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

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Date:   Fri, 21 May 2021 12:23:52 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     mark.rutland@....com, jpoimboe@...hat.com, ardb@...nel.org,
        jthierry@...hat.com, catalin.marinas@....com, will@...nel.org,
        jmorris@...ei.org, pasha.tatashin@...een.com,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability
 checks in the unwinder



On 5/21/21 11:11 AM, Mark Brown wrote:
> On Sat, May 15, 2021 at 11:00:17PM -0500, madvenka@...ux.microsoft.com wrote:
> 
>> Other reliability checks will be added in the future.
> 
> ...
> 
>> +	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
> actually setting the reliable flag here without even a comment.  Equally
> well there's no actual use of this until arch_stack_walk_reliable() gets
> implemented so it's not like it's causing any problems and it gives us
> the structure to start building up the rest of the checks.
> 

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

enum {
	FRAME_NORMAL = 0,
	FRAME_UNALIGNED,
	FRAME_NOT_ACCESSIBLE,
	FRAME_RECURSION,
	FRAME_GRAPH_ERROR,
	FRAME_INVALID_TEXT_ADDRESS,
	FRAME_UNRELIABLE_FUNCTION,
	FRAME_NUM_STATUS,
} frame_status;

struct stackframe {
	...
	enum frame_status status;
};

unwind_frame()
{
	frame->status = FRAME_NORMAL;

	Then, for each situation, change the status appropriately.
}

Eventually, arch_stack_walk_reliable() could just declare the stack trace
as unreliable if status != FRAME_NORMAL.

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

Is that acceptable?

> 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.

The above enum-based solution addresses Josh's comment as well.

Let me know if this is good.

Thanks!

Madhavan