Re: [RFC PATCH v8 3/4] arm64: Introduce stack trace reliability checks in the unwinder

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
To: Mark Brown <broonie@kernel.org>
Cc: mark.rutland@arm.com, jpoimboe@redhat.com, ardb@kernel.org,
	nobuta.keiya@fujitsu.com, sjitindarsingh@gmail.com,
	catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org,
	pasha.tatashin@soleen.com, jthierry@redhat.com,
	linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH v8 3/4] arm64: Introduce stack trace reliability checks in the unwinder
Date: Thu, 26 Aug 2021 18:31:26 -0500	[thread overview]
Message-ID: <77c43173-95c9-6ce5-ad11-219d38a66e34@linux.microsoft.com> (raw)
In-Reply-To: <YSe50PuWM/mjNwAj@sirena.org.uk>

On 8/26/21 10:57 AM, Mark Brown wrote:
> On Thu, Aug 12, 2021 at 02:06:02PM -0500, madvenka@linux.microsoft.com wrote:
> 
>> +	if (frame->need_reliable && !unwind_is_reliable(frame)) {
>> +		/* Cannot unwind to the next frame reliably. */
>> +		frame->failed = true;
>> +		return false;
>> +	}
> 
> This means we only collect reliability information in the case
> where we're specifically doing a reliable stacktrace.  For
> example when printing stack traces on the console it might be
> useful to print a ? or something if the frame is unreliable as a
> hint to the reader that the information might be misleading.
> Could we therefore change the flag here to a reliability one and
> our need_reliable check so that we always run
> unwind_is_reliable()?
> 
> I'm not sure if we need to abandon the trace on first error when
> doing a reliable trace but I can see it's a bit safer so perhaps
> better to do so.  If we don't abandon then we don't require the
> need_reliable check at all.
> 

I think that the caller should be able to specify that the stack trace
should be abandoned. Like Livepatch.

So, we could always do the reliability check. But keep need_reliable.

Thanks.

Madhavan