Re: [RFC PATCH 0/3] arm64: Implement reliable stack trace

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Oct 16, 2020 at 01:15:34PM +0100, Mark Brown wrote:
> 
> Yes, exactly - just copying the existing implementations and hoping that
> it's sensible/relevant and covers everything that's needed.  It's not
> entirely clear what a reliable stacktrace is expected to do that a
> normal stacktrace doesn't do beyond returning an error code.

While in the end there may not be much of a difference between normal
and reliable stacktraces beyond returning an error code, it still
requires beefing up the unwinder's error detection abilities.

> > > The searching for a defined thread entry point for example isn't
> > > entirely visible in the implementations.
> 
> > For now I'll speak only of x86, because I don't quite remember how
> > powerpc does it.
> 
> > For thread entry points, aka the "end" of the stack:
> 
> > - For ORC, the end of the stack is either pt_regs, or -- when unwinding
> >   from kthreads, idle tasks, or irqs/exceptions in entry code --
> >   UNWIND_HINT_EMPTY (found by the unwinder's check for orc->end.
> 
> >   [ Admittedly the implementation needs to be cleaned up a bit.  EMPTY
> >     is too broad and needs to be split into UNDEFINED and ENTRY. ]
> 
> > - For frame pointers, by convention, the end of the stack for all tasks
> >   is a defined stack offset: end of stack page - sizeof(pt_regs).
> 
> > And yes, all that needs to be documented.
> 
> Ah, I'd have interpreted "defined thread entry point" as meaning
> expecting to find specific functions appering at the end of the stack
> rather than meaning positively identifying the end of the stack - for
> arm64 we use a NULL frame pointer to indicate this in all situations.
> In that case that's one bit that is already clear.

I think a NULL frame pointer isn't going to be robust enough.  For
example NULL could easily be introduced by a corrupt stack, or by asm
frame pointer misuse.

-- 
Josh