Commit 9df3f508 authored by Mark Rutland's avatar Mark Rutland Committed by Will Deacon
Browse files

arm64: avoid redundant PAC stripping in __builtin_return_address() 

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:

  689eae42 ("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: default avatarMark Rutland <mark.rutland@arm.com>
Cc: Amit Daniel Kachhap <amit.kachhap@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kristina Martsenko <kristina.martsenko@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20230412160134.306148-2-mark.rutland@arm.com


Signed-off-by: default avatarWill Deacon <will@kernel.org>
parent b5ecc19e

arch/arm64/Kconfig

+14 −0
Original line number Diff line number Diff line
@@ -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

arch/arm64/include/asm/compiler.h

+3 −0
Original line number Diff line number Diff line
@@ -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 */