Re: [RFC PATCH v1 1/1] arm64: Unwinder enhancements for reliable stack trace

[Mark Brown <broonie@kernel.org>]

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On Tue, Feb 23, 2021 at 01:20:49PM -0600, Madhavan T. Venkataraman wrote:
> On 2/23/21 1:02 PM, Mark Brown wrote:
> > On Tue, Feb 23, 2021 at 12:12:43PM -0600, madvenka@linux.microsoft.com wrote:

> >> Reliable stack trace function
> >> =============================
> >>
> >> Implement arch_stack_walk_reliable(). This function walks the stack like
> >> the existing stack trace functions with a couple of additional checks:

> > Again, this should be at least one separate patch.  How does this ensure
> > that we don't have any issues with any of the various probe mechanisms?
> > If there's no need to explicitly check anything that should be called
> > out in the changelog.

> I am trying to do this in an incremental fashion. I have to study the probe
> mechanisms a little bit more before I can come up with a solution. But
> if you want to see that addressed in this patch set, I could do that.
> It will take a little bit of time. That is all.

Handling of the probes stuff seems like it's critical to reliable stack
walk so we shouldn't claim to have support for reliable stack walk
without it.  If it was a working implementation we could improve that'd
be one thing but this would be buggy which is a different thing.

> >> +	(void) on_accessible_stack(task, stackframe, &info);

> > Shouldn't we return NULL if we are not on an accessible stack?

> The prev_fp has already been checked by the unwinder in the previous
> frame. That is why I don't check the return value. If that is acceptable,
> I will add a comment.

TBH if you're adding the comment it seems like you may as well add the
check, it's not like it's expensive and it means there's no possibility
that some future change could result in this assumption being broken.

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