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

Leo Yan
2019-07-03T01:36:10Z

Hi Adrian,

On Tue, Jul 02, 2019 at 02:07:40PM +0300, Adrian Hunter wrote:
> 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.

Thanks for suggestions.

I checked report and script commands, as you said, both command will
always set session->itrace_synth_opts. For these two commands, we can
safely remove the NULL test.

Because perf tool contains many sub commands, so I don't have much
confidence it's very safe to remove the NULL test for all cases; e.g.
there have cases which will process aux trace buffer but without
itrace options; for this case, session->itrace_synth_opts might be NULL.

For either way (remove NULL test or keep NULL test), I don't want to
introduce regression and extra efforts by my patch. So want to double
confirm with you for this :)

Thanks,
Leo Yan

> > 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;
> > }
> >
> >