linux-kernel - Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability checks in the unwinder

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Date:   Fri, 21 May 2021 13:48:17 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>,
        mark.rutland@....com, ardb@...nel.org, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org, jmorris@...ei.org,
        pasha.tatashin@...een.com, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability
 checks in the unwinder

On Fri, May 21, 2021 at 06:53:18PM +0100, Mark Brown wrote:
> On Fri, May 21, 2021 at 12:47:13PM -0500, Madhavan T. Venkataraman wrote:
> > On 5/21/21 12:42 PM, Mark Brown wrote:
> 
> > > Like I say we may come up with some use for the flag in error cases in
> > > future so I'm not opposed to keeping the accounting there.
> 
> > So, should I leave it the way it is now? Or should I not set reliable = false
> > for errors? Which one do you prefer?
> 
> > Josh,
> 
> > Are you OK with not flagging reliable = false for errors in unwind_frame()?
> 
> I think it's fine to leave it as it is.

Either way works for me, but if you remove those 'reliable = false'
statements for stack corruption then, IIRC, the caller would still have
some confusion between the end of stack error (-ENOENT) and the other
errors (-EINVAL).

So the caller would have to know that -ENOENT really means success.
Which, to me, seems kind of flaky.

BTW, not sure if you've seen what we do in x86, but we have a
'frame->error' which gets set for an error, and which is cumulative
across frames.  So non-fatal reliable-type errors don't necessarily have
to stop the unwind.  The end result is the same as your patch, but it
seems less confusing to me because the 'error' is cumulative.  But that
might be personal preference and I'd defer to the arm64 folks.

-- 
Josh