Re: [PATCH] vsprintf: Remove accidental VLA usage

[Tycho Andersen <tycho@tycho.ws>]

Hi Kees,

On Wed, Mar 07, 2018 at 03:07:14PM -0800, Kees Cook wrote:
> The "sym" calculation is actually a fixed size, but since the max()
> macro uses some extensive tricks for safety, it ends up looking like a
> variable size. This replaces max() with a simple max macro which is
> sufficient for the calculation of the array size.
> 
> Seen with -Wvla. Fixed as part of the directive to remove all VLAs from
> the kernel: https://lkml.org/lkml/2018/3/7/621
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  lib/vsprintf.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index d7a708f82559..f420ab1477cb 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -744,8 +744,9 @@ char *resource_string(char *buf, char *end, struct resource *res,
>  #define FLAG_BUF_SIZE		(2 * sizeof(res->flags))
>  #define DECODED_BUF_SIZE	sizeof("[mem - 64bit pref window disabled]")
>  #define RAW_BUF_SIZE		sizeof("[mem - flags 0x]")
> -	char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE,
> -		     2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)];
> +#define SIMPLE_MAX(x, y)	((x) > (y) ? (x) : (y))

It's probably worth hoisting this out into some other header. When I
was looking at this a while ago, this problem happens in a few places,
see e.g. net/ipv4/proc.c:TCPUDP_MIB_MAX.

Cheers,

Tycho