On Mon, 2 Mar 2026 12:35:49 -0500 Sasha Levin <[email protected]> wrote:
> https://lore.kernel.org/all/caphj_v+j6ydb_wx2nhxu6kh466dt_nydsas-1i_y8s7tqy-...@mail.gmail.com/ > > ## 1. Bug Summary > > A WARNING is triggered in `__arm_kprobe_ftrace()` at `kernel/kprobes.c:1147` > when fault injection causes `ftrace_set_filter_ip()` to return `-ENOMEM` > during kprobe arming via `perf_event_open()`. This is a false-positive > warning — the error path itself is correct and the error propagates cleanly > to userspace, but the `WARN_ONCE()` macro fires a kernel warning splat that > is inappropriate for a recoverable allocation failure. The affected subsystem > is kprobes/ftrace. Severity: warning only (no crash, hang, or data > corruption). Thanks for analysis and Nice catch Zw Tang! This looks good analysis to me. > > ## 2. Stack Trace Analysis > > ``` > WARNING: kernel/kprobes.c:1147 at arm_kprobe+0x563/0x620, CPU#0 > Call Trace: > <TASK> > enable_kprobe+0x1fc/0x2c0 > enable_trace_kprobe+0x227/0x4b0 > kprobe_register+0x84/0xc0 > perf_trace_event_init+0x527/0xa20 > perf_kprobe_init+0x156/0x200 > perf_kprobe_event_init+0x101/0x1c0 > perf_try_init_event+0x145/0xa10 > perf_event_alloc+0x1f91/0x5390 > __do_sys_perf_event_open+0x557/0x2d50 > do_syscall_64+0x129/0x1160 > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > </TASK> > ``` > > The crash point is `__arm_kprobe_ftrace()` at `kernel/kprobes.c:1147`, > inlined into `arm_kprobe()`. The calling chain is process context: > `perf_event_open()` syscall -> `perf_kprobe_event_init()` -> > `enable_trace_kprobe()` -> `enable_kprobe()` -> `arm_kprobe()` -> > `__arm_kprobe_ftrace()`. R12 holds `0xfffffff4` which is `-12` (`-ENOMEM`), > confirming the allocation failure injected by fault injection. > > ## 3. Root Cause Analysis > > The root cause is an overly aggressive `WARN_ONCE()` in > `__arm_kprobe_ftrace()` at `kernel/kprobes.c:1147`: > > ```c > ret = ftrace_set_filter_ip(ops, (unsigned long)p->addr, 0, 0); > if (WARN_ONCE(ret < 0, "Failed to arm kprobe-ftrace at %pS (error %d)\n", > p->addr, ret)) > return ret; > ``` > > Prior to commit 9c89bb8e3272 ("kprobes: treewide: Cleanup the error messages > for kprobes"), this was a simple `pr_debug()`. That commit promoted it to > `WARN_ONCE()` as part of a treewide message cleanup, under the rationale that > failures here indicate unexpected conditions. However, > `ftrace_set_filter_ip()` calls into memory allocation paths (e.g., > `ftrace_hash_move_and_update_ops()` -> `__ftrace_hash_update_ipmodify()` or > `ftrace_hash` allocation), and those allocations can legitimately fail under > memory pressure or fault injection. > > The error handling is actually correct — the `-ENOMEM` propagates back > through `arm_kprobe()` -> `enable_kprobe()` and ultimately causes the > `perf_event_open()` syscall to return an error to userspace. The only problem > is the spurious `WARN_ONCE()` which triggers a kernel warning splat and stack > trace for what is a recoverable, non-buggy situation. > > The same issue also applies to the `WARN()` on line 1152 for > `register_ftrace_function()`, which can also fail with `-ENOMEM`. > > ## 4. Affected Versions > > This issue was introduced by commit 9c89bb8e3272 ("kprobes: treewide: Cleanup > the error messages for kprobes"), which first appeared in v5.16-rc1. All > kernel versions from v5.16 onward are affected, including the reporter's > v7.0.0-rc1. > > This is a regression from v5.15, where the same failure path used > `pr_debug()` and did not emit any warning. > > ## 5. Relevant Commits and Fixes > > Introducing commit: > 9c89bb8e3272 ("kprobes: treewide: Cleanup the error messages for kprobes") > Author: Masami Hiramatsu <[email protected]> > Merged in v5.16-rc1 > > This commit changed `pr_debug()` to `WARN_ONCE()` in `__arm_kprobe_ftrace()` > for the `ftrace_set_filter_ip()` failure path, and changed a `pr_debug()` to > `WARN()` for the `register_ftrace_function()` failure path. > > No fix for this issue exists in mainline or stable as of the reporter's > kernel version. > > The suggested fix is to downgrade both `WARN_ONCE()` (line 1147) and `WARN()` > (line 1152) in `__arm_kprobe_ftrace()` back to `pr_warn_once()` / `pr_warn()` > respectively. This preserves the improved error messages from 9c89bb8e3272 > while avoiding spurious warning splats on recoverable failures. The error > code is already propagated correctly to the caller. > > ## 6. Prior Discussions > > No prior reports of this specific issue were found on lore.kernel.org. No > related mailing list discussions or proposed patches addressing the WARN > severity in `__arm_kprobe_ftrace()` were found. > > ## 7. Suggested Actions > > 1. The fix is to downgrade the WARN_ONCE/WARN in __arm_kprobe_ftrace() > (kernel/kprobes.c lines 1147 and 1152) to pr_warn_once/pr_warn. > Specifically: > > - Line 1147: Change WARN_ONCE(ret < 0, ...) to a simple > if (ret < 0) { pr_warn_once(...); return ret; } > > - Line 1152: Change WARN(ret < 0, ...) to a simple > if (ret < 0) { pr_warn(...); goto err_ftrace; } Sounds reasonable. Let's fix it. > > 2. This is a low-severity issue that only manifests with fault injection > enabled. No data corruption or crash occurs — the error is correctly > propagated to userspace. The warning is cosmetic but noisy and can > cause false-positive syzbot/syzkaller reports. > Yeah, it is just too aggressive warnings. Let me remove if the error can be handled correctly. Thank you, -- Masami Hiramatsu (Google) <[email protected]>
