Re: [BUG] kprobes: WARNING in __arm_kprobe_ftrace when kprobe-ftrace arming fails with -ENOMEM under fault injection

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon,  2 Mar 2026 12:35:49 -0500
Sasha Levin <sashal@kernel.org> wrote:

> https://lore.kernel.org/all/CAPHJ_V+J6YDb_wX2nhXU6kh466Dt_nyDSas-1i_Y8s7tqY-Mzw@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 <mhiramat@kernel.org>
>   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) <mhiramat@kernel.org>