linux-hardening - Re: [PATCH 6/7] arch: __get_wchan || STACKTRACE

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Date:   Thu, 14 Oct 2021 19:03:07 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH 6/7] arch: __get_wchan || STACKTRACE_SUPPORT

On Fri, Oct 08, 2021 at 03:45:59PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 08, 2021 at 01:40:52PM +0100, Mark Rutland wrote:
> > [Adding Josh, since there might be a concern here from a livepatch pov]
> > 
> 
> > > +static unsigned long __get_wchan(struct task_struct *p)
> > > +{
> > > +	unsigned long entry = 0;
> > > +
> > > +	stack_trace_save_tsk(p, &entry, 1, 0);
> > 
> > This assumes stack_trace_save_tsk() will skip sched functions, but I
> > don't think that's ever been a requirement? It's certinaly not
> > documented anywhere that I could find, and arm64 doesn't do so today,
> > and this patch causes wchan to just log `__switch_to` for everything.
> 
> Confused, arm64 has arch_stack_walk() and should thus use
> kernel/stacktrace.c's stack_trace_consume_entry_nosched.

Looking at this arm64's *current* get_wchan() unwinds once before
checking in_sched_functions(), so it skips __switch_to(). As of this
patch, we check in_sched_functions() first, which stops the unwind
immediately as __switch_to() isn't marked as __sched.

I think x86 gets away with this because switch_to() is asm, and that
tail-calls __switch_to() when returning.

Does switch_to() and below need to be marked __sched?

Thanks,
Mark.