netdev - Re: [RFC PATCH bpf-next] bpf: Iterate through all PT

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Date:   Wed, 12 Aug 2020 15:26:43 +0000
From:   Song Liu <songliubraving@...com>
To:     Jiri Olsa <jolsa@...nel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [RFC PATCH bpf-next] bpf: Iterate through all PT_NOTE sections
 when looking for build id



> On Aug 12, 2020, at 5:31 AM, Jiri Olsa <jolsa@...nel.org> wrote:
> 
> Currently when we look for build id within bpf_get_stackid helper
> call, we check the first NOTE section and we fail if build id is
> not there.
> 
> However on some system (Fedora) there can be multiple NOTE sections
> in binaries and build id data is not always the first one, like:
> 
>  $ readelf -a /usr/bin/ls
>  ...
>  [ 2] .note.gnu.propert NOTE             0000000000000338  00000338
>       0000000000000020  0000000000000000   A       0     0     8358
>  [ 3] .note.gnu.build-i NOTE             0000000000000358  00000358
>       0000000000000024  0000000000000000   A       0     0     437c
>  [ 4] .note.ABI-tag     NOTE             000000000000037c  0000037c
>  ...
> 
> So the stack_map_get_build_id function will fail on build id retrieval
> and fallback to BPF_STACK_BUILD_ID_IP.
> 
> This patch is changing the stack_map_get_build_id code to iterate
> through all the NOTE sections and try to get build id data from
> each of them.
> 
> When tracing on sched_switch tracepoint that does bpf_get_stackid
> helper call kernel build, I can see about 60% increase of successful
> build id retrieval. The rest seems fails on -EFAULT.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>

LGTM. Thanks for the fix!

Acked-by: Song Liu <songliubraving@...com>