Commit 67ebc3ab authored Feb 08, 2022 by Kees Cook

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

parent 92df138a

include/linux/fortify-string.h

+11 −2

Original line number	Diff line number	Diff line
		@@ -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);