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

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>, Mark Brown <broonie@kernel.org>
Cc: mark.rutland@arm.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 13:59:16 -0500	[thread overview]
Message-ID: <74d12457-7590-bca2-d1ce-5ff82d7ab0d8@linux.microsoft.com> (raw)
In-Reply-To: <20210521184817.envdg232b2aeyprt@treble>

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.

> 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.
> 

OK. I will wait to see if any arm64 folks have an opinion on this.
I am fine with any approach.

Madhavan