On Tue, Mar 13, 2018 at 2:02 PM, Andrew Morton <a...@linux-foundation.org> wrote: > 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.
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
