linux-kernel - Re: [PATCH bpf-next] bpf: Add preempt_disable to protect get_perf

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Message-ID: <CAADnVQKtOCXdv-LJ-T6K_meAS26C_i4Yc0hOpYS46umsPmuQAQ@mail.gmail.com>
Date: Mon, 22 Sep 2025 19:53:35 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: Song Liu <song@...nel.org>, Jiri Olsa <jolsa@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add preempt_disable to protect get_perf_callchain

On Mon, Sep 22, 2025 at 12:54 AM Tao Chen <chen.dylane@...ux.dev> wrote:
>
> As Alexei suggested, the return value from get_perf_callchain() may be
> reused if another task preempts and requests the stack after BPF program
> switched to migrate disable.
>
> Reported-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
> ---
>  kernel/bpf/stackmap.c | 14 +++++---------
>  1 file changed, 5 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 2e182a3ac4c..07892320906 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -314,8 +314,10 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
>         if (max_depth > sysctl_perf_event_max_stack)
>                 max_depth = sysctl_perf_event_max_stack;
>
> +       preempt_disable();
>         trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
>                                    false, false);
> +       preempt_enable();

This is obviously wrong.
As soon as preemption is enabled, trace can be overwritten.
guard(preempt)();
can fix it, but the length of the preempt disabled section
will be quite big.
The way get_perf_callchain() api is written I don't see
another option though. Unless we refactor it similar
to bpf_try_get_buffers().

pw-bot: cr