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

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Date:   Thu, 14 Oct 2021 11:48:25 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>, keescook@...omium.org,
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Subject: Re: [PATCH 6/7] arch: __get_wchan || STACKTRACE_SUPPORT

On Thu, Oct 14, 2021 at 07:03:07PM +0100, Mark Rutland wrote:
> 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?

Yes, I would think so, for arches where they otherwise show up on the
stacktrace.

-- 
Josh