linux-kernel - Re: [RFC PATCH v5 1/2] arm64: Introduce stack trace reliability checks in the unwinder

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Message-ID: <2877744f-83ab-3f18-71e3-d406cfdd793d@linux.microsoft.com>
Date:   Fri, 25 Jun 2021 12:18:38 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>, jpoimboe@...hat.com,
        ardb@...nel.org, nobuta.keiya@...itsu.com, catalin.marinas@....com,
        will@...nel.org, jmorris@...ei.org, pasha.tatashin@...een.com,
        jthierry@...hat.com, linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 1/2] arm64: Introduce stack trace reliability
 checks in the unwinder



On 6/25/21 12:05 PM, Madhavan T. Venkataraman wrote:
> 
> 
> On 6/25/21 10:51 AM, Mark Brown wrote:
>> On Fri, Jun 25, 2021 at 10:39:57AM -0500, Madhavan T. Venkataraman wrote:
>>> On 6/24/21 9:40 AM, Mark Rutland wrote:
>>
>>>> At a high-level, I'm on-board with keeping track of this per unwind
>>>> step, but if we do that then I want to be abel to use this during
>>>> regular unwinds (e.g. so that we can have a backtrace idicate when a
>>>> step is not reliable, like x86 does with '?'), and to do that we need to
>>>> be a little more accurate.
>>
>>> The only consumer of frame->reliable is livepatch. So, in retrospect, my
>>> original per-frame reliability flag was an overkill. I was just trying to
>>> provide extra per-frame debug information which is not really a requirement
>>> for livepatch.
>>
>> It's not a requirement for livepatch but if it's there a per frame
>> reliability flag would have other uses - for example Mark has mentioned
>> the way x86 prints a ? next to unreliable entries in oops output for
>> example, that'd be handy for people debugging issues and would have the
>> added bonus of ensuring that there's more constant and widespread
>> exercising of the reliability stuff than if it's just used for livepatch
>> which is a bit niche.
>>
> 
> I agree. That is why I introduced the per-frame flag.
> 
> So, let us try a different approach.
> 
> First, let us get rid of the frame->reliable flag from this patch series. That flag
> can be implemented when all of the pieces are in place for per-frame debug and tracking.
> 
> For consumers such as livepatch that don't really care about per-frame stuff, let us
> solve it more cleanly via the return value of unwind_frame().
> 
> Currently, the return value from unwind_frame() is a tri-state return value which is
> somewhat confusing.
> 
> 	0	means continue unwinding
> 	-error	means stop unwinding. However,
> 			-ENOENT means successful termination
> 			Other values mean an error has happened.
> 
> Instead, let unwind_frame() return one of 3 values:
> 
> enum {
> 	UNWIND_CONTINUE,
> 	UNWIND_CONTINUE_WITH_ERRORS,
> 	UNWIND_STOP,
> };
> 

Sorry. I need to add one more value to this. So, the enum will be:

enum {
	UNWIND_CONTINUE,
	UNWIND_CONTINUE_WITH_ERRORS,
	UNWIND_STOP,
	UNWIND_STOP_WITH_ERRORS,
};

UNWIND_CONTINUE (what used to be a return value of 0)
	Continue with the unwind.

UNWIND_CONTINUE_WITH_ERRORS (new return value)
	Errors encountered. But the errors are not fatal errors like stack corruption.

UNWIND_STOP (what used to be -ENOENT)
	Successful termination of unwind.

UNWIND_STOP_WITH_ERRORS (what used to be -EINVAL, etc)
	Unsuccessful termination.

Sorry I missed this the last time.

So, to reiterate:

All consumers will stop unwinding when they see UNWIND_STOP and UNWIND_STOP_WITH_ERRORS.

Debug type consumers can choose to continue when they see UNWIND_CONTINUE_WITH_ERRORS.

Livepatch type consumers will only continue on UNWIND_CONTINUE.

This way, my patch series does not have a dependency on the per-frame enhancements.

Thanks!

Madhavan