Re: [PATCH bpf-next] bpf: Remove RCU lock for perf callchain buffer

[Tao Chen]

在 2026/1/29 00:57, Tao Chen 写道:
> As Andrii suggested, BPF can guarantee perf callchain buffer won't
> be released during use, for bpf_get_stack_id, BPF stack map will
> keep them alive by delaying put_callchain_buffer() until freeing time
> or for bpf_get_stack/bpf_get_task_stack, BPF program itself will hold
> these buffers alive again, until freeing time which is delayed until
> after RCU Tasks Trace + RCU grace period.
>
> Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Signed-off-by: Tao Chen <chen.dylane@xxxxxxxxx>
> ---
>   kernel/bpf/stackmap.c | 12 +-----------
>   1 file changed, 1 insertion(+), 11 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index da3d328f5c1..2e682d8697b 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -460,9 +460,6 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>   
>   	max_depth = stack_map_calculate_max_depth(size, elem_size, flags);
>   
> -	if (may_fault)
> -		rcu_read_lock(); /* need RCU for perf's callchain below */
> -
>   	if (trace_in) {
>   		trace = trace_in;
>   		trace->nr = min_t(u32, trace->nr, max_depth);
> @@ -473,11 +470,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>   					   crosstask, false, 0);
>   	}
>   
> -	if (unlikely(!trace) || trace->nr < skip) {
> -		if (may_fault)
> -			rcu_read_unlock();
> +	if (unlikely(!trace) || trace->nr < skip)
>   		goto err_fault;
> -	}
>   
>   	trace_nr = trace->nr - skip;
>   	copy_len = trace_nr * elem_size;
> @@ -493,10 +487,6 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>   		memcpy(buf, ips, copy_len);
>   	}
>   
> -	/* trace/ips should not be dereferenced after this point */
> -	if (may_fault)
> -		rcu_read_unlock();
> -
>   	if (user_build_id)
>   		stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
>   

Hi Andrii,

I send this patch as a separate submission, and I'm not sure when the 
patchset will be merged. After this merged, i will update v9. Is it ok?

The previous patchset:
https://lore.kernel.org/bpf/CAEf4BzbubXxLHsO_2aVBG_oYE1Sc9iafLyvcX-PEjcQ7EXWCqA@xxxxxxxxxxxxxx/

And i also saw Qing reported a syzbot issue recently.
https://lore.kernel.org/bpf/20260128084538.1889001-1-wangqing7171@xxxxxxxxx/

--
Best Regards
Tao Chen