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Message-ID: <CAGXu5jL=93keuGEjNaLYHM-hc1EeaaW2SmnnW5ja=SH1jPJwGw@mail.gmail.com> Date: Thu, 8 Mar 2018 10:02:01 -0800 From: Kees Cook <keescook@...omium.org> To: Josh Poimboeuf <jpoimboe@...hat.com> Cc: 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 7:02 AM, Josh Poimboeuf <jpoimboe@...hat.com> 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: Oooooh magic! Very nice. I couldn't figure out how to do this when I stared at it. Yes, let me respin. (I assume I can add your S-o-b?) -Kees > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > index 3fd291503576..ec863726da29 100644 > --- a/include/linux/kernel.h > +++ b/include/linux/kernel.h > @@ -782,42 +782,32 @@ ftrace_vprintk(const char *fmt, va_list ap) > static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } > #endif /* CONFIG_TRACING */ > > -/* > - * min()/max()/clamp() macros that also do > - * strict type-checking.. See the > - * "unnecessary" pointer comparison. > - */ > -#define __min(t1, t2, min1, min2, x, y) ({ \ > - t1 min1 = (x); \ > - t2 min2 = (y); \ > - (void) (&min1 == &min2); \ > - min1 < min2 ? min1 : min2; }) > +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) \ > > -#define __max(t1, t2, max1, max2, x, y) ({ \ > - t1 max1 = (x); \ > - t2 max2 = (y); \ > - (void) (&max1 == &max2); \ > - max1 > max2 ? max1 : max2; }) > +#define __max(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_max_macro) > > /** > * max - return maximum of two values of the same or compatible types > * @x: first value > * @y: second value > */ > -#define max(x, y) \ > - __max(typeof(x), typeof(y), \ > - __UNIQUE_ID(max1_), __UNIQUE_ID(max2_), \ > - x, y) > +#define max(x, y) __max(typeof(x), typeof(y), x, y) > > /** > * min3 - return minimum of three values > @@ -869,10 +859,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } > * @x: first value > * @y: second value > */ > -#define min_t(type, x, y) \ > - __min(type, type, \ > - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ > - x, y) > +#define min_t(type, x, y) __min(type, type, x, y) > > /** > * max_t - return maximum of two values, using the specified type > @@ -880,10 +867,7 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } > * @x: first value > * @y: second value > */ > -#define max_t(type, x, y) \ > - __max(type, type, \ > - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ > - x, y) > +#define max_t(type, x, y) __max(type, type, x, y) \ > > /** > * clamp_t - return a value clamped to a given range using a given type -- Kees Cook Pixel Security
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