On Tue, 10 Jun 2025 18:50:05 +0530 Naresh Kamboju naresh.kamboju@linaro.org wrote:
On Mon, 9 Jun 2025 at 18:39, Masami Hiramatsu mhiramat@kernel.org wrote:
On Thu, 5 Jun 2025 17:12:10 +0530 Naresh Kamboju naresh.kamboju@linaro.org wrote:
Regressions found on qemu-x86_64 with compat mode (64-bit kernel running on 32-bit userspace) while running LTP tracing test suite on Linux next-20250605 tag kernel.
Regressions found on
- LTP tracing
Regression Analysis:
- New regression? Yes
- Reproducible? Intermittent
Test regression: qemu-x86_64-compat mode ltp tracing Oops int3 kernel panic
Reported-by: Linux Kernel Functional Testing lkft@linaro.org
## Test log ftrace-stress-test: <12>[ 21.971153] /usr/local/bin/kirk[277]: starting test ftrace-stress-test (ftrace_stress_test.sh 90) <4>[ 58.997439] Oops: int3: 0000 [#1] SMP PTI <4>[ 58.998089] CPU: 0 UID: 0 PID: 323 Comm: sh Not tainted 6.15.0-next-20250605 #1 PREEMPT(voluntary) <4>[ 58.998152] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 <4>[ 58.998260] RIP: 0010:_raw_spin_lock+0x5/0x50
Interesting. This hits a stray int3 for ftrace on _raw_spin_lock.
Here is the compiled code of _raw_spin_lock.
ffffffff825daa00 <_raw_spin_lock>: ffffffff825daa00: f3 0f 1e fa endbr64 ffffffff825daa04: e8 47 a6 d5 fe call ffffffff81335050 <__fentry__>
Since int3 exception happens after decoded int3 (1 byte), the RIP `_raw_spin_lock+0x05` is not an instruction boundary.
<4>[ 58.998563] Code: 5d e9 ff 12 00 00 66 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f <1f> 44 00 00 55 48 89 e5 53 48 89 fb bf 01 00 00 00 e8 15 12 e4 fe
And the call is already modified back to a 5-bytes nop when we dump the code. Thus it may hit the intermediate int3 for transforming code.
e8 47 a6 d5 fe (first step) cc 47 a6 d5 fe (second step) cc 1f 44 00 00 <- hit? (third step) 0f 1f 44 00 00 <- handle int3
It is very unlikely scenario (and I'm not sure qemu can correctly emulate it.) But if a CPU hits the int3 (cc) on _raw_spin_lock()+0x4 before anoter CPU' runs third step in smp_text_poke_batch_finish(), and before the CPU runs smp_text_poke_int3_handler(), the CPU' runs the thrid step and sets text_poke_array_refs 0, the smp_text_poke_int3_handler() returns 0 and causes the same problem.
<CPU0> <CPU1> Start smp_text_poke_batch_finish(). Finish second step. Hit int3 (*) Finish third step. Run smp_text_poke_sync_each_cpu().(**) Clear text_poke_array_refs[cpu0] Start smp_text_poke_int3_handler() Failed to get text_poke_array_refs[cpu0] Oops: int3
But as I said it is very unlikely, because as far as I know;
(*) smp_text_poke_int3_handler() is called directly from exc_int3() which is a kind of NMI, so other interrupt should not run. (**) In the third step, smp_text_poke_batch_finish() sends IPI for sync core after removing int3. Thus any int3 exception handling should be finished.
Is this bug reproducible easier recently?
Yes. It is easy to reproduce.
Good, can you test the following 2 patches (I'll send a series)? I think [1/2] may avoid the kernel crash, but still shows a warning, and [2/2] may fix it if my guess is correct.
Thank you,
Thanks,
-- Masami Hiramatsu (Google) mhiramat@kernel.org
- Naresh