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Subject[PATCH] 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@chromium.org>
---
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

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