Date: Tue, 13 Mar 2018 15:14:34 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, "Gustavo A. R. Silva" <gustavo@...eddedor.com>, "Tobin C. Harding" <me@...in.cc>, Steven Rostedt <rostedt@...dmis.org>, Jonathan Corbet <corbet@....net>, Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>, "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Borislav Petkov <bp@...e.de>, Randy Dunlap <rdunlap@...radead.org>, Ian Abbott <abbotti@....co.uk>, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>, Petr Mladek <pmladek@...e.com>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Pantelis Antoniou <pantelis.antoniou@...sulko.com>, Linux Btrfs <linux-btrfs@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max() On Tue, Mar 13, 2018 at 2:02 PM, Andrew Morton <akpm@...ux-foundation.org> wrote: > On Mon, 12 Mar 2018 21:28:57 -0700 Kees Cook <keescook@...omium.org> wrote: > >> On Mon, Mar 12, 2018 at 4:57 PM, Linus Torvalds >> <torvalds@...ux-foundation.org> wrote: >> > On Mon, Mar 12, 2018 at 3:55 PM, Andrew Morton >> > <akpm@...ux-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
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