From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f65.google.com ([209.85.160.65]:33546 "EHLO mail-pl0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751083AbeCHVkt (ORCPT ); Thu, 8 Mar 2018 16:40:49 -0500 Received: by mail-pl0-f65.google.com with SMTP id c11-v6so4077859plo.0 for ; Thu, 08 Mar 2018 13:40:48 -0800 (PST) Date: Thu, 8 Mar 2018 13:40:45 -0800 From: Kees Cook To: Andrew Morton Cc: Josh Poimboeuf , Rasmus Villemoes , "Gustavo A. R. Silva" , "Tobin C. Harding" , rostedt@goodmis.org, corbet@lwn.net, Chris Mason , Josef Bacik , David Sterba , "David S. Miller" , Alexey Kuznetsov , Hideaki YOSHIFUJI , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Masahiro Yamada , Borislav Petkov , Randy Dunlap , Ian Abbott , Sergey Senozhatsky , Petr Mladek , Andy Shevchenko , Pantelis Antoniou , linux-btrfs@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com Subject: [PATCH] kernel.h: Skip single-eval logic on literals in min()/max() Message-ID: <20180308214045.GA6787@beast> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: When max() is used in stack array size calculations from literal values (e.g. "char foo[max(sizeof(struct1), sizeof(struct2))]", the compiler thinks this is a dynamic calculation due to the single-eval logic, which is not needed in the literal case. This change removes several accidental stack VLAs from an x86 allmodconfig build: $ diff -u before.txt after.txt | grep ^- -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array ‘ids’ [-Wvla] -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ‘namebuf’ [-Wvla] -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array ‘sym’ [-Wvla] -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla] -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla] -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array ‘buff64’ [-Wvla] Based on an earlier patch from Josh Poimboeuf. Signed-off-by: Kees Cook --- include/linux/kernel.h | 42 ++++++++++++++++++++++++++++++------------ 1 file changed, 30 insertions(+), 12 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 3fd291503576..e0b39d461582 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -787,37 +787,57 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } * strict type-checking.. See the * "unnecessary" pointer comparison. */ -#define __min(t1, t2, min1, min2, x, y) ({ \ +#define __single_eval_min(t1, t2, min1, min2, x, y) ({ \ t1 min1 = (x); \ t2 min2 = (y); \ (void) (&min1 == &min2); \ min1 < min2 ? min1 : min2; }) +/* + * In the case of builtin constant values, there is no need to do the + * double-evaluation protection, so the raw comparison can be made. + * This allows min()/max() to be used in stack array allocations and + * avoid the compiler thinking it is a dynamic value leading to an + * accidental VLA. + */ +#define __min(t1, t2, 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), \ + __single_eval_min(t1, t2, \ + __UNIQUE_ID(max1_), \ + __UNIQUE_ID(max2_), \ + x, y)) + /** * 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) ({ \ +#define __single_eval_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_constant_p(x) && \ + __builtin_constant_p(y) && \ + __builtin_types_compatible_p(t1, t2), \ + (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y), \ + __single_eval_max(t1, t2, \ + __UNIQUE_ID(max1_), \ + __UNIQUE_ID(max2_), \ + x, y)) /** * 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 @@ -871,7 +891,6 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } */ #define min_t(type, x, y) \ __min(type, type, \ - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ x, y) /** @@ -882,7 +901,6 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } */ #define max_t(type, x, y) \ __max(type, type, \ - __UNIQUE_ID(min1_), __UNIQUE_ID(min2_), \ x, y) /** -- 2.7.4 -- Kees Cook Pixel Security