Date: Thu, 8 Mar 2018 15:33:21 -0800 From: Kees Cook <keescook@...omium.org> To: Rasmus Villemoes <linux@...musvillemoes.dk> Cc: Josh Poimboeuf <jpoimboe@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, Jonathan Corbet <corbet@....net>, "Gustavo A. R. Silva" <gustavo@...eddedor.com>, Steven Rostedt <rostedt@...dmis.org>, 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>, "Tobin C. Harding" <me@...in.cc>, 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 0/3] Remove accidental VLA usage On Thu, Mar 8, 2018 at 2:12 PM, Rasmus Villemoes <linux@...musvillemoes.dk> wrote: > On 8 March 2018 at 21:39, Kees Cook <keescook@...omium.org> wrote: >> However, this works for me: >> >> #define __new_max(t1, t2, max1, max2, x, y) \ >> __builtin_choose_expr(__builtin_constant_p(x) && \ >> __builtin_constant_p(y) && \ >> __builtin_types_compatible_p(t1, t2), \ >> (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y), \ >> __max(t1, t2, max1, max2, x, y)) >> >> #define new_max(x, y) \ >> __new_max(typeof(x), typeof(y), \ >> __UNIQUE_ID(max1_), __UNIQUE_ID(max2_), \ >> x, y) > > Yes, that would seem to do the trick. > > Thinking out loud: do we really want or need the > __builtin_types_compatible condition when x and y are compile-time > constants? I think it would be nice to be able to use max(16, > sizeof(bla)) without having to cast either the literal or the sizeof. > Just omitting the type compatibility check might be dangerous, but > perhaps it could be relaxed to a check that both values are > representable in their common promoted type. Something like > > (type_signed(t1) == type_signed(t2)) || ((t1)x >= 0 && (t2)y >= 0) > > should be safe (if the types have same signedness, or the value of > signed type is positive), though it doesn't allow a few corner cases > (e.g. short vs. unsigned char is always ok due to promotion to int, > and also if the signed type is strictly wider than the unsigned type). I agree, it would be nice. However, I think it'd be better to continue to depend on max_t() for these kinds of cases though. For example: char foo[max_t(size_t, 6, sizeof(something))]; Works with the proposed patch. Also, I think this mismatch would already be triggering warnings, so we shouldn't have any currently. -Kees -- Kees Cook Pixel Security
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