Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

From: Andrew Morton <[email-masked]>
Date: Tue, 13 Mar 2018 14:02:48 -0700

On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <[email-masked]> wrote:

> On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds
> <[email-masked]> wrote:
> > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
> > <[email-masked]> 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.