Re: [PATCH v1 10/11] perf intel-pt: Smatch: Fix potential NULL pointer dereference

From: Adrian Hunter <adrian.hunter@intel.com>
To: Leo Yan <leo.yan@linaro.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Andi Kleen <ak@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Jin Yao <yao.jin@linux.intel.com>,
	Song Liu <songliubraving@fb.com>,
	Alexios Zavras <alexios.zavras@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Changbin Du <changbin.du@intel.com>,
	Eric Saint-Etienne <eric.saint.etienne@oracle.com>,
	Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
	Thomas Richter <tmricht@linux.ibm.com>,
	Alexey Budankov <alexey.budankov@linux.intel.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v1 10/11] perf intel-pt: Smatch: Fix potential NULL pointer dereference
Date: Tue, 2 Jul 2019 14:07:40 +0300	[thread overview]
Message-ID: <cfef1777-141e-4223-e0c1-1a3f3aee1d3c@intel.com> (raw)
In-Reply-To: <20190702103420.27540-11-leo.yan@linaro.org>

On 2/07/19 1:34 PM, Leo Yan wrote:
> Based on the following report from Smatch, fix the potential
> NULL pointer dereference check.

It never is NULL.  Remove the NULL test if you want:

-	if (session->itrace_synth_opts && session->itrace_synth_opts->set) {
+	if (session->itrace_synth_opts->set) {

But blindly making changes like below is questionable.

> 
>   tools/perf/util/intel-pt.c:3200
>   intel_pt_process_auxtrace_info() error: we previously assumed
>   'session->itrace_synth_opts' could be null (see line 3196)
> 
>   tools/perf/util/intel-pt.c:3206
>   intel_pt_process_auxtrace_info() warn: variable dereferenced before
>   check 'session->itrace_synth_opts' (see line 3200)
> 
> tools/perf/util/intel-pt.c
> 3196         if (session->itrace_synth_opts && session->itrace_synth_opts->set) {
> 3197                 pt->synth_opts = *session->itrace_synth_opts;
> 3198         } else {
> 3199                 itrace_synth_opts__set_default(&pt->synth_opts,
> 3200                                 session->itrace_synth_opts->default_no_sample);
>                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^
> 3201                 if (!session->itrace_synth_opts->default_no_sample &&
> 3202                     !session->itrace_synth_opts->inject) {
> 3203                         pt->synth_opts.branches = false;
> 3204                         pt->synth_opts.callchain = true;
> 3205                 }
> 3206                 if (session->itrace_synth_opts)
>                          ^^^^^^^^^^^^^^^^^^^^^^^^^^
> 3207                         pt->synth_opts.thread_stack =
> 3208                                 session->itrace_synth_opts->thread_stack;
> 3209         }
> 
> To dismiss the potential NULL pointer dereference, this patch validates
> the pointer 'session->itrace_synth_opts' before access its elements.
> 
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
>  tools/perf/util/intel-pt.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/intel-pt.c b/tools/perf/util/intel-pt.c
> index 550db6e77968..88b567bdf1f9 100644
> --- a/tools/perf/util/intel-pt.c
> +++ b/tools/perf/util/intel-pt.c
> @@ -3195,7 +3195,7 @@ int intel_pt_process_auxtrace_info(union perf_event *event,
>  
>  	if (session->itrace_synth_opts && session->itrace_synth_opts->set) {
>  		pt->synth_opts = *session->itrace_synth_opts;
> -	} else {
> +	} else if (session->itrace_synth_opts) {
>  		itrace_synth_opts__set_default(&pt->synth_opts,
>  				session->itrace_synth_opts->default_no_sample);
>  		if (!session->itrace_synth_opts->default_no_sample &&
> @@ -3203,8 +3203,7 @@ int intel_pt_process_auxtrace_info(union perf_event *event,
>  			pt->synth_opts.branches = false;
>  			pt->synth_opts.callchain = true;
>  		}
> -		if (session->itrace_synth_opts)
> -			pt->synth_opts.thread_stack =
> +		pt->synth_opts.thread_stack =
>  				session->itrace_synth_opts->thread_stack;
>  	}
>  
> 