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 14:11:40 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     Mark Brown <broonie@...nel.org>, 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 01:59:16PM -0500, Madhavan T. Venkataraman wrote:
> 
> 
> On 5/21/21 1:48 PM, Josh Poimboeuf wrote:
> > 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).
> > 
> 
> I will leave it the way it is. That is, I will do reliable = false on errors
> like you suggested.
> 
> > So the caller would have to know that -ENOENT really means success.
> > Which, to me, seems kind of flaky.
> > 
> 
> Actually, that is why -ENOENT was introduced - to indicate successful
> stack trace termination. A return value of 0 is for continuing with
> the stack trace. A non-zero value is for terminating the stack trace.
> 
> So, either we return a positive value (say 1) to indicate successful
> termination. Or, we return -ENOENT to say no more stack frames left.
> I guess -ENOENT was chosen.

I see.  So it's a tri-state return value, and frame->reliable is
intended to be a private interface not checked by the callers.

That makes sense, and probably fine, it's just perhaps a bit nonstandard
compared to most Linux interfaces.

-- 
Josh