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

[Sasha Levin]

https://lore.kernel.org/all/CAPHJ_V+J6YDb_wX2nhXU6kh466Dt_nyDSas-1i_Y8s7tqY-Mzw@xxxxxxxxxxxxxx/

## 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).

## 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@xxxxxxxxxx>
  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; }

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.