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

From: Mark Brown <broonie@kernel.org>
To: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc: mark.rutland@arm.com, jpoimboe@redhat.com, ardb@kernel.org,
	jthierry@redhat.com, catalin.marinas@arm.com, will@kernel.org,
	jmorris@namei.org, pasha.tatashin@soleen.com,
	linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability checks in the unwinder
Date: Fri, 21 May 2021 18:42:42 +0100	[thread overview]
Message-ID: <20210521174242.GD5825@sirena.org.uk> (raw)
In-Reply-To: <a2a32666-c27e-3a0f-06b2-b7a2baa7e0f1@linux.microsoft.com>

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On Fri, May 21, 2021 at 12:23:52PM -0500, Madhavan T. Venkataraman wrote:
> On 5/21/21 11:11 AM, Mark Brown wrote:
> > On Sat, May 15, 2021 at 11:00:17PM -0500, madvenka@linux.microsoft.com wrote:

> >> +	frame->reliable = true;

> > All these checks are good checks but as you say there's more stuff that
> > we need to add (like your patch 2 here) so I'm slightly nervous about

> OK. So how about changing the field from a flag to an enum that says exactly
> what happened with the frame?

TBH I think the code is fine, or rather will be fine when it gets as far
as actually being used - this was more a comment about when we flip this
switch.

> Also, the caller can get an exact idea of why the stack trace failed.

I'm not sure anything other than someone debugging things will care
enough to get the code out and then decode it so it seems like it'd be
more trouble than it's worth, we're unlikely to be logging the code as
standard.

> > The other thing I guess is the question of if we want to bother flagging
> > frames as unrelaible when we return an error; I don't see an issue with
> > it and it may turn out to make it easier to do something in the future
> > so I'm fine with that

> Initially, I thought that there is no need to flag it for errors. But Josh
> had a comment that the stack trace is indeed unreliable on errors. Again, the
> word unreliable is the one causing the problem.

My understanding there is that arch_stack_walk_reliable() should be
returning an error if either the unwinder detected an error or if any
frame in the stack is flagged as unreliable so from the point of view of
users it's just looking at the error code, it's more that there's no
need for arch_stack_walk_reliable() to consider the reliability
information if an error has been detected and nothing else looks at the
reliability information.

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.

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