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

Daniel Thompson
2019-07-03T10:00:41Z

On Wed, Jul 03, 2019 at 09:35:54AM +0800, Leo Yan wrote:
> 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 :)

Review is useful to ensure the chosen solution is correct but
unless I missed something the non-regression reasoning here is easy
easy. In its original form and despite the check, the code will
always dereference session->itrace_synth_opts, therefore removing
the check cannot makes things worse.

Daniel.

PS Of course we do also have to check that
itrace_synth_opts__set_default() isn't a macro... but it isn't.

> > > 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;
> > > }
> > >
> > >
> >