Re: [PATCH bpf-next v5 1/3] perf: Refactor get_perf_callchain

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

在 2025/11/10 00:58, bot+bpf-ci@kernel.org 写道:
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index fd1d91017..edd3058e4 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -67,6 +67,7 @@ struct perf_callchain_entry_ctx {
>>   	u32				nr;
>>   	short				contexts;
>>   	bool				contexts_maxed;
>> +	bool				add_mark;
>>   };
> 
> [ ... ]
> 
>> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
>> index 808c0d7a3..fb1f26be2 100644
>> --- a/kernel/events/callchain.c
>> +++ b/kernel/events/callchain.c
> 
> [ ... ]
> 
>> +void __get_perf_callchain_user(struct perf_callchain_entry_ctx *ctx, struct pt_regs *regs)
>> +{
>> +	int start_entry_idx;
>> +
>> +	if (!user_mode(regs)) {
>> +		if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
>> +			return;
>> +		regs = task_pt_regs(current);
>> +	}
>> +
>> +	if (ctx->add_mark)
>> +		perf_callchain_store_context(ctx, PERF_CONTEXT_USER);
>> +
>> +	start_entry_idx = ctx->nr;
>                            ^^^^^^^^
>> +	perf_callchain_user(ctx, regs);
>> +	fixup_uretprobe_trampoline_entries(ctx->entry, start_entry_idx);
>> +}
> 
> Can this break uprobe trampoline fixups? The original code used
> entry->nr for start_entry_idx, but the refactored version uses ctx->nr.
> 
> These two counters track different things:
> - entry->nr counts ALL entries (incremented by both
>    perf_callchain_store_context() and perf_callchain_store())
> - ctx->nr only counts regular callchain entries (incremented only by
>    perf_callchain_store(), not by perf_callchain_store_context())
> 
> When add_mark is true, perf_callchain_store_context() increments
> entry->nr but not ctx->nr, causing them to become out of sync.
> 
> For example, if kernel callchain collection runs first with add_mark
> set:
> 
>    Initial: ctx->nr = entry->nr = 0
> 
>    In __get_perf_callchain_kernel():
>      perf_callchain_store_context() stores PERF_CONTEXT_KERNEL marker
>        -> entry->nr becomes 1, ctx->nr still 0
>      perf_callchain_kernel() stores N kernel frames
>        -> entry->nr becomes 1+N, ctx->nr becomes N
> 
>    In __get_perf_callchain_user():
>      perf_callchain_store_context() stores PERF_CONTEXT_USER marker
>        -> entry->nr becomes 2+N, ctx->nr still N
>      start_entry_idx = ctx->nr assigns N (should be 2+N)
>      fixup_uretprobe_trampoline_entries() scans from wrong index
> 
> This causes fixup_uretprobe_trampoline_entries() to scan kernel
> callchain entries instead of user callchain entries, missing the
> uretprobe trampolines that need to be fixed up.
> 
> Should this use ctx->entry->nr instead of ctx->nr?
> 

Use ctx->entry->nr looks better, will change it.
> 
> ---
> 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/19211481431


-- 
Best Regards
Tao Chen