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From: Kees Cook <keescook@chromium.org>
To: Kees Cook <keescook@chromium.org>
Cc: Miguel Ojeda <ojeda@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Nathan Chancellor <nathan@kernel.org>,
	George Burgess IV <gbiv@google.com>,
	linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org,
	llvm@lists.linux.dev
Subject: [PATCH v7 7/8] fortify: Make sure strlen() may still be used as a constant expression
Date: Tue,  8 Feb 2022 14:53:49 -0800	[thread overview]
Message-ID: <20220208225350.1331628-8-keescook@chromium.org> (raw)
In-Reply-To: <20220208225350.1331628-1-keescook@chromium.org>

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>
---
 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


  parent reply	other threads:[~2022-02-08 22:54 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-02-08 22:53 [PATCH v7 0/8] fortify: Add Clang support Kees Cook
2022-02-08 22:53 ` [PATCH v7 1/8] fortify: Replace open-coded __gnu_inline attribute Kees Cook
2022-02-08 22:59   ` Nick Desaulniers
2022-02-08 22:53 ` [PATCH v7 2/8] Compiler Attributes: Add __pass_object_size for Clang Kees Cook
2022-02-08 23:00   ` Nick Desaulniers
2022-02-08 22:53 ` [PATCH v7 3/8] Compiler Attributes: Add __overloadable " Kees Cook
2022-02-08 22:53 ` [PATCH v7 4/8] Compiler Attributes: Add __diagnose_as " Kees Cook
2022-02-08 22:53 ` [PATCH v7 5/8] fortify: Make pointer arguments const Kees Cook
2022-02-08 23:02   ` Nick Desaulniers
2022-02-08 22:53 ` [PATCH v7 6/8] fortify: Use __diagnose_as() for better diagnostic coverage Kees Cook
2022-02-08 23:04   ` Nick Desaulniers
2022-02-08 22:53 ` Kees Cook [this message]
2022-02-08 23:17   ` [PATCH v7 7/8] fortify: Make sure strlen() may still be used as a constant expression Nick Desaulniers
2022-02-08 23:58     ` Kees Cook
2022-02-08 22:53 ` [PATCH v7 8/8] fortify: Add Clang support Kees Cook
2022-02-08 23:25   ` Nick Desaulniers

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