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:

  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@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>

1 files changed, 3 insertions, 0 deletions

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 */