linux-kernel - [PATCH v2] cfi: Use __builtin_function

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Message-Id: <20220405221618.633743-1-samitolvanen@google.com>
Date:   Tue,  5 Apr 2022 15:16:18 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v2] cfi: Use __builtin_function_start

Clang 14 added support for the __builtin_function_start function,
which allows us to implement the function_nocfi macro without
architecture-specific inline assembly and in a way that also works
with static initializers.

Change CONFIG_CFI_CLANG to depend on Clang >= 14, define
function_nocfi using __builtin_function_start, and remove the arm64
inline assembly implementation.

Link: https://github.com/llvm/llvm-project/commit/ec2e26eaf63558934f5b73a6e530edc453cf9508
Link: https://github.com/ClangBuiltLinux/linux/issues/1353
Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Reviewed-by: Mark Rutland <mark.rutland@....com>
Tested-by: Mark Rutland <mark.rutland@....com>
Acked-by: Will Deacon <will@...nel.org> # arm64
---
v2:
- Changed CFI_CLANG to depend on Clang 14 instead of
  __builtin_function_start.
- Squashed all three patches into one.

---
 arch/Kconfig                      |  5 +----
 arch/arm64/include/asm/compiler.h | 16 ----------------
 include/linux/compiler-clang.h    | 10 ++++++++++
 3 files changed, 11 insertions(+), 20 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 29b0167c088b..c1627bce4a3a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -723,10 +723,7 @@ config ARCH_SUPPORTS_CFI_CLANG
 config CFI_CLANG
 	bool "Use Clang's Control Flow Integrity (CFI)"
 	depends on LTO_CLANG && ARCH_SUPPORTS_CFI_CLANG
-	# Clang >= 12:
-	# - https://bugs.llvm.org/show_bug.cgi?id=46258
-	# - https://bugs.llvm.org/show_bug.cgi?id=47479
-	depends on CLANG_VERSION >= 120000
+	depends on CLANG_VERSION >= 140000
 	select KALLSYMS
 	help
 	  This option enables Clang’s forward-edge Control Flow Integrity
diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
index dc3ea4080e2e..6fb2e6bcc392 100644
--- a/arch/arm64/include/asm/compiler.h
+++ b/arch/arm64/include/asm/compiler.h
@@ -23,20 +23,4 @@
 #define __builtin_return_address(val)					\
 	(void *)(ptrauth_clear_pac((unsigned long)__builtin_return_address(val)))
 
-#ifdef CONFIG_CFI_CLANG
-/*
- * With CONFIG_CFI_CLANG, the compiler replaces function address
- * references with the address of the function's CFI jump table
- * entry. The function_nocfi macro always returns the address of the
- * actual function instead.
- */
-#define function_nocfi(x) ({						\
-	void *addr;							\
-	asm("adrp %0, " __stringify(x) "\n\t"				\
-	    "add  %0, %0, :lo12:" __stringify(x)			\
-	    : "=r" (addr));						\
-	addr;								\
-})
-#endif
-
 #endif /* __ASM_COMPILER_H */
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index babb1347148c..c84fec767445 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -69,6 +69,16 @@
 #define __nocfi		__attribute__((__no_sanitize__("cfi")))
 #define __cficanonical	__attribute__((__cfi_canonical_jump_table__))
 
+#if defined(CONFIG_CFI_CLANG)
+/*
+ * With CONFIG_CFI_CLANG, the compiler replaces function address
+ * references with the address of the function's CFI jump table
+ * entry. The function_nocfi macro always returns the address of the
+ * actual function instead.
+ */
+#define function_nocfi(x)	__builtin_function_start(x)
+#endif
+
 /*
  * Turn individual warnings and errors on and off locally, depending
  * on version.
-- 
2.35.1.1094.g7c7d902a7c-goog