Re: [PATCH] perf: write bpf_prog (infos|btfs)_cnt to data file

[Ian Rogers]

On Fri, Nov 7, 2025 at 9:32 AM Thomas Falcon <thomas.falcon@xxxxxxxxx> wrote:
>
> With commit f0d0f978f3f58 ("perf header: Don't write empty BPF/BTF
> info"), the write_bpf_( prog_info() | btf() ) functions exit
> without writing anything if env->bpf_prog.(infos| btfs)_cnt is zero.
>
> process_bpf_( prog_info() | btf() ), however, still expect a "count"
> value to exist in the data file. If btf information is empty, for
> example, process_bpf_btf will read garbage or some other data as the
> number of btf nodes in the data file. As a result, the data file will
> not be processed correctly.
>
> Instead, write the count to the data file and exit if it is zero.
>
> Fixes: f0d0f978f3f58 ("perf header: Don't write empty BPF/BTF info")
> Signed-off-by: Thomas Falcon <thomas.falcon@xxxxxxxxx>

Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks!
Ian

> ---
>  tools/perf/util/header.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
>
> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> index db2ad19fa50d..54968881481c 100644
> --- a/tools/perf/util/header.c
> +++ b/tools/perf/util/header.c
> @@ -1022,12 +1022,9 @@ static int write_bpf_prog_info(struct feat_fd *ff,
>
>         down_read(&env->bpf_progs.lock);
>
> -       if (env->bpf_progs.infos_cnt == 0)
> -               goto out;
> -
>         ret = do_write(ff, &env->bpf_progs.infos_cnt,
>                        sizeof(env->bpf_progs.infos_cnt));
> -       if (ret < 0)
> +       if (ret < 0 || env->bpf_progs.infos_cnt == 0)
>                 goto out;
>
>         root = &env->bpf_progs.infos;
> @@ -1067,13 +1064,10 @@ static int write_bpf_btf(struct feat_fd *ff,
>
>         down_read(&env->bpf_progs.lock);
>
> -       if (env->bpf_progs.btfs_cnt == 0)
> -               goto out;
> -
>         ret = do_write(ff, &env->bpf_progs.btfs_cnt,
>                        sizeof(env->bpf_progs.btfs_cnt));
>
> -       if (ret < 0)
> +       if (ret < 0 || env->bpf_progs.btfs_cnt == 0)
>                 goto out;
>
>         root = &env->bpf_progs.btfs;
> --
> 2.47.3
>