Re: [PATCH v8 0/2] Introducing trace buffer mapping by user-space

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Dec 20, 2023 at 08:29:32AM -0500, Steven Rostedt wrote:
> On Wed, 20 Dec 2023 13:06:06 +0000
> Vincent Donnefort <vdonnefort@google.com> wrote:
> 
> > > @@ -771,10 +772,20 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
> > >  static void rb_wake_up_waiters(struct irq_work *work)
> > >  {
> > >  	struct rb_irq_work *rbwork = container_of(work, struct rb_irq_work, work);
> > > -	struct ring_buffer_per_cpu *cpu_buffer =
> > > -		container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
> > > +	struct ring_buffer_per_cpu *cpu_buffer;
> > > +	struct trace_buffer *buffer;
> > > +	int cpu;
> > >  
> > > -	rb_update_meta_page(cpu_buffer);
> > > +	if (rbwork->is_cpu_buffer) {
> > > +		cpu_buffer = container_of(rbwork, struct ring_buffer_per_cpu, irq_work);
> > > +		rb_update_meta_page(cpu_buffer);
> > > +	} else {
> > > +		buffer = container_of(rbwork, struct trace_buffer, irq_work);
> > > +		for_each_buffer_cpu(buffer, cpu) {
> > > +			cpu_buffer = buffer->buffers[cpu];
> > > +			rb_update_meta_page(cpu_buffer);
> > > +		}
> > > +	}  
> > 
> > Arg, somehow never reproduced the problem :-\. I suppose you need to cat
> > trace/trace_pipe and mmap(trace/cpuX/trace_pipe) at the same time?
> 
> It triggered as soon as I ran "trace-cmd start -e sched_switch"
> 
> In other words, it broke the non mmap case. This function gets called for
> both the buffer and cpu_buffer irq_work entries. You added the
> container_of() to get access to cpu_buffer, when the rbwork could also be
> for the main buffer too. The main buffer has no meta page, and it triggered
> a NULL pointer dereference, as "cpu_buffer->mapped" returned true (because
> it was on something of the buffer structure that wasn't zero), and then here:
> 
> 	if (cpu_buffer->mapped) {
> 		WRITE_ONCE(cpu_buffer->meta_page->reader.read, 0);
> 
> It dereferenced cpu_buffer->meta_page->reader
> 
> which is only God knows what!
> 
> > 
> > Updating the meta-page is only useful if the reader we are waking up is a
> > user-space one, which would only happen with the cpu_buffer version of this
> > function. We could limit the update of the meta_page only to this case?
> 
> I rather not add another irq_work entry. This workaround should be good
> enough.

I meant, to only do in rb_wake_up_waiters()

  if (rbwork->is_cpu_buffer) 
  	rb_update_meta_page(cpu_buffer)

And skip the meta-page update for the !is_cpu_buffer case?

> 
> Thanks,
> 
> -- Steve