Messages in this thread Patch in this message | | | From | Guenter Roeck <> | Subject | [PATCH v2] x86/retpoline: Add clang support | Date | Wed, 7 Feb 2018 13:52:18 -0800 |
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clang has its own set of compiler options for retpoline support.
Link: https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69 Link: https://github.com/llvm-mirror/llvm/commit/fd5a8723ce9f2a6b250e85972ef859e4253ea95d Link: https://github.com/llvm-mirror/llvm/commit/59b64490fda69d29bb42cfdf7eec37bcc31ff833 Cc: David Woodhouse <dwmw@amazon.co.uk> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: gnomes@lxorguk.ukuu.org.uk Cc: Rik van Riel <riel@redhat.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: thomas.lendacky@amd.com Cc: Peter Zijlstra <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Jiri Kosina <jikos@kernel.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Kees Cook <keescook@google.com> Cc: Tim Chen <tim.c.chen@linux.intel.com> Cc: Greg Kroah-Hartman <gregkh@linux-foundation.org> Cc: Paul Turner <pjt@google.com> Signed-off-by: Guenter Roeck <linux@roeck-us.net> --- v2: llvm has been updated to use the same thunk names as gcc. Tested with: clang version 7.0.0 (https://git.llvm.org/git/clang.git/ 848874aed95a913fb45f363120500cebfe54e2ef) (https://git.llvm.org/git/llvm.git/ 3afd566557f3616881505db0d69f5d19bf55ae14) cross-checked with gcc 7.3.0 (x86_64-linux-gcc.br_real (Buildroot 2018.02-rc1) 7.3.0).
Tested with 64-bit builds only; 32-bit images fail to build with clang with various unrelated errors and are difficult to test.
I had to change '+=' to '=' below since make otherwise sets RETPOLINE_CFLAGS to " ", and the subsequent ifneq would always match. This is also the reason for the "ifeq ($(RETPOLINE_CFLAGS),)". If there is another/different/better way to handle this, please let me know.
There are curently lots of warnings when building an image with clang.
./include/linux/init.h:134:6: warning: unknown attribute 'indirect_branch' ignored
I was inclined to add "&& !defined(__clang__)" to the condition for the __noretpoline define to fix the problem, but concluded that this should be a separate patch unless it can be addressed in clang.
arch/x86/Makefile | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/arch/x86/Makefile b/arch/x86/Makefile index fad55160dcb9..c1a35cea88e1 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -232,7 +232,12 @@ KBUILD_CFLAGS += -fno-asynchronous-unwind-tables # Avoid indirect branches in kernel to deal with Spectre ifdef CONFIG_RETPOLINE - RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register) + # gcc + RETPOLINE_CFLAGS = $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register) + ifeq ($(RETPOLINE_CFLAGS),) + # clang + RETPOLINE_CFLAGS = $(call cc-option,-mretpoline-external-thunk) + endif ifneq ($(RETPOLINE_CFLAGS),) KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE endif -- 2.7.4
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