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Message-ID: <78428dec-270e-5d74-0160-83feb5c0ff02@rasmusvillemoes.dk> Date: Thu, 8 Mar 2018 20:57:16 +0100 From: Rasmus Villemoes <linux@...musvillemoes.dk> To: Josh Poimboeuf <jpoimboe@...hat.com>, Kees Cook <keescook@...omium.org> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, corbet@....net, gustavo@...eddedor.com, 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@...r.kernel.org, netdev@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH 0/3] Remove accidental VLA usage On 2018-03-08 16:02, Josh Poimboeuf wrote: > On Wed, Mar 07, 2018 at 07:30:44PM -0800, Kees Cook wrote: >> This series adds SIMPLE_MAX() to be used in places where a stack array >> is actually fixed, but the compiler still warns about VLA usage due to >> confusion caused by the safety checks in the max() macro. >> >> I'm sending these via -mm since that's where I've introduced SIMPLE_MAX(), >> and they should all have no operational differences. > > What if we instead simplify the max() macro's type checking so that GCC > can more easily fold the array size constants? The below patch seems to > work: > > +extern long __error_incompatible_types_in_min_macro; > +extern long __error_incompatible_types_in_max_macro; > + > +#define __min(t1, t2, x, y) \ > + __builtin_choose_expr(__builtin_types_compatible_p(t1, t2), \ > + (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y), \ > + (t1)__error_incompatible_types_in_min_macro) > > /** > * min - return minimum of two values of the same or compatible types > * @x: first value > * @y: second value > */ > -#define min(x, y) \ > - __min(typeof(x), typeof(y), \ > - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ > - x, y) > +#define min(x, y) __min(typeof(x), typeof(y), x, y) \ > But this introduces the the-chosen-one-of-x-and-y-gets-evaluated-twice problem. Maybe we don't care? But until we get a __builtin_assert_this_has_no_side_effects() I think that's a little dangerous. Rasmus
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