linux-hardening - [PATCH v7 7/8] fortify: Make sure strlen() may still be used as a constant expression

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Date:   Tue,  8 Feb 2022 14:53:49 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        George Burgess IV <gbiv@...gle.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: [PATCH v7 7/8] 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@...omium.org>
---
 include/linux/fortify-string.h | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index db1ad1c1c79a..f77cf22e2d60 100644
--- a/include/linux/fortify-string.h
+++ b/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);
-- 
2.30.2