linux-kernel - Re: v4.10: kernel stack frame pointer .. has bad value (null)

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Message-ID: <a38ce0ab-ec12-2871-e005-08170fcd5c9b@zytor.com>
Date:   Wed, 22 Feb 2017 12:51:11 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Pavel Machek <pavel@....cz>,
        kernel list <linux-kernel@...r.kernel.org>, mingo@...nel.org,
        luto@...nel.org, bp@...en8.de, brgerst@...il.com,
        dvlasenk@...hat.com, torvalds@...ux-foundation.org,
        peterz@...radead.org, tglx@...utronix.de
Subject: Re: v4.10: kernel stack frame pointer .. has bad value (null)

On 02/22/17 08:45, Josh Poimboeuf wrote:
>>
>> FWIW, it would be really darned nice to not have all those zeroes in a
>> 32-bit stack frame dump.
> 
> Yeah, I'll fix that.
> 
>> Is not a zero stack frame pointer value an end of stack token?
> 
> There's no end of stack "token" per se, though any frame pointer value
> outside the bounds of the stack will terminate the stack trace (and that
> still happened here).
> 

Well, my understanding is that at least gdb and perhaps other unwinders
consider a zero stack frame pointer to be an indicator that the stack
has reached its end.  That's why I'm wondering if this is possible in
this case or if it is unlikely because of the value.

> The warning is because the stack trace didn't make it all the way to the
> "end" location of the stack (right before the syscall pt_regs location).
> The warning is part of the effort to ensure reliable stacks.

It would be useful to get an understanding why...

	-hpa