fortify: Make sure strlen() may still be used as a constant expression

In preparation for enabling Clang FORTIFY_SOURCE support, redefine
strlen() as a macro that tests for being a constant expression
so that strlen() can still be used in static initializers, which is
lost when adding __pass_object_size and __overloadable.

An example of this usage can be seen here:
	https://lore.kernel.org/all/202201252321.dRmWZ8wW-lkp@intel.com/

Notably, this constant expression feature of strlen() is not available
for architectures that build with -ffreestanding. This means the kernel
currently does not universally expect strlen() to be used this way, but
since there _are_ some build configurations that depend on it, retain
the characteristic for Clang FORTIFY_SOURCE builds too.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220208225350.1331628-8-keescook@chromium.org

include/linux/fortify-string.h

@@ -2,6 +2,8 @@
 #ifndef _LINUX_FORTIFY_STRING_H_
 #define _LINUX_FORTIFY_STRING_H_
 
+#include <linux/const.h>
+
 #define __FORTIFY_INLINE extern __always_inline __gnu_inline
 #define __RENAME(x) __asm__(#x)
 
@@ -95,9 +97,16 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const p, __kernel_size_t m
 	return ret;
 }
 
-/* defined after fortified strnlen to reuse it. */
+/*
+ * Defined after fortified strnlen to reuse it. However, it must still be
+ * possible for strlen() to be used on compile-time strings for use in
+ * static initializers (i.e. as a constant expression).
+ */
+#define strlen(p)							\
+	__builtin_choose_expr(__is_constexpr(__builtin_strlen(p)),	\
+		__builtin_strlen(p), __fortify_strlen(p))
 __FORTIFY_INLINE __diagnose_as(__builtin_strlen, 1)
-__kernel_size_t strlen(const char * const p)
+__kernel_size_t __fortify_strlen(const char * const p)
 {
 	__kernel_size_t ret;
 	size_t p_size = __builtin_object_size(p, 1);