Re: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump

[Shi, Yang]

On 2/10/2016 4:10 AM, Will Deacon wrote:
> On Wed, Feb 10, 2016 at 11:52:31AM +0000, James Morse wrote:
> > On 10/02/16 10:29, Will Deacon wrote:
> > > On Tue, Feb 09, 2016 at 01:26:22PM -0800, Yang Shi wrote:
> > > > dump_backtrace may be called in kthread context, which is not bound to a single
> > > > cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug
> > > > report:
> > >
> > > If we're preemptible here, it means that our irq_stack_ptr is potentially
> > > bogus. Whilst this isn't an issue for kthreads, it does feel like we
> > > could make this slightly more robust in the face of potential frame
> > > corruption. Maybe just zero the IRQ stack pointer if we're in preemptible
> > > context?
> >
> > Switching between stacks is only valid if we are tracing ourselves while on the
> > irq_stack, we should probably prevent it for other tasks too.
> >
> > Something like (untested):
> > ---------------------
> > if (tsk == current && in_atomic())
> > 	irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());

One follow up question, is it possible to have both tsk != current and 
on_irq_stack is true at the same time? If it is possible, this may be a 
problem in unwind_frame called by profile_pc which has tsk being NULL.

Thanks,
Yang

> > else
> > 	irq_stack_ptr = 0;
> > ---------------------
> >
> > This would work when we trace ourselves while on the irq_stack, but break*
> > tracing a running task on a remote cpu (khungtaskd doesn't do this).
> >
> > The same fix would apply to unwind_frame(), we have 'tsk' in both functions.
> >
> > Thoughts?
>
> in_atomic is a misnomer:
>
>    https://lwn.net/Articles/274695/
>
> ;)
>
> So we might be better off zeroing the pointer if tsk != current ||
> preemptible(). But yeah, I think we're in general agreement about this.
>
> Will