From: Ard Biesheuvel <ardb+git@google.com>
To: linux-efi@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org, Ard Biesheuvel <ardb@kernel.org>
Subject: [PATCH] efi/libstub: arm64: Don't use the platform register x18
Date: Thu, 15 Oct 2026 13:54:06 +0200 [thread overview]
Message-ID: <20261015115405.212211-2-ardb+git@google.com> (raw)
From: Ard Biesheuvel <ardb@kernel.org>
The UEFI specification designates x18, the platform register, as "do not
use" on AArch64, so that UEFI code is compatible both with an OS that
assigns a fixed role to x18, and with one that uses it as a temporary
register.
The arm64 EFI stub and the EFI zboot decompressor only avoid x18 if the
kernel is built with shadow call stacks, as that is the only case where
-ffixed-x18 is added to KBUILD_CFLAGS, which the stub inherits.
Otherwise, the compiler is free to use x18 as a temporary register, and
it does so.
So pass -ffixed-x18 unconditionally when building the stub for arm64,
so that the image can be run by loaders that rely on x18 being left
alone, such as a kernel that uses it for its shadow call stack.
Cc: <stable@vger.kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
drivers/firmware/efi/libstub/Makefile | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 564773c89d14..e13d4dfe27c5 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -21,7 +21,7 @@ cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ $(CC_FLAGS_DIALECT) \
# arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
# disable the stackleak plugin
-cflags-$(CONFIG_ARM64) += -fpie $(DISABLE_KSTACK_ERASE) \
+cflags-$(CONFIG_ARM64) += -fpie $(DISABLE_KSTACK_ERASE) -ffixed-x18 \
-fno-unwind-tables -fno-asynchronous-unwind-tables
cflags-$(CONFIG_ARM) += -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \
-DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
--
2.56.0.385.gd3acb90ef8-goog
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