[PATCH 1/3] arm64: avoid redundant PAC stripping in __builtin_return_address()

[Mark Rutland]

In old versions of GCC and Clang, __builtin_return_address() did not
strip the PAC. This was not the behaviour we desired, and so we wrapped
this with code to strip the PAC in commit:

  689eae42afd7a916 ("arm64: mask PAC bits of __builtin_return_address")

Since then, both GCC and Clang decided that __builtin_return_address()
*should* strip the PAC, and the existing behaviour was a bug.

GCC was fixed in 11.1.0, with those fixes backported to 10.2.0, 9.4.0,
8.5.0, but not earlier:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94891

Clang was fixed in 12.0.0, though this was not backported:

  https://reviews.llvm.org/D75044

When using a compiler whose __builtin_return_address() strips the PAC,
our wrapper to strip the PAC is redundant. Similarly, when pointer
authentication is not in use within the kernel pointers will not have a
PAC, and so there's no point stripping those pointers.

To avoid this redundant work, this patch updates the
__builtin_return_address() wrapper to only be used when in-kernel
pointer authentication is configured and the compiler's
__builtin_return_address() does not strip the PAC.

This is a cleanup/optimization, and not a fix that requires backporting.
Stripping a PAC should be an idempotent operation, and so redundantly
stripping the PAC is not harmful.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Amit Daniel Kachhap <amit.kachhap at arm.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: James Morse <james.morse at arm.com>
Cc: Kristina Martsenko <kristina.martsenko at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/Kconfig                | 14 ++++++++++++++
 arch/arm64/include/asm/compiler.h |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 1023e896d46b..d69b624b3083 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -362,6 +362,20 @@ config ARCH_PROC_KCORE_TEXT
 config BROKEN_GAS_INST
 	def_bool !$(as-instr,1:\n.inst 0\n.rept . - 1b\n\nnop\n.endr\n)
 
+config BUILTIN_RETURN_ADDRESS_STRIPS_PAC
+	bool
+	# Clang's __builtin_return_adddress() strips the PAC since 12.0.0
+	# https://reviews.llvm.org/D75044
+	default y if CC_IS_CLANG && (CLANG_VERSION >= 120000)
+	# GCC's __builtin_return_address() strips the PAC since 11.1.0,
+	# and this was backported to 10.2.0, 9.4.0, 8.5.0, but not earlier
+	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94891
+	default y if CC_IS_GCC && (GCC_VERSION >= 110100)
+	default y if CC_IS_GCC && (GCC_VERSION >= 100200) && (GCC_VERSION < 110000)
+	default y if CC_IS_GCC && (GCC_VERSION >=  90400) && (GCC_VERSION < 100000)
+	default y if CC_IS_GCC && (GCC_VERSION >=  80500) && (GCC_VERSION <  90000)
+	default n
+
 config KASAN_SHADOW_OFFSET
 	hex
 	depends on KASAN_GENERIC || KASAN_SW_TAGS
diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
index 6fb2e6bcc392..9033a0b2f46a 100644
--- a/arch/arm64/include/asm/compiler.h
+++ b/arch/arm64/include/asm/compiler.h
@@ -20,7 +20,10 @@
 	((ptr & BIT_ULL(55)) ? (ptr | ptrauth_kernel_pac_mask()) :	\
 			       (ptr & ~ptrauth_user_pac_mask()))
 
+#if defined(CONFIG_ARM64_PTR_AUTH_KERNEL) &&				\
+    !defined(CONFIG_BUILTIN_RETURN_ADDRESS_STRIPS_PAC)
 #define __builtin_return_address(val)					\
 	(void *)(ptrauth_clear_pac((unsigned long)__builtin_return_address(val)))
+#endif
 
 #endif /* __ASM_COMPILER_H */
-- 
2.30.2