not called before `filp_close` in `put_binfmt_handler`. This is a functional change that affects the binfmt_misc filesystem.",
  "WorthFuzzing": true
}