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Message-ID: <20180308233758.GA22120@beast>
Date: Thu, 8 Mar 2018 15:37:58 -0800
From: Kees Cook <keescook@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Cc: 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>,
LKML <linux-kernel@...r.kernel.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: [PATCH v2] kernel.h: Skip single-eval logic on literals in
min()/max()
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 <keescook@...omium.org>
---
v2:
- fix copy/paste-o max1_/max2_ (ijc)
- clarify "compile-time" constant in comment (Rasmus)
- clean up formatting on min_t()/max_t()
---
include/linux/kernel.h | 50 ++++++++++++++++++++++++++++++++------------------
1 file changed, 32 insertions(+), 18 deletions(-)
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fd291503576..108cdf7bd484 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 compile-time 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(min1_), \
+ __UNIQUE_ID(min2_), \
+ 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
@@ -869,10 +889,7 @@ 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 +897,7 @@ 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
--
2.7.4
--
Kees Cook
Pixel Security
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