linux-kernel - Re: [PATCH] perf: write bpf_prog (infos|btfs)

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Message-ID: <aRbYjFLCpLAFrMWD@google.com>
Date: Thu, 13 Nov 2025 23:21:48 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Thomas Falcon <thomas.falcon@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: write bpf_prog (infos|btfs)_cnt to data file

On Thu, Nov 13, 2025 at 09:52:40AM -0300, Arnaldo Carvalho de Melo wrote:
> On Tue, Nov 11, 2025 at 02:38:07PM -0800, Namhyung Kim wrote:
> > 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?
> 
> Ok, agreed, I'm taking your request as an Acked-by, ok?

Sure, thanks!
Namhyung

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