linux-kernel - Re: [PATCH] powerpc: mpc5200: Remove VLA usage

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Date:   Mon, 02 Jul 2018 11:33:32 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Anatolij Gustschin <agust@...x.de>,
        Paul Mackerras <paulus@...ba.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc: mpc5200: Remove VLA usage

Kees Cook <keescook@...omium.org> writes:

> On Fri, Jun 29, 2018 at 2:02 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Fri, Jun 29, 2018 at 8:53 PM, Kees Cook <keescook@...omium.org> wrote:
>>> In the quest to remove all stack VLA usage from the kernel[1], this
>>> switches to using a stack size large enough for the saved routine and
>>> adds a sanity check.
>>>
>>> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
>>>
>>> Signed-off-by: Kees Cook <keescook@...omium.org>
>>
>> This seems particularly nice, not only avoids it the dynamic stack
>> allocation, it
>> also makes sure the new 0x500 handler doesn't overflow into the 0x600
>> exception handler.
>>
>> It would help to explain how you arrived at that '256 byte' number in
>> the changelog though.
>
> Honestly, I just counted instructions, multiplied by 8 and rounded up
> to the next nearest power of 2, and the result felt right for giving
> some level of flexibility for code growth before tripping the WARN. :P
>
> I'm happy to adjust, of course. :)

What if we write it:

       char saved_0x500[0x600 - 0x500];

Hopefully the compiler is smart enough not to generate a VLA for that :)

cheers