On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <keesc...@chromium.org> wrote:
> On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds
> <torva...@linux-foundation.org> wrote:
> > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
> > <a...@linux-foundation.org> wrote:
> >> Replacing the __builtin_choose_expr() with ?: works of course.
> > Hmm. That sounds like the right thing to do. We were so myopically
> > staring at the __builtin_choose_expr() problem that we overlooked the
> > obvious solution.
> > Using __builtin_constant_p() together with a ?: is in fact our common
> > pattern, so that should be fine. The only real reason to use
> > __builtin_choose_expr() is if you want to get the *type* to vary
> > depending on which side you choose, but that's not an issue for
> > min/max.
> This doesn't solve it for -Wvla, unfortunately. That was the point of
> Josh's original suggestion of __builtin_choose_expr().
> Try building with KCFLAGS=-Wval and checking net/ipv6/proc.c:
> net/ipv6/proc.c: In function ‘snmp6_seq_show_item’:
> net/ipv6/proc.c:198:2: warning: ISO C90 forbids array ‘buff’ whose
> size can’t be evaluated [-Wvla]
> unsigned long buff[SNMP_MIB_MAX];
PITA. Didn't we once have a different way of detecting VLAs? Some
post-compilation asm parser, iirc.
I suppose the world wouldn't end if we had a gcc version ifdef in
kernel.h. We'll get to remove it in, oh, ten years.