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

[Kees Cook]

On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> 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 âsnmp6_seq_show_itemâ:
net/ipv6/proc.c:198:2: warning: ISO C90 forbids array âbuffâ whose
size canât be evaluated [-Wvla]
  unsigned long buff[SNMP_MIB_MAX];
  ^~~~~~~~


-Kees

-- 
Kees Cook
Pixel Security