From 4b053019792d5c71f8b2e988c34c6352816c5aac Mon Sep 17 00:00:00 2001 From: Rasmus Villemoes Date: Mon, 8 Aug 2022 16:12:30 +0200 Subject: arm64: explicitly disable pointer authentication instructions The Yocto project builds their aarch64 cross-compiler with the configure knob --enable-standard-branch-protection, which means that their gcc behaves as if -mbranch-protection=standard is passed; the default (lacking that configure knob) is -mbranch-protection=none. This means that when building U-Boot using the Yocto toolchain, most functions end up containing paciasp/autiasp/bti instructions. However, since U-Boot is not an ordinary userspace application, there's no OS kernel which has set up the required authentication keys, so these instructions do nothing at all (even on arm64 hardware that does have the pointer authentication capability). They do however make the image larger. It is theoretically possible for U-Boot to make use of the pointer authentication protection - cf. the linux kernel's CONFIG_ARM64_PTR_AUTH_KERNEL - but it is far from trivial, and it's hard to see just what threat model it would protect against in a bootloader context. Regardless, we certainly have none of the required infrastructure now, so explicitly pass -mbranch-protection=none to ensure those useless instructions do not get emitted. For a toolchain not configured with --enable-standard-branch-protection, this changes nothing. For the Yocto toolchain, this reduces the size of both SPL and U-Boot proper by about 3% for my imx8mp target. If you don't have a Yocto toolchain, the effect can easily be reproduced by applying this patch and changing =none to =standard. Signed-off-by: Rasmus Villemoes Reviewed-by: Simon Glass --- arch/arm/cpu/armv8/config.mk | 1 + 1 file changed, 1 insertion(+) diff --git a/arch/arm/cpu/armv8/config.mk b/arch/arm/cpu/armv8/config.mk index 6f90931..ca06ed3 100644 --- a/arch/arm/cpu/armv8/config.mk +++ b/arch/arm/cpu/armv8/config.mk @@ -3,6 +3,7 @@ # (C) Copyright 2002 # Gary Jennejohn, DENX Software Engineering, PLATFORM_RELFLAGS += -fno-common -ffixed-x18 +PLATFORM_RELFLAGS += $(call cc-option,-mbranch-protection=none) PF_NO_UNALIGNED := $(call cc-option, -mstrict-align) PLATFORM_CPPFLAGS += $(PF_NO_UNALIGNED) -- cgit v1.1