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

[Namhyung Kim]

On Fri, Nov 07, 2025 at 11:31:50AM -0600, Thomas Falcon 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.

Oh, I'm afraid it'd create a compatibility problem.  But I think this is
a right behavior and it's should be fine if it goes to the stable soon.

Arnaldo, can you please take this into perf-tools tree for v6.18?

Thanks,
Namhyung

> 
> Fixes: f0d0f978f3f58 ("perf header: Don't write empty BPF/BTF info")
> Signed-off-by: Thomas Falcon <thomas.falcon@xxxxxxxxx>
> ---
>  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
>