linux-kernel - Re: [PATCH 2/2] perf trace: Handle NULL pointer dereference in trace__syscall

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Message-ID: <20190618063937.GB24549@leoy-ThinkPad-X240s>
Date:   Tue, 18 Jun 2019 14:39:37 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH 2/2] perf trace: Handle NULL pointer dereference in
 trace__syscall_info()

On Mon, Jun 17, 2019 at 02:32:03PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jun 17, 2019 at 05:11:40PM +0800, Leo Yan escreveu:
> > trace__init_bpf_map_syscall_args() invokes trace__syscall_info() to
> > retrieve system calls information, it always passes NULL for 'evsel'
> > argument; when id is an invalid value then the logging will try to
> > output event name, this triggers NULL pointer dereference.
> > 
> > This patch directly uses string "unknown" for event name when 'evsel'
> > is NULL pointer.
> > 
> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
> > ---
> >  tools/perf/builtin-trace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> > index 5cd74651db4c..49dfb2fd393b 100644
> > --- a/tools/perf/builtin-trace.c
> > +++ b/tools/perf/builtin-trace.c
> > @@ -1764,7 +1764,7 @@ static struct syscall *trace__syscall_info(struct trace *trace,
> >  		static u64 n;
> >  
> >  		pr_debug("Invalid syscall %d id, skipping (%s, %" PRIu64 ")\n",
> > -			 id, perf_evsel__name(evsel), ++n);
> > +			 id, evsel ? perf_evsel__name(evsel) : "unknown", ++n);
> >  		return NULL;
> 
> What do you think of this instead?

Yes, I agree the below change is right thing to do.  FWIW:

Reviewed-by: Leo Yan <leo.yan@...aro.org>

BTW, my patch followed the code in [1], after apply below your change,
could consider to simplify code in [1] for without checking 'evsel' is
NULL pointer anymore.

Thanks,
Leo

[1] https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/builtin-report.c?h=perf/core#n301

> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 68beef8f47ff..1d6af95b9207 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -590,6 +590,9 @@ const char *perf_evsel__name(struct perf_evsel *evsel)
>  {
>  	char bf[128];
>  
> +	if (!evsel)
> +		goto out_unknown;
> +
>  	if (evsel->name)
>  		return evsel->name;
>  
> @@ -629,7 +632,10 @@ const char *perf_evsel__name(struct perf_evsel *evsel)
>  
>  	evsel->name = strdup(bf);
>  
> -	return evsel->name ?: "unknown";
> +	if (evsel->name)
> +		return evsel->name;
> +out_unknown:
> +	return "unknown";
>  }
>  
>  const char *perf_evsel__group_name(struct perf_evsel *evsel)