Re: [PATCH bpf-next 8/9] bpf: Remove trace_in argument from __bpf_get_stack

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jul 20, 2026 at 09:09:45AM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] bpf_get_stack_pe permanently truncates the shared perf event callchain for user stacks.
> --
> 
> --- Patch [8]: [PATCH bpf-next 8/9] bpf: Remove trace_in argument from __bpf_get_stack ---
> commit 42ed70b803080e71468d115e2948bdba98a800f5
> Author: Jiri Olsa <jolsa@kernel.org>
> 
> bpf: Remove trace_in argument from __bpf_get_stack
> 
> This commit removes the trace_in argument from __bpf_get_stack and
> handles the trace_in case directly in a new __bpf_get_stack_pe function.
> This simplifies __bpf_get_stack for upcoming preemption fixes.
> 
> > diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> > index eea7b781300ca..57cd4c33403bd 100644
> > --- a/kernel/bpf/stackmap.c
> > +++ b/kernel/bpf/stackmap.c
> [ ... ]
> > @@ -978,14 +1001,13 @@ BPF_CALL_4(bpf_get_stack_pe, struct bpf_perf_event_data_kern *, ctx,
> >  		skip += nr_kernel;
> >  		if (skip > BPF_F_SKIP_FIELD_MASK)
> >  			goto clear;
> > -
> >  		flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
> > -		err = __bpf_get_stack(regs, NULL, trace, buf, size, flags, false /* !may_fault */);
> > +		err = __bpf_get_stack_pe(trace, buf, size, flags);
> 
> [Severity: High]
> This is a pre-existing issue, but does this permanently truncate the shared
> perf event callchain for user stacks?
> 
> In bpf_get_stack_pe(), the kernel stack branch explicitly saves and restores
> trace->nr around the call to __bpf_get_stack_pe().
> 
> However, the user stack branch directly calls __bpf_get_stack_pe() without
> saving the original trace->nr value. Inside __bpf_get_stack_pe(), the trace
> size is modified in place:
> 
> static int __bpf_get_stack_pe(...)
> {
>         ...
>         max_depth = stack_map_calculate_max_depth(size, elem_size, flags);
>         trace->nr = min_t(u32, trace->nr, max_depth);
>         ...
> }
> 
> If a BPF program attached to a perf event requests a user stack trace and
> provides a buffer smaller than the actual callchain length, trace->nr is
> truncated.
> 
> When the perf subsystem later processes this event and outputs it to the
> perf ring buffer, could this result in incomplete or corrupted profiling
> data due to the truncated user space callchain length?

right, will fix that in next version

jirka