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From:   Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Date:   Mon, 17 Jun 2019 14:32:03 -0300
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        bpf@vger.kernel.org
Subject: Re: [PATCH 2/2] perf trace: Handle NULL pointer dereference in
 trace__syscall_info()
Message-ID: <20190617173203.GA23094@kernel.org>
References: <20190617091140.24372-1-leo.yan@linaro.org>
 <20190617091140.24372-2-leo.yan@linaro.org>
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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@linaro.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?

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)