linux-kernel - Re: [PATCH] bpf: fix sample_flags for bpf_perf_event

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Date:   Mon, 17 Oct 2022 19:27:44 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Sumanth Korikkar <sumanthk@...ux.ibm.com>
Cc:     olsajiri@...il.com, bpf@...r.kernel.org, gor@...ux.ibm.com,
        hca@...ux.ibm.com, iii@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-tip-commits@...r.kernel.org, namhyung@...nel.org,
        peterz@...radead.org, svens@...ux.ibm.com, tip-bot2@...utronix.de,
        tmricht@...ux.ibm.com, x86@...nel.org
Subject: Re: [PATCH] bpf: fix sample_flags for bpf_perf_event_output

Hello,


The commit that this patch is fixing[1] also causes yet another segfault for
'perf-script' of tracepoint records.  For example:

    $ sudo timeout 3 perf record -e exceptions:page_fault_user
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.228 MB perf.data (74 samples) ]
    $ sudo perf script
    Segmentation fault

Reverting this patch and the original bug commit[1] fixes the issue.  I haven't
deep dive yet because I'm not familiar with this area.  Anybody has any idea
about this?

[1] 838d9bb62d13 ("perf: Use sample_flags for raw_data")


Thanks,
SJ

On Fri, 7 Oct 2022 10:13:27 +0200 Sumanth Korikkar <sumanthk@...ux.ibm.com> wrote:

> * Raw data is also filled by bpf_perf_event_output.
> * Add sample_flags to indicate raw data.
> * This eliminates the segfaults as shown below:
>   Run ./samples/bpf/trace_output
>   BUG pid 9 cookie 1001000000004 sized 4
>   BUG pid 9 cookie 1001000000004 sized 4
>   BUG pid 9 cookie 1001000000004 sized 4
>   Segmentation fault (core dumped)
> 
> Fixes: 838d9bb62d13 ("perf: Use sample_flags for raw_data")
> Acked-by: Namhyung Kim <namhyung@...nel.org>
> Signed-off-by: Sumanth Korikkar <sumanthk@...ux.ibm.com>
> ---
>  kernel/trace/bpf_trace.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 49fb9ec8366d..1ed08967fb97 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -687,6 +687,7 @@ BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
>  
>  	perf_sample_data_init(sd, 0, 0);
>  	sd->raw = &raw;
> +	sd->sample_flags |= PERF_SAMPLE_RAW;
>  
>  	err = __bpf_perf_event_output(regs, map, flags, sd);
>  
> @@ -745,6 +746,7 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
>  	perf_fetch_caller_regs(regs);
>  	perf_sample_data_init(sd, 0, 0);
>  	sd->raw = &raw;
> +	sd->sample_flags |= PERF_SAMPLE_RAW;
>  
>  	ret = __bpf_perf_event_output(regs, map, flags, sd);
>  out:
> -- 
> 2.36.1