linux-kernel - Re: [RFC PATCH v2 1/8] arm64: Implement stack trace termination record

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Message-ID: <8591e34a-c181-f3ff-e691-a6350225e5b4@linux.microsoft.com>
Date:   Thu, 18 Mar 2021 15:26:13 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     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 1/8] arm64: Implement stack trace termination
 record

On 3/18/21 10:09 AM, Mark Brown wrote:
> On Mon, Mar 15, 2021 at 11:57:53AM -0500, madvenka@...ux.microsoft.com wrote:
> 
>> In summary, task pt_regs->stackframe is where a successful stack trace ends.
> 
>>         .if \el == 0
>> -       mov     x29, xzr
>> +       stp     xzr, xzr, [sp, #S_STACKFRAME]
>>         .else
>>         stp     x29, x22, [sp, #S_STACKFRAME]
>> -       add     x29, sp, #S_STACKFRAME
>>         .endif
>> +       add     x29, sp, #S_STACKFRAME
> 
> For both user and kernel threads this patch (at least by itself) results
> in an additional record being reported in stack traces with a NULL
> function pointer since it keeps the existing record where it is and adds
> this new fixed record below it.  This is addressed for the kernel later
> in the series, by "arm64: Terminate the stack trace at TASK_FRAME and
> EL0_FRAME", but will still be visible to other unwinders such as
> debuggers.  I'm not sure that this *matters* but it might and should at
> least be called out more explicitly.
> 
> If we are going to add the extra record there would probably be less
> potential for confusion if we pointed it at some sensibly named dummy
> function so anything or anyone that does see it on the stack doesn't get
> confused by a NULL.
> 

I agree. I will think about this some more. If no other solution presents
itself, I will add the dummy function.

Madhavan

> 
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