On Wed, Dec 20, 2023 at 08:29:32AM -0500, Steven Rostedt wrote:
> On Wed, 20 Dec 2023 13:06:06 +0000
> Vincent Donnefort <vdonnef...@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
