linux-kernel - Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack trace unreliable

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Date:   Tue, 23 Mar 2021 15:23:33 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>, jpoimboe@...hat.com,
        jthierry@...hat.com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 5/8] arm64: Detect an FTRACE frame and mark a stack
 trace unreliable



On 3/23/21 1:27 PM, Mark Brown wrote:
> On Tue, Mar 23, 2021 at 12:23:34PM -0500, Madhavan T. Venkataraman wrote:
>> On 3/23/21 12:02 PM, Mark Rutland wrote:
> 
>>> 3. Figure out exception boundary handling. I'm currently working to
>>>    simplify the entry assembly down to a uniform set of stubs, and I'd
>>>    prefer to get that sorted before we teach the unwinder about
>>>    exception boundaries, as it'll be significantly simpler to reason
>>>    about and won't end up clashing with the rework.
> 
>> So, here is where I still have a question. Is it necessary for the unwinder
>> to know the exception boundaries? Is it not enough if it knows if there are
>> exceptions present? For instance, using something like num_special_frames
>> I suggested above?
> 
> For reliable stack trace we can live with just flagging things as
> unreliable when we know there's an exception boundary somewhere but (as
> Mark mentioned elsewhere) being able to actually go through a subset of
> exception boundaries safely is likely to help usefully improve the
> performance of live patching, and for defensiveness we want to try to
> detect during an actual unwind anyway so it ends up being a performance
> improvment and double check rather than saving us code.  Better
> understanding of what's going on in the presence of exceptions may also
> help other users of the unwinder which can use stacks which aren't
> reliable get better results.
> 

Actually, I was not suggesting that the counter replace the unwinder
intelligence to recognize exception boundaries. I was only suggesting
the use of the counter for arch_stack_walk_reliable().

But I am fine with not implementing the counter for now.

Madhavan