From: Jiri Olsa <olsajiri@gmail.com>
To: sashiko-reviews@lists.linux.dev
Cc: bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next 8/9] bpf: Remove trace_in argument from __bpf_get_stack
Date: Mon, 20 Jul 2026 22:31:58 +0200 [thread overview]
Message-ID: <al6Fvrxd19Qip2o9@krava> (raw)
In-Reply-To: <20260720090945.C778F1F000E9@smtp.kernel.org>
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
next prev parent reply other threads:[~2026-07-20 20:32 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-07-20 8:53 [PATCH bpf-next 0/9] bpf: Disable preemption in stack map code Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 1/9] bpf: Factor stackid_init function from __bpf_get_stackid Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 2/9] bpf: Factor stackid_fastpath " Jiri Olsa
2026-07-20 9:09 ` sashiko-bot
2026-07-20 20:31 ` Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 3/9] bpf: Factor stackid_new_bucket " Jiri Olsa
2026-07-20 9:01 ` sashiko-bot
2026-07-20 20:31 ` Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 4/9] bpf: Use stack id functions instead of __bpf_get_stackid Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 5/9] bpf: Disable preemption in bpf_get_stackid Jiri Olsa
2026-07-20 9:04 ` sashiko-bot
2026-07-20 20:31 ` Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 6/9] bpf: Factor callchain_store function from __bpf_get_stack Jiri Olsa
2026-07-20 9:07 ` sashiko-bot
2026-07-20 20:31 ` Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 7/9] bpf: Factor callchain_finalize " Jiri Olsa
2026-07-20 9:08 ` sashiko-bot
2026-07-20 20:31 ` Jiri Olsa
2026-07-20 8:53 ` [PATCH bpf-next 8/9] bpf: Remove trace_in argument " Jiri Olsa
2026-07-20 9:09 ` sashiko-bot
2026-07-20 20:31 ` Jiri Olsa [this message]
2026-07-20 8:53 ` [PATCH bpf-next 9/9] bpf: Disable preemption in __bpf_get_stack Jiri Olsa
2026-07-20 9:18 ` sashiko-bot
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