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

[Leo Yan]

On Wed, Jul 03, 2019 at 11:00:32AM +0100, Daniel Thompson wrote:
> 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.

Fair point and it's smart to connect with function
itrace_synth_opts__set_default(). :)

Thanks, 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@xxxxxxxxxx>
> > > > ---
> > > >  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;
> > > >  	}
> > > >  
> > > > 
> > >