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

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Date:   Mon, 2 Jul 2018 12:16:31 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...db.de>,
        Paul Mackerras <paulus@...ba.org>,
        Anatolij Gustschin <agust@...x.de>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc: mpc5200: Remove VLA usage

On Mon, Jul 02, 2018 at 11:33:32AM +1000, Michael Ellerman wrote:
> What if we write it:
> 
>        char saved_0x500[0x600 - 0x500];
> 
> Hopefully the compiler is smart enough not to generate a VLA for that :)

It is a VLA if the array size is not an integer constant expression.  This
is defined by C; the compiler has nothing to do with it.  0x600-0x500 is
an integer constant expression, so this is not a VLA.

But if you meant if GCC will ever do a dynamic stack allocation for a fixed
size local variable: yes indeed, I hope not!

(Sometimes GCC can avoid this even with VLAs; but in this example we do
not even have a VLA, so it's easier than that :-) )


Segher