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

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Date:   Mon, 2 Jul 2018 08:52:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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

On Sun, Jul 1, 2018 at 6:33 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
> 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 :)

Sure, that's fine. I'll send an updated patch.

-Kees

-- 
Kees Cook
Pixel Security