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

[Kees Cook]

On Tue, Mar 13, 2018 at 2:02 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
>> 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];
>>   ^~~~~~~~
>
> 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.

For fixing only 6 VLAs, we don't need all this effort. When it looked
like we could get away with just a "better" max(), sure. ;)

I'll send a "const_max()" which will refuse to work on
non-constant-values (so it doesn't get accidentally used on variables
that could be exposed to double-evaluation), and will work for stack
array declarations (to avoid the overly-sensitive -Wvla checks).

-Kees

-- 
Kees Cook
Pixel Security