AI Trajectory

null

{
  "EnableConfigs": [
    "PERF_EVENTS"
  ],
  "FocusSymbols": [
    "perf_mmap_rb",
    "perf_mmap_aux",
    "perf_mmap",
    "perf_mmap_close"
  ],
  "KMSANReasoning": "The patch addresses resource accounting bugs in perf mmap handling (`perf_mmap_rb()` and `perf_mmap_aux()`). Previously, re-mapping an existing perf buffer or AUX area erroneously called `perf_mmap_account()`, leading to duplicate charging of locked/pinned memory against `user-\u003elocked_vm` and `mm-\u003epinned_vm`. The patch removes `perf_mmap_account()` from the duplicate-mmap success path in `perf_mmap_rb()` and moves it inside the initial allocation branch in `perf_mmap_aux()`.\n\nThis change solely affects accounting counters (`locked_vm` and `pinned_vm`) and does not touch data initialization, copy-to-user paths, struct padding, buffer limits, or branching based on uninitialized memory. There is no risk of uninitialized memory reads or information leaks to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN/sanitizer testing is sufficient.",
  "NeedsKMSAN": false,
  "Reasoning": "The patch modifies memory accounting in perf_mmap_rb() and perf_mmap_aux() in kernel/events/core.c to avoid duplicate accounting of locked and pinned VM when mmapping an existing perf ring buffer or AUX buffer multiple times. These paths are directly reachable from userspace via perf_event_open and mmap syscalls in standard QEMU environments, making it worth fuzzing for resource accounting, reference counting, and lifecycle regressions.",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit b606583a22fc1a3e3ee225c8e5e8d99f0795179f\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Tue Sep 22 22:11:38 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/kernel/events/core.c b/kernel/events/core.c\nindex fe33fe15689d0..82f42799696ca 100644\n--- a/kernel/events/core.c\n+++ b/kernel/events/core.c\n@@ -7311,7 +7311,6 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,\n \t\t\t * Success -- managed to mmap() the same buffer\n \t\t\t * multiple times.\n \t\t\t */\n-\t\t\tperf_mmap_account(vma, user_extra, extra);\n \t\t\trefcount_inc(\u0026event-\u003emmap_count);\n \t\t\treturn 0;\n \t\t}\n@@ -7399,7 +7398,6 @@ static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,\n \n \tif (rb_has_aux(rb)) {\n \t\trefcount_inc(\u0026rb-\u003eaux_mmap_count);\n-\n \t} else {\n \t\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra)) {\n \t\t\trefcount_dec(\u0026rb-\u003emmap_count);\n@@ -7420,9 +7418,9 @@ static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,\n \n \t\trefcount_set(\u0026rb-\u003eaux_mmap_count, 1);\n \t\trb-\u003eaux_mmap_locked = extra;\n+\t\tperf_mmap_account(vma, user_extra, extra);\n \t}\n \n-\tperf_mmap_account(vma, user_extra, extra);\n \trefcount_inc(\u0026event-\u003emmap_count);\n \n \treturn 0;\n"
}

core

null

{
  "EnableConfigs": [
    "PERF_EVENTS"
  ],
  "FocusSymbols": [
    "perf_mmap_rb",
    "perf_mmap_aux",
    "perf_mmap",
    "perf_mmap_close"
  ],
  "Reasoning": "The patch modifies memory accounting in perf_mmap_rb() and perf_mmap_aux() in kernel/events/core.c to avoid duplicate accounting of locked and pinned VM when mmapping an existing perf ring buffer or AUX buffer multiple times. These paths are directly reachable from userspace via perf_event_open and mmap syscalls in standard QEMU environments, making it worth fuzzing for resource accounting, reference counting, and lifecycle regressions.",
  "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 b606583a22fc1a3e3ee225c8e5e8d99f0795179f
Author: syz-cluster <triage@syzkaller.com>
Date:   Tue Sep 22 22:11:38 2026 +0000

    syz-cluster: applied patch under review

diff --git a/kernel/events/core.c b/kernel/events/core.c
index fe33fe15689d0..82f42799696ca 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7311,7 +7311,6 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
 			 * Success -- managed to mmap() the same buffer
 			 * multiple times.
 			 */
-			perf_mmap_account(vma, user_extra, extra);
 			refcount_inc(&event->mmap_count);
 			return 0;
 		}
@@ -7399,7 +7398,6 @@ static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,
 
 	if (rb_has_aux(rb)) {
 		refcount_inc(&rb->aux_mmap_count);
-
 	} else {
 		if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
 			refcount_dec(&rb->mmap_count);
@@ -7420,9 +7418,9 @@ static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,
 
 		refcount_set(&rb->aux_mmap_count, 1);
 		rb->aux_mmap_locked = extra;
+		perf_mmap_account(vma, user_extra, extra);
 	}
 
-	perf_mmap_account(vma, user_extra, extra);
 	refcount_inc(&event->mmap_count);
 
 	return 0;

gemini-3.8-flash

null

null

							input: 3125
							output: 36
							thoughts: 525
						

**Analyzing a Performance Event Mmap Patch**

Okay, let's dive into this patch. My initial thought is to examine the `diff` output, specifically the changes within `kernel/events/core.c`. I see the addition and removal of lines around lines 7311 and 7399. The core of this seems to be related to the `perf_mmap_rb` and `perf_mmap_aux` functions, both concerning memory mapping for performance events.

Looking at the code, it seems the `perf_mmap_account` calls were moved. Specifically, in `perf_mmap_rb`, it appears the accounting call has been removed when a mmap occurs. The accounting is unchanged, however, in `perf_mmap_aux` and now happens after the `rb` is set. This looks like a refactoring, possibly to streamline resource allocation or to improve the logic flow related to mmap'd buffers. I need to understand the implications of this change. Is this a bug fix? An optimization?

Now I need to understand what `perf_mmap_account` does in the context of `vma`, `user_extra`, and `extra`. I will need to delve deeper into the surrounding code and the function itself. Given my experience, I'll bet it's related to tracking or accounting for the memory usage of the mmap'd regions, but I need to make sure I understand the precise implications of moving this call. I'll check its location and context. Are there any concurrency issues introduced or resolved by this? I will check the related commits for any context of this. I will look for any references to the surrounding logic that may have been affected. I should check any related locking or reference counting that might be affected by this change.

Overall, it seems like a relatively focused change. My suspicion is that this is a subtle correction, possibly related to some edge case handling or optimization, but I have to verify that before I accept this patch. I need to understand why they were moved, and whether any side effects were considered.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7290,
  "LineCount": 80
}

{
  "Contents": "7290:\t\t */\n7291:\t\tif (nr_pages != 0 \u0026\u0026 !is_power_of_2(nr_pages))\n7292:\t\t\treturn -EINVAL;\n7293:\t\n7294:\t\tWARN_ON_ONCE(event-\u003ectx-\u003eparent_ctx);\n7295:\t\n7296:\t\tif (event-\u003erb) {\n7297:\t\t\tif (data_page_nr(event-\u003erb) != nr_pages)\n7298:\t\t\t\treturn -EINVAL;\n7299:\t\n7300:\t\t\t/*\n7301:\t\t\t * If this event doesn't have mmap_count, we're attempting to\n7302:\t\t\t * create an alias of another event's mmap(); this would mean\n7303:\t\t\t * both events will end up scribbling the same user_page;\n7304:\t\t\t * which makes no sense.\n7305:\t\t\t */\n7306:\t\t\tif (!refcount_read(\u0026event-\u003emmap_count))\n7307:\t\t\t\treturn -EBUSY;\n7308:\t\n7309:\t\t\tif (refcount_inc_not_zero(\u0026event-\u003erb-\u003emmap_count)) {\n7310:\t\t\t\t/*\n7311:\t\t\t\t * Success -- managed to mmap() the same buffer\n7312:\t\t\t\t * multiple times.\n7313:\t\t\t\t */\n7314:\t\t\t\trefcount_inc(\u0026event-\u003emmap_count);\n7315:\t\t\t\treturn 0;\n7316:\t\t\t}\n7317:\t\n7318:\t\t\t/*\n7319:\t\t\t * Raced against perf_mmap_close()'s\n7320:\t\t\t * refcount_dec_and_mutex_lock() remove the\n7321:\t\t\t * event and continue as if !event-\u003erb\n7322:\t\t\t */\n7323:\t\t\tring_buffer_attach(event, NULL);\n7324:\t\t}\n7325:\t\n7326:\t\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra))\n7327:\t\t\treturn -EPERM;\n7328:\t\n7329:\t\tif (vma-\u003evm_flags \u0026 VM_WRITE)\n7330:\t\t\trb_flags |= RING_BUFFER_WRITABLE;\n7331:\t\n7332:\t\trb = rb_alloc(nr_pages,\n7333:\t\t\t      event-\u003eattr.watermark ? event-\u003eattr.wakeup_watermark : 0,\n7334:\t\t\t      event-\u003ecpu, rb_flags);\n7335:\t\n7336:\t\tif (!rb)\n7337:\t\t\treturn -ENOMEM;\n7338:\t\n7339:\t\trb-\u003emmap_locked = extra;\n7340:\t\n7341:\t\tring_buffer_attach(event, rb);\n7342:\t\n7343:\t\tperf_event_update_time(event);\n7344:\t\tperf_event_init_userpage(event);\n7345:\t\tperf_event_update_userpage(event);\n7346:\t\n7347:\t\tperf_mmap_account(vma, user_extra, extra);\n7348:\t\trefcount_set(\u0026event-\u003emmap_count, 1);\n7349:\t\n7350:\t\treturn 0;\n7351:\t}\n7352:\t\n7353:\tstatic int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,\n7354:\t\t\t\t unsigned long nr_pages)\n7355:\t{\n7356:\t\tconst pgoff_t pgoff_start = vma_start_pgoff(vma);\n7357:\t\tlong extra = 0, user_extra = nr_pages;\n7358:\t\tu64 aux_offset, aux_size;\n7359:\t\tstruct perf_buffer *rb;\n7360:\t\tint ret, rb_flags = 0;\n7361:\t\n7362:\t\trb = event-\u003erb;\n7363:\t\tif (!rb)\n7364:\t\t\treturn -EINVAL;\n7365:\t\n7366:\t\tguard(mutex)(\u0026rb-\u003eaux_mutex);\n7367:\t\n7368:\t\t/*\n7369:\t\t * AUX area mapping: if rb-\u003eaux_nr_pages != 0, it's already\n"
}

gemini-3.8-flash

null

null

							input: 5284
							output: 36
							thoughts: 27
						

Okay, here's my interpretation of what I would be thinking, focusing on the specified lines of `kernel/events/core.c`, crafted for an expert audience:

**Diving Deep into the Core Event Handling**

Alright, let's take a look at lines 7370 to 7440 of `kernel/events/core.c`. I need to get a clear picture of how this section handles the event lifecycle. My immediate goals are to understand the flow for event creation, registration, and dispatch. Based on the file context, I'm expecting to find core functionality related to trace events, perf events, and likely, the generic event infrastructure.

I'm zeroing in on this particular segment to clarify some recent issues I've been seeing with event correlation. I'm hoping to get some details about the event loop's interactions with the CPU's performance monitoring counters. I suspect the issue has to do with some subtle races, so I want to understand how atomic operations are used in this event loop. Given where we are, I expect to find a series of function calls, likely dealing with setting up event buffers and associated metadata structures, along with the management of the event list or queue.

I need to see how the code handles event filters and how they interact with the event dispatching mechanism. It will be helpful to see whether they are implemented with BPF or something else. I'll need to pay close attention to the lock contention, which could be part of the performance problem I'm trying to identify. Specifically, I'm looking at how the code manages interrupts with lock protection. I want to see if the architecture design is correct for the intended workload.

I'm ready to dig in and pinpoint where these events are registered, what data is collected, and how they are routed to the appropriate consumers. I'm expecting some clever use of RCU or per-CPU data structures to minimize locking overhead, but I also expect to see robust synchronization to prevent data corruption. This should give me a good handle on where to go next, and hopefully a clearer sense of the current state of event processing in the kernel.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7370,
  "LineCount": 70
}

{
  "Contents": "7370:\t\t * mapped, all subsequent mappings should have the same size\n7371:\t\t * and offset. Must be above the normal perf buffer.\n7372:\t\t */\n7373:\t\taux_offset = READ_ONCE(rb-\u003euser_page-\u003eaux_offset);\n7374:\t\taux_size = READ_ONCE(rb-\u003euser_page-\u003eaux_size);\n7375:\t\n7376:\t\tif (aux_offset \u003c perf_data_size(rb) + PAGE_SIZE)\n7377:\t\t\treturn -EINVAL;\n7378:\t\n7379:\t\tif (aux_offset != pgoff_start \u003c\u003c PAGE_SHIFT)\n7380:\t\t\treturn -EINVAL;\n7381:\t\n7382:\t\t/* already mapped with a different offset */\n7383:\t\tif (rb_has_aux(rb) \u0026\u0026 rb-\u003eaux_pgoff != pgoff_start)\n7384:\t\t\treturn -EINVAL;\n7385:\t\n7386:\t\tif (aux_size != nr_pages * PAGE_SIZE)\n7387:\t\t\treturn -EINVAL;\n7388:\t\n7389:\t\t/* already mapped with a different size */\n7390:\t\tif (rb_has_aux(rb) \u0026\u0026 rb-\u003eaux_nr_pages != nr_pages)\n7391:\t\t\treturn -EINVAL;\n7392:\t\n7393:\t\tif (!is_power_of_2(nr_pages))\n7394:\t\t\treturn -EINVAL;\n7395:\t\n7396:\t\tif (!refcount_inc_not_zero(\u0026rb-\u003emmap_count))\n7397:\t\t\treturn -EINVAL;\n7398:\t\n7399:\t\tif (rb_has_aux(rb)) {\n7400:\t\t\trefcount_inc(\u0026rb-\u003eaux_mmap_count);\n7401:\t\t} else {\n7402:\t\t\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra)) {\n7403:\t\t\t\trefcount_dec(\u0026rb-\u003emmap_count);\n7404:\t\t\t\treturn -EPERM;\n7405:\t\t\t}\n7406:\t\n7407:\t\t\tWARN_ON(!rb \u0026\u0026 event-\u003erb);\n7408:\t\n7409:\t\t\tif (vma-\u003evm_flags \u0026 VM_WRITE)\n7410:\t\t\t\trb_flags |= RING_BUFFER_WRITABLE;\n7411:\t\n7412:\t\t\tret = rb_alloc_aux(rb, event, pgoff_start, nr_pages,\n7413:\t\t\t\t\t   event-\u003eattr.aux_watermark, rb_flags);\n7414:\t\t\tif (ret) {\n7415:\t\t\t\trefcount_dec(\u0026rb-\u003emmap_count);\n7416:\t\t\t\treturn ret;\n7417:\t\t\t}\n7418:\t\n7419:\t\t\trefcount_set(\u0026rb-\u003eaux_mmap_count, 1);\n7420:\t\t\trb-\u003eaux_mmap_locked = extra;\n7421:\t\t\tperf_mmap_account(vma, user_extra, extra);\n7422:\t\t}\n7423:\t\n7424:\t\trefcount_inc(\u0026event-\u003emmap_count);\n7425:\t\n7426:\t\treturn 0;\n7427:\t}\n7428:\t\n7429:\tstatic int perf_mmap(struct file *file, struct vm_area_struct *vma)\n7430:\t{\n7431:\t\tstruct perf_event *event = file-\u003eprivate_data;\n7432:\t\tunsigned long vma_size, nr_pages;\n7433:\t\tmapped_f mapped;\n7434:\t\tint ret;\n7435:\t\n7436:\t\t/*\n7437:\t\t * Don't allow mmap() of inherited per-task counters. This would\n7438:\t\t * create a performance issue due to all children writing to the\n7439:\t\t * same rb.\n"
}

gemini-3.8-flash

null

null

							input: 6853
							output: 33
							thoughts: 32
						

Here's my analysis of the provided text, rendered in the first person, assuming an expert-level understanding of the field:

**Diving into `perf_mmap_close`**

Alright, let's take a look at what's happening internally within `perf_mmap_close`. Specifically, I'm focusing on the unmapping or closing behavior – I want to understand the exact mechanics. My next step, as always, is to dive straight into the source code for `perf_mmap_close`. Time to see what we're really doing here.

{
  "Expression": "perf_mmap_close",
  "PathPrefix": "kernel/events/core.c"
}

{
  "Output": "kernel/events/core.c=5815=static void _free_event(struct perf_event *event)\n--\nkernel/events/core.c-5826-\t\t *\nkernel/events/core.c:5827:\t\t * Since we have a 0 refcount, perf_mmap_close() will skip\nkernel/events/core.c-5828-\t\t * over us; possibly making our ring_buffer_put() the last.\n--\nkernel/events/core.c=6883=static void ring_buffer_attach(struct perf_event *event,\n--\nkernel/events/core.c-6918-\t/*\nkernel/events/core.c:6919:\t * Avoid racing with perf_mmap_close(AUX): stop the event\nkernel/events/core.c-6920-\t * before swizzling the event::rb pointer; if it's getting\n--\nkernel/events/core.c=7016=static void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb);\n--\nkernel/events/core.c-7025- */\nkernel/events/core.c:7026:static void perf_mmap_close(struct vm_area_struct *vma)\nkernel/events/core.c-7027-{\n--\nkernel/events/core.c=7147=static const struct vm_operations_struct perf_mmap_vmops = {\nkernel/events/core.c-7148-\t.open\t\t= perf_mmap_open,\nkernel/events/core.c:7149:\t.close\t\t= perf_mmap_close, /* non mergeable */\nkernel/events/core.c-7150-\t.pfn_mkwrite\t= perf_mmap_pfn_mkwrite,\n--\nkernel/events/core.c=7154=static int map_range(struct perf_buffer *rb, struct vm_area_struct *vma)\n--\nkernel/events/core.c-7196-\t * We manage the lifetime of these mappings with internal refcounts (see\nkernel/events/core.c:7197:\t * perf_mmap_open() and perf_mmap_close()) so we ensure the lifetime of\nkernel/events/core.c-7198-\t * this mapping is maintained correctly.\n--\nkernel/events/core.c=7278=static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,\n--\nkernel/events/core.c-7318-\t\t/*\nkernel/events/core.c:7319:\t\t * Raced against perf_mmap_close()'s\nkernel/events/core.c-7320-\t\t * refcount_dec_and_mutex_lock() remove the\n--\nkernel/events/core.c=7429=static int perf_mmap(struct file *file, struct vm_area_struct *vma)\n--\nkernel/events/core.c-7515-\t\t\t/*\nkernel/events/core.c:7516:\t\t\t * Minimal perf_mmap_close(); there can't be AUX or\nkernel/events/core.c-7517-\t\t\t * other events on account of this being the first.\n--\nkernel/events/core.c-7530-\t\t * no race vs first exposure, so fall-through and call\nkernel/events/core.c:7531:\t\t * perf_mmap_close().\nkernel/events/core.c-7532-\t\t */\n--\nkernel/events/core.c-7534-\nkernel/events/core.c:7535:\tperf_mmap_close(vma);\nkernel/events/core.c-7536-\treturn ret;\n--\nkernel/events/core.c=13056=int perf_pmu_unregister(struct pmu *pmu)\n--\nkernel/events/core.c-13080-\t\t * Can't force remove events when pmu::event_unmapped()\nkernel/events/core.c:13081:\t\t * is used in perf_mmap_close().\nkernel/events/core.c-13082-\t\t */\n--\nkernel/events/core.c=13748=perf_event_set_output(struct perf_event *event, struct perf_event *output_event)\n--\nkernel/events/core.c-13794-\t/*\nkernel/events/core.c:13795:\t * Hold both mmap_mutex to serialize against perf_mmap_close().  Since\nkernel/events/core.c-13796-\t * output_event is already on rb-\u003eevent_list, and the list iteration\n--\nkernel/events/core.c-13815-\nkernel/events/core.c:13816:\t\t/* did we race against perf_mmap_close() */\nkernel/events/core.c-13817-\t\tif (!refcount_read(\u0026rb-\u003emmap_count)) {\n"
}

gemini-3.8-flash

null

null

							input: 8306
							output: 36
							thoughts: 32
						

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

**Diving into Core Event Handling**

Okay, time to dig into the heart of the matter. I'm going to focus on `perf_mmap_close` and `perf_mmap_unaccount` within `kernel/events/core.c`. These seem like the crucial points where the mmap'd buffers are handled, and how the kernel manages its internal accounting with respect to those buffers. I need to understand exactly how the closing and unmapping process functions because there's always a risk of subtle resource leaks or unexpected interactions when dealing with memory mapping in the context of performance monitoring. I'll be looking specifically at how they affect the perf event's state and data structures. It's a fundamental part of the tracing process, and if there are inefficiencies or, worse, errors here, it will have a massive impact.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7010,
  "LineCount": 80
}

{
  "Contents": "7010:\t\n7011:\t\tif (mapped)\n7012:\t\t\tmapped(event, vma-\u003evm_mm);\n7013:\t}\n7014:\t\n7015:\tstatic void perf_pmu_output_stop(struct perf_event *event);\n7016:\tstatic void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb);\n7017:\t\n7018:\t/*\n7019:\t * A buffer can be mmap()ed multiple times; either directly through the same\n7020:\t * event, or through other events by use of perf_event_set_output().\n7021:\t *\n7022:\t * In order to undo the VM accounting done by perf_mmap() we need to destroy\n7023:\t * the buffer here, where we still have a VM context. This means we need\n7024:\t * to detach all events redirecting to us.\n7025:\t */\n7026:\tstatic void perf_mmap_close(struct vm_area_struct *vma)\n7027:\t{\n7028:\t\tstruct perf_event *event = vma-\u003evm_file-\u003eprivate_data;\n7029:\t\tmapped_f unmapped = get_mapped(event, event_unmapped);\n7030:\t\tstruct perf_buffer *rb = ring_buffer_get(event);\n7031:\t\tstruct user_struct *mmap_user = rb-\u003emmap_user;\n7032:\t\n7033:\t\t/* FIXIES vs perf_pmu_unregister() */\n7034:\t\tif (unmapped)\n7035:\t\t\tunmapped(event, vma-\u003evm_mm);\n7036:\t\n7037:\t\t/*\n7038:\t\t * The AUX buffer is strictly a sub-buffer, serialize using aux_mutex\n7039:\t\t * to avoid complications.\n7040:\t\t */\n7041:\t\tif (rb_has_aux(rb) \u0026\u0026 vma_start_pgoff(vma) == rb-\u003eaux_pgoff \u0026\u0026\n7042:\t\t    refcount_dec_and_mutex_lock(\u0026rb-\u003eaux_mmap_count, \u0026rb-\u003eaux_mutex)) {\n7043:\t\t\t/*\n7044:\t\t\t * Stop all AUX events that are writing to this buffer,\n7045:\t\t\t * so that we can free its AUX pages and corresponding PMU\n7046:\t\t\t * data. Note that after rb::aux_mmap_count dropped to zero,\n7047:\t\t\t * they won't start any more (see perf_aux_output_begin()).\n7048:\t\t\t */\n7049:\t\t\tperf_pmu_output_stop(event);\n7050:\t\n7051:\t\t\t/* now it's safe to free the pages */\n7052:\t\t\tatomic_long_sub(rb-\u003eaux_nr_pages - rb-\u003eaux_mmap_locked, \u0026mmap_user-\u003elocked_vm);\n7053:\t\t\tatomic64_sub(rb-\u003eaux_mmap_locked, \u0026vma-\u003evm_mm-\u003epinned_vm);\n7054:\t\n7055:\t\t\t/* this has to be the last one */\n7056:\t\t\trb_free_aux(rb);\n7057:\t\t\tWARN_ON_ONCE(refcount_read(\u0026rb-\u003eaux_refcount));\n7058:\t\n7059:\t\t\tmutex_unlock(\u0026rb-\u003eaux_mutex);\n7060:\t\t}\n7061:\t\n7062:\t\t/*\n7063:\t\t * Drop references in reverse order of perf_mmap() to prevent\n7064:\t\t * rb revival after rb-\u003emmap_count reaches zero.\n7065:\t\t */\n7066:\t\tif (refcount_dec_and_mutex_lock(\u0026event-\u003emmap_count,\n7067:\t\t\t\t\t\t\u0026event-\u003emmap_mutex)) {\n7068:\t\t\tring_buffer_attach(event, NULL);\n7069:\t\t\tmutex_unlock(\u0026event-\u003emmap_mutex);\n7070:\t\t}\n7071:\t\n7072:\t\t/* If there's still other mmap()s of this buffer, we're done. */\n7073:\t\tif (!refcount_dec_and_test(\u0026rb-\u003emmap_count))\n7074:\t\t\tgoto out_put;\n7075:\t\n7076:\t\t/*\n7077:\t\t * No other mmap()s, detach from all other events that might redirect\n7078:\t\t * into the now unreachable buffer. Somewhat complicated by the\n7079:\t\t * fact that rb::event_lock otherwise nests inside mmap_mutex.\n7080:\t\t */\n7081:\tagain:\n7082:\t\trcu_read_lock();\n7083:\t\tlist_for_each_entry_rcu(event, \u0026rb-\u003eevent_list, rb_entry) {\n7084:\t\t\tif (!atomic_long_inc_not_zero(\u0026event-\u003erefcount)) {\n7085:\t\t\t\t/*\n7086:\t\t\t\t * This event is en-route to free_event() which will\n7087:\t\t\t\t * detach it and remove it from the list.\n7088:\t\t\t\t */\n7089:\t\t\t\tcontinue;\n"
}

gemini-3.8-flash

null

null

							input: 9926
							output: 36
							thoughts: 155
						

Okay, here's my interpretation of that code snippet, presented as my own thought process:

**Diving into Memory Accounting in Perf Events**

Alright, let's dissect this. I'm focusing on how memory is managed within the perf subsystem. Specifically, I need to understand the relationship between the `perf_mmap_close` and `perf_mmap_account` functions. I'm keying in on what happens when the reference count on the auxiliary mmap count, `rb->aux_mmap_count`, hits zero, which is managed with the mutex `rb->aux_mutex`.

The critical part is the decrement and lock operation `refcount_dec_and_mutex_lock(&rb->aux_mmap_count, &rb->aux_mutex)`. When the count goes to zero, the following operations occur:

```c
			atomic_long_sub(rb->aux_nr_pages - rb->aux_mmap_locked, &mmap_user->locked_vm);
			atomic64_sub(rb->aux_mmap_locked, &vma->vm_mm->pinned_vm);
```

Basically, these atomic subtractions are the memory de-allocation. The first line is subtracting the number of pages from `locked_vm` to the mmap user struct. The second line is subtracting the mmap locked pages from the virtual memory area. These must represent the release of virtual memory pages.

And then, I also need to go through the `perf_mmap_unaccount(vma, rb)` function. I'll need to see how the code handles the situation of deallocating virtual memory. I think it makes sense to go look at that next.

{
  "File": "kernel/events/core.c",
  "FirstLine": 6990,
  "LineCount": 25
}

{
  "Contents": "6990:\t#define get_mapped(event, func)\t\t\t\\\n6991:\t({\tstruct pmu *pmu;\t\t\t\\\n6992:\t\tmapped_f f = NULL;\t\t\t\\\n6993:\t\tguard(rcu)();\t\t\t\t\\\n6994:\t\tpmu = READ_ONCE(event-\u003epmu);\t\t\\\n6995:\t\tif (pmu)\t\t\t\t\\\n6996:\t\t\tf = pmu-\u003efunc;\t\t\t\\\n6997:\t\tf;\t\t\t\t\t\\\n6998:\t})\n6999:\t\n7000:\tstatic void perf_mmap_open(struct vm_area_struct *vma)\n7001:\t{\n7002:\t\tstruct perf_event *event = vma-\u003evm_file-\u003eprivate_data;\n7003:\t\tmapped_f mapped = get_mapped(event, event_mapped);\n7004:\t\n7005:\t\trefcount_inc(\u0026event-\u003emmap_count);\n7006:\t\trefcount_inc(\u0026event-\u003erb-\u003emmap_count);\n7007:\t\n7008:\t\tif (vma_start_pgoff(vma))\n7009:\t\t\trefcount_inc(\u0026event-\u003erb-\u003eaux_mmap_count);\n7010:\t\n7011:\t\tif (mapped)\n7012:\t\t\tmapped(event, vma-\u003evm_mm);\n7013:\t}\n7014:\t\n"
}

gemini-3.8-flash

null

null

							input: 10875
							output: 34
							thoughts: 17
						

Here's a summary of my thought process:

**Diving into `perf_mmap_unaccount`**

Okay, so I'm trying to understand the workings of `perf_mmap_unaccount`. My first step, naturally, is to pinpoint exactly where this function is defined within the codebase. I need to trace its origins to get a handle on its role and purpose. Knowing the definition is essential before I can dive deeper into its implementation or how it interacts with other components of the performance monitoring framework. Time to start digging!

{
  "Expression": "perf_mmap_unaccount",
  "PathPrefix": "kernel/events/core.c"
}

{
  "Output": "kernel/events/core.c=7015=static void perf_pmu_output_stop(struct perf_event *event);\nkernel/events/core.c:7016:static void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb);\nkernel/events/core.c-7017-\n--\nkernel/events/core.c=7026=static void perf_mmap_close(struct vm_area_struct *vma)\n--\nkernel/events/core.c-7125-\t */\nkernel/events/core.c:7126:\tperf_mmap_unaccount(vma, rb);\nkernel/events/core.c-7127-\n--\nkernel/events/core.c=7261=static void perf_mmap_account(struct vm_area_struct *vma, long user_extra, long extra)\n--\nkernel/events/core.c-7268-\nkernel/events/core.c:7269:static void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb)\nkernel/events/core.c-7270-{\n--\nkernel/events/core.c=7429=static int perf_mmap(struct file *file, struct vm_area_struct *vma)\n--\nkernel/events/core.c-7521-\t\t\t\tmapped(event, vma-\u003evm_mm);\nkernel/events/core.c:7522:\t\t\tperf_mmap_unaccount(vma, event-\u003erb);\nkernel/events/core.c-7523-\t\t\tring_buffer_attach(event, NULL);\t/* drops last rb-\u003erefcount */\n"
}

gemini-3.8-flash

null

null

							input: 11478
							output: 36
							thoughts: 30
						

Okay, here's my interpretation of that request, formatted for a fellow expert:

**Investigating Memory Accounting in Performance Monitoring**

Alright, I need to zoom in on a specific area within the performance monitoring code. My thought is to directly examine the `perf_mmap_account` and `perf_mmap_unaccount` functions. I know they're probably critical for tracking memory mappings and unmappings, essential operations in any system profiling endeavor. I'm focusing in on the vicinity of line 7260 – likely where these functions are directly called or at least where related logic resides. I'm anticipating that examining these function calls in that area will provide valuable insights into how the system accounts for memory usage, particularly related to the use of memory mapped files in performance analysis. It will provide a good foundation for analysis of any memory leaks or performance issues related to memory mapping.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7255,
  "LineCount": 30
}

{
  "Contents": "7255:\t\tlock_limit \u003e\u003e= PAGE_SHIFT;\n7256:\t\tlocked = atomic64_read(\u0026vma-\u003evm_mm-\u003epinned_vm) + *extra;\n7257:\t\n7258:\t\treturn locked \u003c= lock_limit || !perf_is_paranoid() || capable(CAP_IPC_LOCK);\n7259:\t}\n7260:\t\n7261:\tstatic void perf_mmap_account(struct vm_area_struct *vma, long user_extra, long extra)\n7262:\t{\n7263:\t\tstruct user_struct *user = current_user();\n7264:\t\n7265:\t\tatomic_long_add(user_extra, \u0026user-\u003elocked_vm);\n7266:\t\tatomic64_add(extra, \u0026vma-\u003evm_mm-\u003epinned_vm);\n7267:\t}\n7268:\t\n7269:\tstatic void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb)\n7270:\t{\n7271:\t\tstruct user_struct *user = rb-\u003emmap_user;\n7272:\t\n7273:\t\tatomic_long_sub((perf_data_size(rb) \u003e\u003e PAGE_SHIFT) + 1 - rb-\u003emmap_locked,\n7274:\t\t\t\t\u0026user-\u003elocked_vm);\n7275:\t\tatomic64_sub(rb-\u003emmap_locked, \u0026vma-\u003evm_mm-\u003epinned_vm);\n7276:\t}\n7277:\t\n7278:\tstatic int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,\n7279:\t\t\t\tunsigned long nr_pages)\n7280:\t{\n7281:\t\tlong extra = 0, user_extra = nr_pages;\n7282:\t\tstruct perf_buffer *rb;\n7283:\t\tint rb_flags = 0;\n7284:\t\n"
}

gemini-3.8-flash

null

null

							input: 12252
							output: 36
							thoughts: 1141
						

**Analysis of a Potential Memory Accounting Leak in the Perf Subsystem**

Alright, let's break this down. It seems there's a subtle but significant issue lurking in the `perf_mmap_rb` and `perf_mmap_aux` functions, both residing within the `kernel/events/core.c` file. Previously, the code wasn't correctly accounting for multiple mmaps of the same ring buffer.

I observed that the `perf_mmap_account` function, responsible for tracking `pinned_vm` and `user->locked_vm` (critical resources!), was being called on *every* mmap operation, regardless of whether it was the first time the buffer was mapped. However, `perf_mmap_unaccount` was only called on the *last* close/unmap. This means that if you repeatedly `mmap()`ed and `close()`ed the same perf ring buffer (a common scenario), these counters would leak, leading to potential resource exhaustion or underflow. The same was true for AUX buffers.

The patch itself appears to address this double-accounting. The fix restricts `perf_mmap_account` calls to the initial allocation of a buffer, effectively preventing the leaks.

Now, as an expert in the field, I need to evaluate the implications of this patch. Is this code reachable? Absolutely! `perf_event_open` followed by `mmap()` is core functionality exposed to user space. It's a standard and well-trodden path within the perf subsystem and perfectly reachable within QEMU or a standard Linux environment.

Does this meet the "WorthFuzzing" criteria? Yes, without a doubt. The patch directly touches memory management logic within a core subsystem (perf is a core subsystem and heavily uses memory management primitives), and it involves modifications to syscall handling (`perf_event_open`, `mmap`). This is exactly the kind of change that warrants fuzzing. We're talking about a reachable execution path, touching critical resource accounting. Syzkaller, in particular, would likely have a field day with concurrent `mmap`, `munmap`, `fork`, and `dup` calls to perf fds.

Given that this is a memory management issue with user-facing syscalls, this patch directly meets the positive criteria for fuzzing.

Finally, what are the key functions modified and directly involved? We have `perf_mmap_rb` and `perf_mmap_aux`, which are called by `perf_mmap`. Importantly, these are `static` functions within `kernel/events/core.c`, but since `perf_mmap` is public, the code is still fully reachable via a standard `perf_event_open` + `mmap` syscall.

Therefore, the `FocusSymbols` are:
*   `perf_mmap_rb`
*   `perf_mmap_aux`
*   `perf_mmap`

{
  "File": "kernel/events/core.c",
  "FirstLine": 7430,
  "LineCount": 70
}

{
  "Contents": "7430:\t{\n7431:\t\tstruct perf_event *event = file-\u003eprivate_data;\n7432:\t\tunsigned long vma_size, nr_pages;\n7433:\t\tmapped_f mapped;\n7434:\t\tint ret;\n7435:\t\n7436:\t\t/*\n7437:\t\t * Don't allow mmap() of inherited per-task counters. This would\n7438:\t\t * create a performance issue due to all children writing to the\n7439:\t\t * same rb.\n7440:\t\t */\n7441:\t\tif (event-\u003ecpu == -1 \u0026\u0026 event-\u003eattr.inherit)\n7442:\t\t\treturn -EINVAL;\n7443:\t\n7444:\t\tif (!(vma-\u003evm_flags \u0026 VM_SHARED))\n7445:\t\t\treturn -EINVAL;\n7446:\t\n7447:\t\tret = security_perf_event_read(event);\n7448:\t\tif (ret)\n7449:\t\t\treturn ret;\n7450:\t\n7451:\t\tvma_size = vma-\u003evm_end - vma-\u003evm_start;\n7452:\t\tnr_pages = vma_size / PAGE_SIZE;\n7453:\t\n7454:\t\tif (nr_pages \u003e INT_MAX)\n7455:\t\t\treturn -ENOMEM;\n7456:\t\n7457:\t\tif (vma_size != PAGE_SIZE * nr_pages)\n7458:\t\t\treturn -EINVAL;\n7459:\t\n7460:\t\tscoped_guard (mutex, \u0026event-\u003emmap_mutex) {\n7461:\t\t\t/*\n7462:\t\t\t * This relies on __pmu_detach_event() taking mmap_mutex after marking\n7463:\t\t\t * the event REVOKED. Either we observe the state, or __pmu_detach_event()\n7464:\t\t\t * will detach the rb created here.\n7465:\t\t\t */\n7466:\t\t\tif (event-\u003estate \u003c= PERF_EVENT_STATE_REVOKED)\n7467:\t\t\t\treturn -ENODEV;\n7468:\t\n7469:\t\t\tif (!vma_start_pgoff(vma))\n7470:\t\t\t\tret = perf_mmap_rb(vma, event, nr_pages);\n7471:\t\t\telse\n7472:\t\t\t\tret = perf_mmap_aux(vma, event, nr_pages);\n7473:\t\t\tif (ret)\n7474:\t\t\t\treturn ret;\n7475:\t\n7476:\t\t\t/*\n7477:\t\t\t * Since pinned accounting is per vm we cannot allow fork() to copy our\n7478:\t\t\t * vma.\n7479:\t\t\t */\n7480:\t\t\tvm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP);\n7481:\t\t\tvma-\u003evm_ops = \u0026perf_mmap_vmops;\n7482:\t\n7483:\t\t\tmapped = get_mapped(event, event_mapped);\n7484:\t\t\tif (mapped)\n7485:\t\t\t\tmapped(event, vma-\u003evm_mm);\n7486:\t\n7487:\t\t\t/*\n7488:\t\t\t * Try to map it into the page table. On fail undo the above,\n7489:\t\t\t * as the callsite expects full cleanup in this case and\n7490:\t\t\t * therefore does not invoke vmops::close().\n7491:\t\t\t */\n7492:\t\t\tret = map_range(event-\u003erb, vma);\n7493:\t\t\tif (likely(!ret))\n7494:\t\t\t\treturn 0;\n7495:\t\n7496:\t\t\t/* Error path */\n7497:\t\n7498:\t\t\t/*\n7499:\t\t\t * If this is the first mmap(), then event-\u003emmap_count should\n"
}

gemini-3.8-flash

null

null

							input: 15068
							output: 31
							thoughts: 32
						

Here's my take on the thought process, tailored for an expert audience and presented as a first-person summary:

**Diving into the Perf Event File Operations**

Okay, so I'm looking at how performance events are managed at the file operation level, and it's hitting me: `perf_mmap` is *the* callback function responsible for handling `mmap` calls on performance event file descriptors. That's the entry point! Time to dig deeper. I need to understand the complete picture of the file operations associated with perf event fds. My next logical step is to examine the `perf_fops` structure. This will reveal the full suite of file operations implemented for these perf event file descriptors. That's where the magic should be. Let's see what's in there.

{
  "Expression": "perf_mmap",
  "PathPrefix": "kernel/events/core.c"
}

{
  "Output": "kernel/events/core.c=5815=static void _free_event(struct perf_event *event)\n--\nkernel/events/core.c-5826-\t\t *\nkernel/events/core.c:5827:\t\t * Since we have a 0 refcount, perf_mmap_close() will skip\nkernel/events/core.c-5828-\t\t * over us; possibly making our ring_buffer_put() the last.\n--\nkernel/events/core.c=6883=static void ring_buffer_attach(struct perf_event *event,\n--\nkernel/events/core.c-6918-\t/*\nkernel/events/core.c:6919:\t * Avoid racing with perf_mmap_close(AUX): stop the event\nkernel/events/core.c-6920-\t * before swizzling the event::rb pointer; if it's getting\n--\nkernel/events/core.c=6988=typedef void (*mapped_f)(struct perf_event *event, struct mm_struct *mm);\n--\nkernel/events/core.c-6999-\nkernel/events/core.c:7000:static void perf_mmap_open(struct vm_area_struct *vma)\nkernel/events/core.c-7001-{\n--\nkernel/events/core.c=7015=static void perf_pmu_output_stop(struct perf_event *event);\nkernel/events/core.c:7016:static void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb);\nkernel/events/core.c-7017-\n--\nkernel/events/core.c-7021- *\nkernel/events/core.c:7022: * In order to undo the VM accounting done by perf_mmap() we need to destroy\nkernel/events/core.c-7023- * the buffer here, where we still have a VM context. This means we need\n--\nkernel/events/core.c-7025- */\nkernel/events/core.c:7026:static void perf_mmap_close(struct vm_area_struct *vma)\nkernel/events/core.c-7027-{\n--\nkernel/events/core.c-7062-\t/*\nkernel/events/core.c:7063:\t * Drop references in reverse order of perf_mmap() to prevent\nkernel/events/core.c-7064-\t * rb revival after rb-\u003emmap_count reaches zero.\n--\nkernel/events/core.c-7125-\t */\nkernel/events/core.c:7126:\tperf_mmap_unaccount(vma, rb);\nkernel/events/core.c-7127-\n--\nkernel/events/core.c-7131-\nkernel/events/core.c:7132:static vm_fault_t perf_mmap_pfn_mkwrite(struct vm_fault *vmf)\nkernel/events/core.c-7133-{\n--\nkernel/events/core.c-7137-\nkernel/events/core.c:7138:static int perf_mmap_may_split(struct vm_area_struct *vma, unsigned long addr)\nkernel/events/core.c-7139-{\n--\nkernel/events/core.c-7146-\nkernel/events/core.c:7147:static const struct vm_operations_struct perf_mmap_vmops = {\nkernel/events/core.c:7148:\t.open\t\t= perf_mmap_open,\nkernel/events/core.c:7149:\t.close\t\t= perf_mmap_close, /* non mergeable */\nkernel/events/core.c:7150:\t.pfn_mkwrite\t= perf_mmap_pfn_mkwrite,\nkernel/events/core.c:7151:\t.may_split\t= perf_mmap_may_split,\nkernel/events/core.c-7152-};\n--\nkernel/events/core.c=7154=static int map_range(struct perf_buffer *rb, struct vm_area_struct *vma)\n--\nkernel/events/core.c-7196-\t * We manage the lifetime of these mappings with internal refcounts (see\nkernel/events/core.c:7197:\t * perf_mmap_open() and perf_mmap_close()) so we ensure the lifetime of\nkernel/events/core.c-7198-\t * this mapping is maintained correctly.\n--\nkernel/events/core.c-7201-\t\tunsigned long va = vma-\u003evm_start + PAGE_SIZE * pagenum;\nkernel/events/core.c:7202:\t\tstruct page *page = perf_mmap_to_page(rb,\nkernel/events/core.c-7203-\t\t\t\tvma_start_pgoff(vma) + pagenum);\n--\nkernel/events/core.c-7209-\nkernel/events/core.c:7210:\t\t/* Map readonly, perf_mmap_pfn_mkwrite() called on write fault. */\nkernel/events/core.c-7211-\t\terr = remap_pfn_range(vma, va, page_to_pfn(page), PAGE_SIZE,\n--\nkernel/events/core.c-7225-\nkernel/events/core.c:7226:static bool perf_mmap_calc_limits(struct vm_area_struct *vma, long *user_extra, long *extra)\nkernel/events/core.c-7227-{\n--\nkernel/events/core.c-7260-\nkernel/events/core.c:7261:static void perf_mmap_account(struct vm_area_struct *vma, long user_extra, long extra)\nkernel/events/core.c-7262-{\n--\nkernel/events/core.c-7268-\nkernel/events/core.c:7269:static void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb)\nkernel/events/core.c-7270-{\n--\nkernel/events/core.c-7277-\nkernel/events/core.c:7278:static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,\nkernel/events/core.c-7279-\t\t\tunsigned long nr_pages)\n--\nkernel/events/core.c-7318-\t\t/*\nkernel/events/core.c:7319:\t\t * Raced against perf_mmap_close()'s\nkernel/events/core.c-7320-\t\t * refcount_dec_and_mutex_lock() remove the\n--\nkernel/events/core.c-7325-\nkernel/events/core.c:7326:\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra))\nkernel/events/core.c-7327-\t\treturn -EPERM;\n--\nkernel/events/core.c-7346-\nkernel/events/core.c:7347:\tperf_mmap_account(vma, user_extra, extra);\nkernel/events/core.c-7348-\trefcount_set(\u0026event-\u003emmap_count, 1);\n--\nkernel/events/core.c-7352-\nkernel/events/core.c:7353:static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,\nkernel/events/core.c-7354-\t\t\t unsigned long nr_pages)\n--\nkernel/events/core.c-7401-\t} else {\nkernel/events/core.c:7402:\t\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra)) {\nkernel/events/core.c-7403-\t\t\trefcount_dec(\u0026rb-\u003emmap_count);\n--\nkernel/events/core.c-7420-\t\trb-\u003eaux_mmap_locked = extra;\nkernel/events/core.c:7421:\t\tperf_mmap_account(vma, user_extra, extra);\nkernel/events/core.c-7422-\t}\n--\nkernel/events/core.c-7428-\nkernel/events/core.c:7429:static int perf_mmap(struct file *file, struct vm_area_struct *vma)\nkernel/events/core.c-7430-{\n--\nkernel/events/core.c-7469-\t\tif (!vma_start_pgoff(vma))\nkernel/events/core.c:7470:\t\t\tret = perf_mmap_rb(vma, event, nr_pages);\nkernel/events/core.c-7471-\t\telse\nkernel/events/core.c:7472:\t\t\tret = perf_mmap_aux(vma, event, nr_pages);\nkernel/events/core.c-7473-\t\tif (ret)\n--\nkernel/events/core.c-7480-\t\tvm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP);\nkernel/events/core.c:7481:\t\tvma-\u003evm_ops = \u0026perf_mmap_vmops;\nkernel/events/core.c-7482-\n--\nkernel/events/core.c-7500-\t\t * be stable at 1. It is only modified by:\nkernel/events/core.c:7501:\t\t * perf_mmap_{open,close}() and perf_mmap().\nkernel/events/core.c-7502-\t\t *\n--\nkernel/events/core.c-7515-\t\t\t/*\nkernel/events/core.c:7516:\t\t\t * Minimal perf_mmap_close(); there can't be AUX or\nkernel/events/core.c-7517-\t\t\t * other events on account of this being the first.\n--\nkernel/events/core.c-7521-\t\t\t\tmapped(event, vma-\u003evm_mm);\nkernel/events/core.c:7522:\t\t\tperf_mmap_unaccount(vma, event-\u003erb);\nkernel/events/core.c-7523-\t\t\tring_buffer_attach(event, NULL);\t/* drops last rb-\u003erefcount */\n--\nkernel/events/core.c-7530-\t\t * no race vs first exposure, so fall-through and call\nkernel/events/core.c:7531:\t\t * perf_mmap_close().\nkernel/events/core.c-7532-\t\t */\n--\nkernel/events/core.c-7534-\nkernel/events/core.c:7535:\tperf_mmap_close(vma);\nkernel/events/core.c-7536-\treturn ret;\n--\nkernel/events/core.c=7585=static const struct file_operations perf_fops = {\n--\nkernel/events/core.c-7590-\t.compat_ioctl\t\t= perf_compat_ioctl,\nkernel/events/core.c:7591:\t.mmap\t\t\t= perf_mmap,\nkernel/events/core.c-7592-\t.fasync\t\t\t= perf_fasync,\n--\nkernel/events/core.c=9627=static void perf_event_cgroup(struct cgroup *cgrp)\n--\nkernel/events/core.c-9681-\nkernel/events/core.c:9682:struct perf_mmap_event {\nkernel/events/core.c-9683-\tstruct vm_area_struct\t*vma;\n--\nkernel/events/core.c=9705=static int perf_event_mmap_match(struct perf_event *event,\n--\nkernel/events/core.c-9707-{\nkernel/events/core.c:9708:\tstruct perf_mmap_event *mmap_event = data;\nkernel/events/core.c-9709-\tstruct vm_area_struct *vma = mmap_event-\u003evma;\n--\nkernel/events/core.c=9716=static void perf_event_mmap_output(struct perf_event *event,\n--\nkernel/events/core.c-9718-{\nkernel/events/core.c:9719:\tstruct perf_mmap_event *mmap_event = data;\nkernel/events/core.c-9720-\tstruct perf_output_handle handle;\n--\nkernel/events/core.c-9782-\nkernel/events/core.c:9783:static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)\nkernel/events/core.c-9784-{\n--\nkernel/events/core.c=9988=void perf_event_mmap(struct vm_area_struct *vma)\nkernel/events/core.c-9989-{\nkernel/events/core.c:9990:\tstruct perf_mmap_event mmap_event;\nkernel/events/core.c-9991-\n--\nkernel/events/core.c-9994-\nkernel/events/core.c:9995:\tmmap_event = (struct perf_mmap_event){\nkernel/events/core.c-9996-\t\t.vma\t= vma,\n--\nkernel/events/core.c=12966=static void __pmu_detach_event(struct pmu *pmu, struct perf_event *event,\n--\nkernel/events/core.c-12976-\t *\nkernel/events/core.c:12977:\t * Notably, perf_mmap() relies on the ordering here.\nkernel/events/core.c-12978-\t */\n--\nkernel/events/core.c-12981-\t\t/*\nkernel/events/core.c:12982:\t\t * Mostly an empty lock sequence, such that perf_mmap(), which\nkernel/events/core.c-12983-\t\t * relies on mmap_mutex, is sure to observe the state change.\n--\nkernel/events/core.c=13056=int perf_pmu_unregister(struct pmu *pmu)\n--\nkernel/events/core.c-13080-\t\t * Can't force remove events when pmu::event_unmapped()\nkernel/events/core.c:13081:\t\t * is used in perf_mmap_close().\nkernel/events/core.c-13082-\t\t */\n--\nkernel/events/core.c=13748=perf_event_set_output(struct perf_event *event, struct perf_event *output_event)\n--\nkernel/events/core.c-13794-\t/*\nkernel/events/core.c:13795:\t * Hold both mmap_mutex to serialize against perf_mmap_close().  Since\nkernel/events/core.c-13796-\t * output_event is already on rb-\u003eevent_list, and the list iteration\n--\nkernel/events/core.c-13815-\nkernel/events/core.c:13816:\t\t/* did we race against perf_mmap_close() */\nkernel/events/core.c-13817-\t\tif (!refcount_read(\u0026rb-\u003emmap_count)) {\n"
}

gemini-3.8-flash

null

null

							input: 19211
							output: 31
							thoughts: 204
						

**Analyzing Perf mmap Functions**

Okay, let's dive into this. My initial focus is on the `perf_mmap` family of functions: `perf_mmap`, `perf_mmap_rb`, `perf_mmap_aux`, `perf_mmap_close`, and `perf_mmap_open`. My primary question is whether `perf_mmap_rb` and `perf_mmap_aux` could be considered FocusSymbols – key functions that encapsulate crucial logic. The more I think about it, the more I believe they can. Actually, `perf_mmap`, `perf_mmap_rb`, and `perf_mmap_aux` all seem like candidates. And I must include `perf_mmap_close` in that discussion, as it is intrinsically linked to the lifecycle and resource accounting of these mmap'ed perf buffers.

So, I need to evaluate if `perf_mmap_rb`, `perf_mmap_aux`, and the base `perf_mmap` are indeed good focus symbols. My next thought is whether they are lifecycle functions in a broader, more general context. However, upon further reflection, they seem quite specific to the mmap operation on perf event file descriptors. That narrows the scope significantly.

Now, let's determine the configuration options that enable the `kernel/events/core.c` file. I'll need to search the `Makefile` within the `kernel/events/` directory to ascertain that information.

{
  "File": "kernel/events/Makefile",
  "FirstLine": 1,
  "LineCount": 20
}

{
  "Contents": "   1:\t# SPDX-License-Identifier: GPL-2.0\n   2:\tobj-y := core.o ring_buffer.o callchain.o\n   3:\t\n   4:\tobj-$(CONFIG_HAVE_HW_BREAKPOINT) += hw_breakpoint.o\n   5:\tobj-$(CONFIG_HW_BREAKPOINT_KUNIT_TEST) += hw_breakpoint_test.o\n   6:\tobj-$(CONFIG_UPROBES) += uprobes.o\n"
}

gemini-3.8-flash

null

null

							input: 19892
							output: 24
							thoughts: 19
						

Here's my thought process, as requested:

**Navigating the Kernel Build System**

Okay, so I need to figure out how the `kernel/events/` directory gets built. That's a good starting point for understanding how a specific subsystem is integrated into the larger kernel image. My first instinct, as always, is to check the `kernel/Makefile`. It's the central hub for the kernel build process, and I'm pretty sure that's where I'll find the relevant rules. I'll need to open that up and start searching, probably looking for references to that directory specifically. From there, I'll see what targets depend on it and what rules govern its compilation. I'm expecting to find either explicit rules for `kernel/events/` or, at the very least, a definition that includes it as part of a broader build configuration. Let's dig in and see what the makefile reveals.

{
  "Expression": "events",
  "PathPrefix": "kernel/Makefile"
}

{
  "Output": "kernel/Makefile=26=endif\nkernel/Makefile-27-\nkernel/Makefile:28:# Prevents flicker of uninteresting __do_softirq()/__local_bh_disable_ip()\nkernel/Makefile-29-# in coverage traces.\n--\nkernel/Makefile=128=obj-$(CONFIG_CFI) += cfi.o\nkernel/Makefile-129-\nkernel/Makefile:130:obj-$(CONFIG_PERF_EVENTS) += events/\nkernel/Makefile-131-\n"
}

gemini-3.8-flash

null

null

							input: 20267
							output: 31
							thoughts: 24
						

Okay, here's my take on those thoughts, tailored for an expert audience, presented in the first person:

**Kernel Configuration Consideration: `CONFIG_PERF_EVENTS`**

Alright, that's it, that's the trigger. When I see `CONFIG_PERF_EVENTS`, my brain immediately jumps to the Kconfig files. Time to confirm if the kernel performance events system is enabled, and maybe poke around at what is set. A quick scan of `Kconfig` files is definitely warranted. I need to understand what's configured, specifically. This configuration variable is the gatekeeper to a whole slew of instrumentation options. Am I going to want more fine-grained performance monitoring and system profiling, or is it going to affect my security posture and footprint? Let's check the setting.

{
  "Expression": "config PERF_EVENTS\\b",
  "PathPrefix": "init/Kconfig"
}

{
  "Output": "init/Kconfig=2183=menu \"Kernel Performance Events And Counters\"\ninit/Kconfig-2184-\ninit/Kconfig:2185:config PERF_EVENTS\ninit/Kconfig-2186-\tbool \"Kernel performance events and counters\"\n"
}

gemini-3.8-flash

null

null

							input: 20568
							output: 33
							thoughts: 18
						

Okay, here's my interpretation of what's going through my mind, rendered in first person, keeping in mind I'm dealing with `init/Kconfig` and assuming a high level of expertise in the field:

**Diving into `init/Kconfig` - Line 2185 and Surrounding**

Alright, time to get my hands dirty. I need to take a look at something specific within the `init/Kconfig` file. Given my familiarity with the kernel build system, I know exactly what kind of information that file holds - the configuration options, dependencies, and help text that determine what features get compiled into the kernel. It’s the backbone of how we tailor the kernel to a specific platform or use case.

I’m zeroing in on line 2185 and the surrounding lines. What's it likely dealing with? Could be a new feature, a subtle dependency tweak, or maybe a bug fix impacting a core initialization function. My immediate thought is to understand the context. What's the section or module being configured around that line? What other related options are nearby? Knowing this surrounding context will be key to understanding the specific intent of the configuration entry and its implications.

I need to quickly grasp the implications for the wider system. Is it a core driver, a specific hardware setting, or something more obscure? Based on the text surrounding 2185, I will be thinking about how it impacts the other kernel subsystems. I'll be looking for any hints about potential conflicts, interactions, or dependencies with other configurations. I'll be mentally tracing the decision tree this part of the Kconfig forms.

Ultimately, I want to assess the impact of this particular config option on kernel behaviour. I'm going to determine if the description and default values are appropriate. I'll check to see if there are missing dependencies, or if there's any unnecessary complexity. My goal is to quickly and precisely understand its role in the system.

{
  "File": "init/Kconfig",
  "FirstLine": 2180,
  "LineCount": 25
}

{
  "Contents": "2180:\t\thelp\n2181:\t\t  See tools/perf/design.txt for details\n2182:\t\n2183:\tmenu \"Kernel Performance Events And Counters\"\n2184:\t\n2185:\tconfig PERF_EVENTS\n2186:\t\tbool \"Kernel performance events and counters\"\n2187:\t\tdefault y if PROFILING\n2188:\t\tdepends on HAVE_PERF_EVENTS\n2189:\t\tselect IRQ_WORK\n2190:\t\thelp\n2191:\t\t  Enable kernel support for various performance events provided\n2192:\t\t  by software and hardware.\n2193:\t\n2194:\t\t  Software events are supported either built-in or via the\n2195:\t\t  use of generic tracepoints.\n2196:\t\n2197:\t\t  Most modern CPUs support performance events via performance\n2198:\t\t  counter registers. These registers count the number of certain\n2199:\t\t  types of hw events: such as instructions executed, cachemisses\n2200:\t\t  suffered, or branches mis-predicted - without slowing down the\n2201:\t\t  kernel or applications. These registers can also trigger interrupts\n2202:\t\t  when a threshold number of events have passed - and can thus be\n2203:\t\t  used to profile the code that runs on that CPU.\n2204:\t\n"
}

gemini-3.8-flash

null

null

							input: 21391
							output: 164
							thoughts: 430
						

**My Analysis of the Perf Event Configuration and Fuzzing Strategy**

Okay, here's the breakdown of my thinking. First, I confirmed that `CONFIG_PERF_EVENTS` depends on `HAVE_PERF_EVENTS`, which on AMD64 architectures hinges on either `ARCH_SUPPORTS_PERF_EVENTS` or specific x86 Kconfig options. This makes `PERF_EVENTS` the core configuration to focus on.

Next, I zeroed in on the critical functions. I double-checked the "FocusSymbols," confirming that `perf_mmap`, `perf_mmap_rb`, `perf_mmap_aux`, and `perf_mmap_close` are the relevant entrypoints. These are the specific functions touched by the modification in `kernel/events/core.c`.

I then went through my mental checklist: Are these functions non-ubiquitous? Yes, they're specifically tied to perf event mmap handling. Are they feature-specific? Absolutely; they relate to perf buffer mmap accounting. Does the patch modify reachable executable code? Yes, and that's critical for fuzzing. Therefore, is it WorthFuzzing? Yes!

Now, the arguments for `set-results`. I determined:

*   `WorthFuzzing`: True.
*   `Reasoning`: This patch is significant because it's altering memory accounting within `perf_mmap_rb()` and `perf_mmap_aux()` – specifically removing redundant accounting for `locked_vm` and `pinned_vm` when mmap is called repeatedly on the same ring or AUX buffer. The code paths are fully reachable, so this is a great place to start fuzzing to find regressions or edge cases in perf buffer accounting and refcounting.
*   `FocusSymbols`: `perf_mmap_rb`, `perf_mmap_aux`, `perf_mmap`, and `perf_mmap_close`.
*   `EnableConfigs`: `PERF_EVENTS`.

With that completed, I'm confident in the reasoning, configuration, and selected symbols. Everything's sound. Time to call `set-results`.

{
  "EnableConfigs": [
    "PERF_EVENTS"
  ],
  "FocusSymbols": [
    "perf_mmap_rb",
    "perf_mmap_aux",
    "perf_mmap",
    "perf_mmap_close"
  ],
  "Reasoning": "The patch modifies memory accounting in perf_mmap_rb() and perf_mmap_aux() in kernel/events/core.c to avoid duplicate accounting of locked and pinned VM when mmapping an existing perf ring buffer or AUX buffer multiple times. These paths are directly reachable from userspace via perf_event_open and mmap syscalls in standard QEMU environments, making it worth fuzzing for resource accounting, reference counting, and lifecycle regressions.",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "PERF_EVENTS"
  ],
  "FocusSymbols": [
    "perf_mmap_rb",
    "perf_mmap_aux",
    "perf_mmap",
    "perf_mmap_close"
  ],
  "Reasoning": "The patch modifies memory accounting in perf_mmap_rb() and perf_mmap_aux() in kernel/events/core.c to avoid duplicate accounting of locked and pinned VM when mmapping an existing perf ring buffer or AUX buffer multiple times. These paths are directly reachable from userspace via perf_event_open and mmap syscalls in standard QEMU environments, making it worth fuzzing for resource accounting, reference counting, and lifecycle regressions.",
  "WorthFuzzing": true
}

{
  "WorthFuzzing": true
}

null

core

null

{
  "KMSANReasoning": "The patch addresses resource accounting bugs in perf mmap handling (`perf_mmap_rb()` and `perf_mmap_aux()`). Previously, re-mapping an existing perf buffer or AUX area erroneously called `perf_mmap_account()`, leading to duplicate charging of locked/pinned memory against `user-\u003elocked_vm` and `mm-\u003epinned_vm`. The patch removes `perf_mmap_account()` from the duplicate-mmap success path in `perf_mmap_rb()` and moves it inside the initial allocation branch in `perf_mmap_aux()`.\n\nThis change solely affects accounting counters (`locked_vm` and `pinned_vm`) and does not touch data initialization, copy-to-user paths, struct padding, buffer limits, or branching based on uninitialized memory. There is no risk of uninitialized memory reads or information leaks to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN/sanitizer testing is sufficient.",
  "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 b606583a22fc1a3e3ee225c8e5e8d99f0795179f
Author: syz-cluster <triage@syzkaller.com>
Date:   Tue Sep 22 22:11:38 2026 +0000

    syz-cluster: applied patch under review

diff --git a/kernel/events/core.c b/kernel/events/core.c
index fe33fe15689d0..82f42799696ca 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7311,7 +7311,6 @@ static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,
 			 * Success -- managed to mmap() the same buffer
 			 * multiple times.
 			 */
-			perf_mmap_account(vma, user_extra, extra);
 			refcount_inc(&event->mmap_count);
 			return 0;
 		}
@@ -7399,7 +7398,6 @@ static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,
 
 	if (rb_has_aux(rb)) {
 		refcount_inc(&rb->aux_mmap_count);
-
 	} else {
 		if (!perf_mmap_calc_limits(vma, &user_extra, &extra)) {
 			refcount_dec(&rb->mmap_count);
@@ -7420,9 +7418,9 @@ static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,
 
 		refcount_set(&rb->aux_mmap_count, 1);
 		rb->aux_mmap_locked = extra;
+		perf_mmap_account(vma, user_extra, extra);
 	}
 
-	perf_mmap_account(vma, user_extra, extra);
 	refcount_inc(&event->mmap_count);
 
 	return 0;

gemini-3.8-flash

null

null

							input: 2337
							output: 36
							thoughts: 74
						

Here's my assessment of the situation:

**KMSAN Fuzzing Justification: An Initial Review**

Okay, so the question is whether this patch series warrants firing up a dedicated KMSAN fuzzing session. My first step is to dive right into the code and see what's actually changed and the context around it. I need to understand the potential impact. I'm going to focus my initial scrutiny on `kernel/events/core.c`, specifically around lines 7300 to 7450. I'll use `read-file` to get that specific section of the code and then analyze the diff and the surrounding logic to assess the risk profile. This will give me a solid foundation for deciding whether a focused fuzzing campaign is the right approach. I need to quickly identify any areas that might be introducing new memory safety issues, race conditions, or other vulnerabilities that KMSAN would be particularly effective at uncovering. This is the critical first step to a responsible and efficient fuzzing strategy.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7290,
  "LineCount": 80
}

{
  "Contents": "7290:\t\t */\n7291:\t\tif (nr_pages != 0 \u0026\u0026 !is_power_of_2(nr_pages))\n7292:\t\t\treturn -EINVAL;\n7293:\t\n7294:\t\tWARN_ON_ONCE(event-\u003ectx-\u003eparent_ctx);\n7295:\t\n7296:\t\tif (event-\u003erb) {\n7297:\t\t\tif (data_page_nr(event-\u003erb) != nr_pages)\n7298:\t\t\t\treturn -EINVAL;\n7299:\t\n7300:\t\t\t/*\n7301:\t\t\t * If this event doesn't have mmap_count, we're attempting to\n7302:\t\t\t * create an alias of another event's mmap(); this would mean\n7303:\t\t\t * both events will end up scribbling the same user_page;\n7304:\t\t\t * which makes no sense.\n7305:\t\t\t */\n7306:\t\t\tif (!refcount_read(\u0026event-\u003emmap_count))\n7307:\t\t\t\treturn -EBUSY;\n7308:\t\n7309:\t\t\tif (refcount_inc_not_zero(\u0026event-\u003erb-\u003emmap_count)) {\n7310:\t\t\t\t/*\n7311:\t\t\t\t * Success -- managed to mmap() the same buffer\n7312:\t\t\t\t * multiple times.\n7313:\t\t\t\t */\n7314:\t\t\t\trefcount_inc(\u0026event-\u003emmap_count);\n7315:\t\t\t\treturn 0;\n7316:\t\t\t}\n7317:\t\n7318:\t\t\t/*\n7319:\t\t\t * Raced against perf_mmap_close()'s\n7320:\t\t\t * refcount_dec_and_mutex_lock() remove the\n7321:\t\t\t * event and continue as if !event-\u003erb\n7322:\t\t\t */\n7323:\t\t\tring_buffer_attach(event, NULL);\n7324:\t\t}\n7325:\t\n7326:\t\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra))\n7327:\t\t\treturn -EPERM;\n7328:\t\n7329:\t\tif (vma-\u003evm_flags \u0026 VM_WRITE)\n7330:\t\t\trb_flags |= RING_BUFFER_WRITABLE;\n7331:\t\n7332:\t\trb = rb_alloc(nr_pages,\n7333:\t\t\t      event-\u003eattr.watermark ? event-\u003eattr.wakeup_watermark : 0,\n7334:\t\t\t      event-\u003ecpu, rb_flags);\n7335:\t\n7336:\t\tif (!rb)\n7337:\t\t\treturn -ENOMEM;\n7338:\t\n7339:\t\trb-\u003emmap_locked = extra;\n7340:\t\n7341:\t\tring_buffer_attach(event, rb);\n7342:\t\n7343:\t\tperf_event_update_time(event);\n7344:\t\tperf_event_init_userpage(event);\n7345:\t\tperf_event_update_userpage(event);\n7346:\t\n7347:\t\tperf_mmap_account(vma, user_extra, extra);\n7348:\t\trefcount_set(\u0026event-\u003emmap_count, 1);\n7349:\t\n7350:\t\treturn 0;\n7351:\t}\n7352:\t\n7353:\tstatic int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,\n7354:\t\t\t\t unsigned long nr_pages)\n7355:\t{\n7356:\t\tconst pgoff_t pgoff_start = vma_start_pgoff(vma);\n7357:\t\tlong extra = 0, user_extra = nr_pages;\n7358:\t\tu64 aux_offset, aux_size;\n7359:\t\tstruct perf_buffer *rb;\n7360:\t\tint ret, rb_flags = 0;\n7361:\t\n7362:\t\trb = event-\u003erb;\n7363:\t\tif (!rb)\n7364:\t\t\treturn -EINVAL;\n7365:\t\n7366:\t\tguard(mutex)(\u0026rb-\u003eaux_mutex);\n7367:\t\n7368:\t\t/*\n7369:\t\t * AUX area mapping: if rb-\u003eaux_nr_pages != 0, it's already\n"
}

gemini-3.8-flash

null

null

							input: 3817
							output: 36
							thoughts: 34
						

Here's my take on those functions, based on a quick glance:

**Diving into the Memory Mapping Plumbing**

Okay, so I need to understand how this `perf` tool handles memory mapping. I'm going to take a closer look at `perf_mmap_aux`, `perf_mmap_account`, and `perf_mmap_close` to get a grip on the whole mmap lifecycle within this performance monitoring context. It's crucial, so I'll follow the logical flow, likely looking for how the auxiliary function supports the others, and how mapping and accounting are handled, and how it is ultimately closed. I need to figure out the interactions between them to see where optimizations or bottlenecks might exist in this code path.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7370,
  "LineCount": 80
}

{
  "Contents": "7370:\t\t * mapped, all subsequent mappings should have the same size\n7371:\t\t * and offset. Must be above the normal perf buffer.\n7372:\t\t */\n7373:\t\taux_offset = READ_ONCE(rb-\u003euser_page-\u003eaux_offset);\n7374:\t\taux_size = READ_ONCE(rb-\u003euser_page-\u003eaux_size);\n7375:\t\n7376:\t\tif (aux_offset \u003c perf_data_size(rb) + PAGE_SIZE)\n7377:\t\t\treturn -EINVAL;\n7378:\t\n7379:\t\tif (aux_offset != pgoff_start \u003c\u003c PAGE_SHIFT)\n7380:\t\t\treturn -EINVAL;\n7381:\t\n7382:\t\t/* already mapped with a different offset */\n7383:\t\tif (rb_has_aux(rb) \u0026\u0026 rb-\u003eaux_pgoff != pgoff_start)\n7384:\t\t\treturn -EINVAL;\n7385:\t\n7386:\t\tif (aux_size != nr_pages * PAGE_SIZE)\n7387:\t\t\treturn -EINVAL;\n7388:\t\n7389:\t\t/* already mapped with a different size */\n7390:\t\tif (rb_has_aux(rb) \u0026\u0026 rb-\u003eaux_nr_pages != nr_pages)\n7391:\t\t\treturn -EINVAL;\n7392:\t\n7393:\t\tif (!is_power_of_2(nr_pages))\n7394:\t\t\treturn -EINVAL;\n7395:\t\n7396:\t\tif (!refcount_inc_not_zero(\u0026rb-\u003emmap_count))\n7397:\t\t\treturn -EINVAL;\n7398:\t\n7399:\t\tif (rb_has_aux(rb)) {\n7400:\t\t\trefcount_inc(\u0026rb-\u003eaux_mmap_count);\n7401:\t\t} else {\n7402:\t\t\tif (!perf_mmap_calc_limits(vma, \u0026user_extra, \u0026extra)) {\n7403:\t\t\t\trefcount_dec(\u0026rb-\u003emmap_count);\n7404:\t\t\t\treturn -EPERM;\n7405:\t\t\t}\n7406:\t\n7407:\t\t\tWARN_ON(!rb \u0026\u0026 event-\u003erb);\n7408:\t\n7409:\t\t\tif (vma-\u003evm_flags \u0026 VM_WRITE)\n7410:\t\t\t\trb_flags |= RING_BUFFER_WRITABLE;\n7411:\t\n7412:\t\t\tret = rb_alloc_aux(rb, event, pgoff_start, nr_pages,\n7413:\t\t\t\t\t   event-\u003eattr.aux_watermark, rb_flags);\n7414:\t\t\tif (ret) {\n7415:\t\t\t\trefcount_dec(\u0026rb-\u003emmap_count);\n7416:\t\t\t\treturn ret;\n7417:\t\t\t}\n7418:\t\n7419:\t\t\trefcount_set(\u0026rb-\u003eaux_mmap_count, 1);\n7420:\t\t\trb-\u003eaux_mmap_locked = extra;\n7421:\t\t\tperf_mmap_account(vma, user_extra, extra);\n7422:\t\t}\n7423:\t\n7424:\t\trefcount_inc(\u0026event-\u003emmap_count);\n7425:\t\n7426:\t\treturn 0;\n7427:\t}\n7428:\t\n7429:\tstatic int perf_mmap(struct file *file, struct vm_area_struct *vma)\n7430:\t{\n7431:\t\tstruct perf_event *event = file-\u003eprivate_data;\n7432:\t\tunsigned long vma_size, nr_pages;\n7433:\t\tmapped_f mapped;\n7434:\t\tint ret;\n7435:\t\n7436:\t\t/*\n7437:\t\t * Don't allow mmap() of inherited per-task counters. This would\n7438:\t\t * create a performance issue due to all children writing to the\n7439:\t\t * same rb.\n7440:\t\t */\n7441:\t\tif (event-\u003ecpu == -1 \u0026\u0026 event-\u003eattr.inherit)\n7442:\t\t\treturn -EINVAL;\n7443:\t\n7444:\t\tif (!(vma-\u003evm_flags \u0026 VM_SHARED))\n7445:\t\t\treturn -EINVAL;\n7446:\t\n7447:\t\tret = security_perf_event_read(event);\n7448:\t\tif (ret)\n7449:\t\t\treturn ret;\n"
}

gemini-3.8-flash

null

null

							input: 5230
							output: 33
							thoughts: 27
						

Okay, let's dive into this. I need to understand the relationship between `perf_mmap_account` and `perf_mmap_close`. My initial thought is to figure out the functions' roles. It's likely that one handles the allocation or setup related to memory mapping for performance data collection (mmap) while the other handles its teardown, perhaps when the perf session ends or a particular buffer is no longer needed. I'll need to look closely at their implementations to understand the specifics: what resources they manage, what locks they might touch, and what data structures they interact with. Understanding the interplay between these two is critical for figuring out how the perf subsystem manages memory and potentially avoids resource leaks or corruption.

{
  "Expression": "perf_mmap_account",
  "PathPrefix": "kernel/events/core.c"
}

{
  "Output": "kernel/events/core.c=7226=static bool perf_mmap_calc_limits(struct vm_area_struct *vma, long *user_extra, long *extra)\n--\nkernel/events/core.c-7260-\nkernel/events/core.c:7261:static void perf_mmap_account(struct vm_area_struct *vma, long user_extra, long extra)\nkernel/events/core.c-7262-{\n--\nkernel/events/core.c=7278=static int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,\n--\nkernel/events/core.c-7346-\nkernel/events/core.c:7347:\tperf_mmap_account(vma, user_extra, extra);\nkernel/events/core.c-7348-\trefcount_set(\u0026event-\u003emmap_count, 1);\n--\nkernel/events/core.c=7353=static int perf_mmap_aux(struct vm_area_struct *vma, struct perf_event *event,\n--\nkernel/events/core.c-7420-\t\trb-\u003eaux_mmap_locked = extra;\nkernel/events/core.c:7421:\t\tperf_mmap_account(vma, user_extra, extra);\nkernel/events/core.c-7422-\t}\n"
}

gemini-3.8-flash

null

null

							input: 5805
							output: 36
							thoughts: 37
						

Okay, here's my interpretation of those directives, structured for an expert like yourself:

**Analyzing Relevant Code Sections**

Alright, my immediate thought is to zero in on specific sections of code to get a handle on what's going on. I'm focusing on lines 7260-7278 and 7226-7260. Given the context, I'm assuming these are crucial for understanding the overall system behavior, likely related to memory management, performance instrumentation, or a similar low-level aspect. I'll need to carefully parse the logic flow, any conditional statements, and data dependencies within those ranges. It's almost guaranteed there are hints here as to what needs debugging or performance analysis.

Additionally, the explicit mention of `perf_mmap_close` is a signal. This is very likely a key function within the Performance Monitoring Unit (PMU) infrastructure, or related memory map handling. I know from experience that understanding how the memory-mapped regions are closed is often central to identifying potential memory leaks, resource contention, or incorrect state management. I'll need to trace the calls to `perf_mmap_close` and understand how the associated resources are being deallocated. This is likely an indicator of how to move forward with the problem.

{
  "File": "kernel/events/core.c",
  "FirstLine": 7226,
  "LineCount": 55
}

{
  "Contents": "7226:\tstatic bool perf_mmap_calc_limits(struct vm_area_struct *vma, long *user_extra, long *extra)\n7227:\t{\n7228:\t\tunsigned long user_locked, user_lock_limit, locked, lock_limit;\n7229:\t\tstruct user_struct *user = current_user();\n7230:\t\n7231:\t\tuser_lock_limit = sysctl_perf_event_mlock \u003e\u003e (PAGE_SHIFT - 10);\n7232:\t\t/* Increase the limit linearly with more CPUs */\n7233:\t\tuser_lock_limit *= num_online_cpus();\n7234:\t\n7235:\t\tuser_locked = atomic_long_read(\u0026user-\u003elocked_vm);\n7236:\t\n7237:\t\t/*\n7238:\t\t * sysctl_perf_event_mlock may have changed, so that\n7239:\t\t *     user-\u003elocked_vm \u003e user_lock_limit\n7240:\t\t */\n7241:\t\tif (user_locked \u003e user_lock_limit)\n7242:\t\t\tuser_locked = user_lock_limit;\n7243:\t\tuser_locked += *user_extra;\n7244:\t\n7245:\t\tif (user_locked \u003e user_lock_limit) {\n7246:\t\t\t/*\n7247:\t\t\t * charge locked_vm until it hits user_lock_limit;\n7248:\t\t\t * charge the rest from pinned_vm\n7249:\t\t\t */\n7250:\t\t\t*extra = user_locked - user_lock_limit;\n7251:\t\t\t*user_extra -= *extra;\n7252:\t\t}\n7253:\t\n7254:\t\tlock_limit = rlimit(RLIMIT_MEMLOCK);\n7255:\t\tlock_limit \u003e\u003e= PAGE_SHIFT;\n7256:\t\tlocked = atomic64_read(\u0026vma-\u003evm_mm-\u003epinned_vm) + *extra;\n7257:\t\n7258:\t\treturn locked \u003c= lock_limit || !perf_is_paranoid() || capable(CAP_IPC_LOCK);\n7259:\t}\n7260:\t\n7261:\tstatic void perf_mmap_account(struct vm_area_struct *vma, long user_extra, long extra)\n7262:\t{\n7263:\t\tstruct user_struct *user = current_user();\n7264:\t\n7265:\t\tatomic_long_add(user_extra, \u0026user-\u003elocked_vm);\n7266:\t\tatomic64_add(extra, \u0026vma-\u003evm_mm-\u003epinned_vm);\n7267:\t}\n7268:\t\n7269:\tstatic void perf_mmap_unaccount(struct vm_area_struct *vma, struct perf_buffer *rb)\n7270:\t{\n7271:\t\tstruct user_struct *user = rb-\u003emmap_user;\n7272:\t\n7273:\t\tatomic_long_sub((perf_data_size(rb) \u003e\u003e PAGE_SHIFT) + 1 - rb-\u003emmap_locked,\n7274:\t\t\t\t\u0026user-\u003elocked_vm);\n7275:\t\tatomic64_sub(rb-\u003emmap_locked, \u0026vma-\u003evm_mm-\u003epinned_vm);\n7276:\t}\n7277:\t\n7278:\tstatic int perf_mmap_rb(struct vm_area_struct *vma, struct perf_event *event,\n7279:\t\t\t\tunsigned long nr_pages)\n7280:\t{\n"
}

gemini-3.8-flash

null

null

							input: 7077
							output: 237
							thoughts: 1709
						

**Analyzing the Perf Event mmap Patch: KASAN vs. KMSAN Considerations**

Okay, let's break this down. First, I need to understand this patch and what it's fixing in the `perf_mmap_rb` and `perf_mmap_aux` functions. The core of the change appears to be removing calls to `perf_mmap_account()` in specific scenarios: when re-mapping an existing buffer in `perf_mmap_rb` and inside the `if` block for `rb_has_aux(rb)` in `perf_mmap_aux`. This suggests a potential issue with double-counting memory usage related to mmap'd perf buffers.

Before this change, in the `perf_mmap_rb` function, the `perf_mmap_account` was being called even if the buffer was already mapped. This led to over-accounting the memory against the `user->locked_vm` and `pinned_vm`. The values passed were `user_extra = nr_pages + 1` and `extra = 0` which were uncalculated/incorrect. Similarly in `perf_mmap_aux` the `perf_mmap_account` was also being called outside of a conditional for existing buffers which led to incorrect accounting.

My concern now is whether this patch introduces any *uninitialized memory* risks that are *only* detectable by KMSAN. I need to distinguish this from issues KASAN, LOCKDEP, or standard bug detectors would catch.

Let's walk through the criteria:

1.  **Kernel structures to userspace:** Nope. The patch isn't modifying or passing any new kernel structures to user space. No data leaks.
2.  **Conditional logic on uninitialized variables:** No. The `user_extra` and `extra` are local and initialized within the functions. It’s resource accounting, and the buffer is allocated separately, so there is no potential for it to lead to a read from uninitialized memory.
3.  **Complex data structure initialization:** No allocations or data structures are added or modified. The allocated buffers are done via separate functions such as `rb_alloc`, `rb_alloc_aux`.
4.  **Bounds checks/lengths leading to uninitialized reads:** No. The patch doesn't alter any buffer boundaries or introduce new access patterns that could lead to reading uninitialized memory.

This is fundamentally about resource accounting—correcting an over-counting problem. The patch fixes incorrect accounting of memory against `user->locked_vm` and `vma->vm_mm->pinned_vm`. It does not introduce any risks of using uninitialized memory. This isn't a bounds issue, a use-after-free issue, or anything that KASAN or lockdep would catch.

**Therefore, `NeedsKMSAN` should be `false`.** This patch exclusively concerns accounting corrections for memory mapping, which KASAN, LOCKDEP, or standard bug detectors are fully capable of handling. KMSAN's focus on detecting uninitialized memory reads isn't relevant here.

{
  "KMSANReasoning": "The patch addresses resource accounting bugs in perf mmap handling (`perf_mmap_rb()` and `perf_mmap_aux()`). Previously, re-mapping an existing perf buffer or AUX area erroneously called `perf_mmap_account()`, leading to duplicate charging of locked/pinned memory against `user-\u003elocked_vm` and `mm-\u003epinned_vm`. The patch removes `perf_mmap_account()` from the duplicate-mmap success path in `perf_mmap_rb()` and moves it inside the initial allocation branch in `perf_mmap_aux()`.\n\nThis change solely affects accounting counters (`locked_vm` and `pinned_vm`) and does not touch data initialization, copy-to-user paths, struct padding, buffer limits, or branching based on uninitialized memory. There is no risk of uninitialized memory reads or information leaks to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN/sanitizer testing is sufficient.",
  "NeedsKMSAN": false
}

{
  "KMSANReasoning": "The patch addresses resource accounting bugs in perf mmap handling (`perf_mmap_rb()` and `perf_mmap_aux()`). Previously, re-mapping an existing perf buffer or AUX area erroneously called `perf_mmap_account()`, leading to duplicate charging of locked/pinned memory against `user-\u003elocked_vm` and `mm-\u003epinned_vm`. The patch removes `perf_mmap_account()` from the duplicate-mmap success path in `perf_mmap_rb()` and moves it inside the initial allocation branch in `perf_mmap_aux()`.\n\nThis change solely affects accounting counters (`locked_vm` and `pinned_vm`) and does not touch data initialization, copy-to-user paths, struct padding, buffer limits, or branching based on uninitialized memory. There is no risk of uninitialized memory reads or information leaks to userspace. Therefore, a dedicated KMSAN fuzzing session is not warranted, and standard KASAN/sanitizer testing is sufficient.",
  "NeedsKMSAN": false
}