Subject: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

From: Kees Cook <[email-masked]>
Date: Fri, 9 Mar 2018 12:05:36 -0800

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 <[email-masked]>
---
v3:
- drop __builtin_types_compatible_p() (Rasmus, Linus)
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 | 48 ++++++++++++++++++++++++++++++------------------
 1 file changed, 30 insertions(+), 18 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fd291503576..a0fca4deb3ab 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -787,37 +787,55 @@ 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),			\
+			      (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),			\
+			      (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 +887,7 @@ 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 +895,7 @@ 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