@@ -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
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@chromium.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(-)