[PATCH] cfi: rust: pass -Zpatchable-function-entry on all architectures

[Alice Ryhl <aliceryhl@google.com>]

The KCFI sanitizer stores the CFI tag of a function just before its
machine code. However, the patchable-function-entry flag can be used to
introduce additional nop instructions before the machine code, taking up
the space that normally holds the CFI tag. In this case, a backwards
offset is applied to the CFI tag to move them out of the way of the nop
instructions. To ensure that C and Rust agree on the offset used by CFI
tags, pass the -Zpatchable-function-entry to rustc whenever it is passed
to the C compiler.

The required rustc version is bumped to 1.81.0 to ensure that the
-Zpatchable-function-entry flag is available when CFI is used.

Fixes: ca627e636551 ("rust: cfi: add support for CFI_CLANG with Rust")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Note that this fix uses rustc-option which has a pending fix:
https://lore.kernel.org/all/20241008-rustc-option-bootstrap-v2-1-e6e155b8f9f3@google.com/
---
 arch/arm64/Makefile     | 2 ++
 arch/loongarch/Makefile | 1 +
 arch/riscv/Makefile     | 2 ++
 init/Kconfig            | 2 +-
 4 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 9efd3f37c2fd..d7ec0bb09fc4 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -143,9 +143,11 @@ CHECKFLAGS	+= -D__aarch64__
 ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS),y)
   KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
   CC_FLAGS_FTRACE := -fpatchable-function-entry=4,2
+  KBUILD_RUSTFLAGS += $(call rustc-option,-Zpatchable-function-entry=4$(comma)2)
 else ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_ARGS),y)
   KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
   CC_FLAGS_FTRACE := -fpatchable-function-entry=2
+  KBUILD_RUSTFLAGS += $(call rustc-option,-Zpatchable-function-entry=2)
 endif
 
 ifeq ($(CONFIG_KASAN_SW_TAGS), y)
diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
index ae3f80622f4c..f9cef31d1f0e 100644
--- a/arch/loongarch/Makefile
+++ b/arch/loongarch/Makefile
@@ -44,6 +44,7 @@ endif
 ifdef CONFIG_DYNAMIC_FTRACE
 KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
 CC_FLAGS_FTRACE := -fpatchable-function-entry=2
+KBUILD_RUSTFLAGS += $(call rustc-option,-Zpatchable-function-entry=2)
 endif
 
 ifdef CONFIG_64BIT
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index d469db9f46f4..65d4dcba309a 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -16,8 +16,10 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
 	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
 ifeq ($(CONFIG_RISCV_ISA_C),y)
 	CC_FLAGS_FTRACE := -fpatchable-function-entry=4
+	KBUILD_RUSTFLAGS += $(call rustc-option,-Zpatchable-function-entry=4)
 else
 	CC_FLAGS_FTRACE := -fpatchable-function-entry=2
+	KBUILD_RUSTFLAGS += $(call rustc-option,-Zpatchable-function-entry=2)
 endif
 endif
 
diff --git a/init/Kconfig b/init/Kconfig
index 530a382ee0fe..43434b681c3f 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1946,7 +1946,7 @@ config RUST
 	depends on !GCC_PLUGIN_RANDSTRUCT
 	depends on !RANDSTRUCT
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
-	depends on !CFI_CLANG || RUSTC_VERSION >= 107900 && HAVE_CFI_ICALL_NORMALIZE_INTEGERS
+	depends on !CFI_CLANG || RUSTC_VERSION >= 108100 && HAVE_CFI_ICALL_NORMALIZE_INTEGERS
 	select CFI_ICALL_NORMALIZE_INTEGERS if CFI_CLANG
 	depends on !CALL_PADDING || RUSTC_VERSION >= 108100
 	depends on !KASAN_SW_TAGS

---
base-commit: 4a335f920bc78e51b1d7d216d11f2ecbb6dd949f
change-id: 20241008-cfi-patchable-all-ddd6275eaf4f

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>