string.h: Use ARRAY_SIZE() for memtostr*()/strtomem*()

The destination argument of memtostr*() and strtomem*() must be a
fixed-size char array at compile time, so there is no need to use
__builtin_object_size() (which is useful for when an argument is
either a pointer or unknown). Instead use ARRAY_SIZE(), which has the
benefit of working around a bug in Clang (fixed[1] in 15+) that got
__builtin_object_size() wrong sometimes.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202501310832.kiAeOt2z-lkp@intel.com/
Suggested-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://github.com/llvm/llvm-project/commit/d8e0a6d5e9dd2311641f9a8a5d2bf90829951ddc [1]
Tested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Kees Cook <kees@kernel.org>

include/linux/string.h

@@ -411,7 +411,8 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
  * must be discoverable by the compiler.
  */
 #define strtomem_pad(dest, src, pad)	do {				\
-	const size_t _dest_len = __builtin_object_size(dest, 1);	\
+	const size_t _dest_len = __must_be_byte_array(dest) +		\
+				 ARRAY_SIZE(dest);			\
 	const size_t _src_len = __builtin_object_size(src, 1);		\
 									\
 	BUILD_BUG_ON(!__builtin_constant_p(_dest_len) ||		\
@@ -434,7 +435,8 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
  * must be discoverable by the compiler.
  */
 #define strtomem(dest, src)	do {					\
-	const size_t _dest_len = __builtin_object_size(dest, 1);	\
+	const size_t _dest_len = __must_be_byte_array(dest) +		\
+				 ARRAY_SIZE(dest);			\
 	const size_t _src_len = __builtin_object_size(src, 1);		\
 									\
 	BUILD_BUG_ON(!__builtin_constant_p(_dest_len) ||		\
@@ -453,7 +455,8 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
  * Note that sizes of @dest and @src must be known at compile-time.
  */
 #define memtostr(dest, src)	do {					\
-	const size_t _dest_len = __builtin_object_size(dest, 1);	\
+	const size_t _dest_len = __must_be_byte_array(dest) +		\
+				 ARRAY_SIZE(dest);			\
 	const size_t _src_len = __builtin_object_size(src, 1);		\
 	const size_t _src_chars = strnlen(src, _src_len);		\
 	const size_t _copy_len = min(_dest_len - 1, _src_chars);	\
@@ -478,7 +481,8 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
  * Note that sizes of @dest and @src must be known at compile-time.
  */
 #define memtostr_pad(dest, src)		do {				\
-	const size_t _dest_len = __builtin_object_size(dest, 1);	\
+	const size_t _dest_len = __must_be_byte_array(dest) +		\
+				 ARRAY_SIZE(dest);			\
 	const size_t _src_len = __builtin_object_size(src, 1);		\
 	const size_t _src_chars = strnlen(src, _src_len);		\
 	const size_t _copy_len = min(_dest_len - 1, _src_chars);	\