Re: [PATCH bpf-next v8 1/3] perf: Add rctx in perf_callchain_entry

[Tao Chen <chen.dylane@linux.dev>]

在 2026/1/26 16:03, bot+bpf-ci@kernel.org 写道:
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index 9ded2e582c60..0330f265173d 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -57,6 +57,7 @@
>>   #include <asm/local.h>
>>
>>   struct perf_callchain_entry {
>> +	int				rctx;
>>   	u64				nr;
>>   	u64				ip[]; /* /proc/sys/kernel/perf_event_max_stack */
>>   };
> 
> Adding rctx before nr changes the structure layout in a way that
> appears to break the perf userspace ABI.
> 
> In perf_output_sample() in kernel/events/core.c, when PERF_SAMPLE_CALLCHAIN
> is set, the code does:
> 
>      int size = 1;
>      size += data->callchain->nr;
>      size *= sizeof(u64);
>      __output_copy(handle, data->callchain, size);
> 
> This copies (1 + nr) * 8 bytes starting from the beginning of the
> structure. Before this patch, offset 0 contained nr (u64), followed
> by ip[]. After this patch, offset 0 contains rctx (int) + 4 bytes
> padding, then nr at offset 8, then ip[] at offset 16.
> 
> With the new layout, this copy sends the following to userspace:
> - First 8 bytes: rctx + padding (kernel-internal data) instead of nr
> - Remaining bytes: nr + ip[0..nr-2] instead of ip[0..nr-1]
> 
> This means the callchain data sent to userspace is shifted by 8 bytes,
> the last IP is truncated, and the rctx value is leaked to userspace.
> 
> Is there a plan to update perf_output_sample() to output starting from
> &entry->nr instead of the structure start? Or am I missing something
> about how this structure is used for userspace output?
> 

As AI said, this change shifts the memory layout, which effectively 
breaks the userspace ABI.

Maybe we can use __output_copy(handle, &data->callchain->nr, size);

> [ ... ]
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21350234831


-- 
Best Regards
Tao Chen