[PATCH] overflow: Provide constant expression struct_size

[Kees Cook]

There have been cases where struct_size() (or flex_array_size()) needs
to be calculated for an initializer, which requires it be a constant
expression. This is possible when the "count" argument is a constant
expression, so provide this ability for the helpers.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/overflow.h | 10 +++++++---
 lib/test_overflow.c      |  7 +++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 59d7228104d0..f1221d11f8e5 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -4,6 +4,7 @@
 
 #include <linux/compiler.h>
 #include <linux/limits.h>
+#include <linux/const.h>
 
 /*
  * We need to compute the minimum and maximum values representable in a given
@@ -221,8 +222,9 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * Return: number of bytes needed or SIZE_MAX on overflow.
  */
 #define flex_array_size(p, member, count)				\
-	size_mul(count,							\
-		 sizeof(*(p)->member) + __must_be_array((p)->member))
+	__builtin_choose_expr(__is_constexpr(count),			\
+		(count) * sizeof(*(p)->member) + __must_be_array((p)->member),	\
+		size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
 
 /**
  * struct_size() - Calculate size of structure with trailing flexible array.
@@ -237,6 +239,8 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
  * Return: number of bytes needed or SIZE_MAX on overflow.
  */
 #define struct_size(p, member, count)					\
-	size_add(sizeof(*(p)), flex_array_size(p, member, count))
+	__builtin_choose_expr(__is_constexpr(count),			\
+		sizeof(*(p)) + flex_array_size(p, member, count),	\
+		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
 
 #endif /* __LINUX_OVERFLOW_H */
diff --git a/lib/test_overflow.c b/lib/test_overflow.c
index 712fb2351c27..d488ffaf56be 100644
--- a/lib/test_overflow.c
+++ b/lib/test_overflow.c
@@ -602,11 +602,18 @@ struct __test_flex_array {
 
 static int __init test_overflow_size_helpers(void)
 {
+	/* Make sure struct_size() can be used in a constant expression. */
+	u8 ce_array[struct_size((struct __test_flex_array *)0, data, 55)];
 	struct __test_flex_array *obj;
 	int count = 0;
 	int err = 0;
 	int var;
 
+	/* Verify constant expression against runtime version. */
+	var = 55;
+	OPTIMIZER_HIDE_VAR(var);
+	err |= sizeof(ce_array) != struct_size(obj, data, var);
+
 #define check_one_size_helper(expected, func, args...)	({	\
 	bool __failure = false;					\
 	size_t _r;						\
-- 
2.30.2