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arm64: avoid redundant PAC stripping in __builtin_return_address()

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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:

  689eae42af ("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@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: Will Deacon <will@kernel.org>
This commit is contained in:
Mark Rutland 2023-04-12 17:01:32 +01:00 committed by Will Deacon
parent b5ecc19e68
commit 9df3f5082f
2 changed files with 17 additions and 0 deletions
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14
arch/arm64/Kconfig
View File

@ -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

3
arch/arm64/include/asm/compiler.h
View File

@ -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 */