Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf()

[Mathieu Poirier]

Hi Peter,

On Thu, Oct 22, 2020 at 04:32:36PM +0100, Suzuki Poulose wrote:
> On 10/22/20 4:06 PM, Peter Zijlstra wrote:
> > On Thu, Oct 22, 2020 at 02:30:21PM +0100, Suzuki Poulose wrote:
> > > On 10/22/20 12:32 PM, Peter Zijlstra wrote:
> > > > On Thu, Oct 22, 2020 at 04:27:52PM +0530, Sai Prakash Ranjan wrote:
> > > > 
> > > > > Looking at the ETR and other places in the kernel, ETF and the
> > > > > ETB are the only places trying to dereference the task(owner)
> > > > > in tmc_enable_etf_sink_perf() which is also called from the
> > > > > sched_in path as in the call trace.
> > > > 
> > > > > @@ -391,6 +392,10 @@ static void *tmc_alloc_etf_buffer(struct coresight_device *csdev,
> > > > >    {
> > > > >    	int node;
> > > > >    	struct cs_buffers *buf;
> > > > > +	struct task_struct *task = READ_ONCE(event->owner);
> > > > > +
> > > > > +	if (!task || is_kernel_event(event))
> > > > > +		return NULL;
> > > > 
> > > > 
> > > > This is *wrong*... why do you care about who owns the events?
> > > > 
> > > 
> > > This is due to the special case of the CoreSight configuration, where
> > > a "sink" (where the trace data is captured) is shared by multiple Trace
> > > units. So, we could share the "sink" for multiple trace units if they
> > > are tracing the events that belong to the same "perf" session. (The
> > > userspace tool could decode the trace data based on the TraceID
> > > in the trace packets). Is there a better way to do this ?
> > 
> > I thought we added sink identification through perf_event_attr::config2
> > ?
> > 
> 
> Correct. attr:config2 identifies the "sink" for the collection. But,
> that doesn't solve the problem we have here. If two separate perf
> sessions use the "same sink", we don't want to mix the
> trace data into the same sink for events from different sessions.
> 
> Thus, we need a way to check if a new event starting the tracing on
> an ETM belongs to the same session as the one already pumping the trace
> into the sink.

Suzuki's depiction of the usecase is accurate.  Using the pid of the process
that created the events comes out of a discussion you and I had in the common
area by the Intel booth at ELC in Edinburgh in the fall of 2018.  At the time I
exposed the problem of having multiple events sharing the same HW resources and
you advised to proceed this way.

That being said it is plausible that I did not expressed myself clearly enough
for you to understand the full extend of the problem.  If that is the case we
are more than willing to revisit that solution.  Do you see a better option than
what has currently been implemented?

Many thanks,
Mathieu

> 
> We use event->owner pid for this check and thats where we encountered
> a NULL event->owner. Looking at the code further, we identified that
> kernel events could also trigger this issue.
> 
> Suzuki