clang has its own set of compiler options for retpoline support. Also, the thunks required by C code have their own function names.
For 64-bit builds, there is only a single thunk, which is easy to support. Support for 32-bit builds is more complicated - in addition to various register thunks, there is also a thunk named __llvm_external_retpoline_push which is more challenging. Play it safe and only support 64-bit clang builds for now. Link: https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69 Cc: David Woodhouse <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: [email protected] Cc: Rik van Riel <[email protected]> Cc: Andi Kleen <[email protected]> Cc: Josh Poimboeuf <[email protected]> Cc: [email protected] Cc: Peter Zijlstra <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Jiri Kosina <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Kees Cook <[email protected]> Cc: Tim Chen <[email protected]> Cc: Greg Kroah-Hartman <[email protected]> Cc: Paul Turner <[email protected]> Signed-off-by: Guenter Roeck <[email protected]> --- Sent as RFC because I am not sure if the 64-bit only solution is acceptable. arch/x86/Makefile | 5 ++++- arch/x86/lib/retpoline.S | 24 ++++++++++++++++++++---- 2 files changed, 24 insertions(+), 5 deletions(-) diff --git a/arch/x86/Makefile b/arch/x86/Makefile index fad55160dcb9..536dd6775988 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -232,7 +232,10 @@ 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) + RETPOLINE_CFLAGS = $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register) + ifeq ($(RETPOLINE_CFLAGS)$(CONFIG_X86_32),) + RETPOLINE_CFLAGS = $(call cc-option,-mretpoline -mretpoline-external-thunk) + endif ifneq ($(RETPOLINE_CFLAGS),) KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE endif diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index 480edc3a5e03..f77738b13481 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -9,14 +9,22 @@ #include <asm/nospec-branch.h> #include <asm/bitsperlong.h> -.macro THUNK reg +.macro _THUNK prefix, reg .section .text.__x86.indirect_thunk -ENTRY(__x86_indirect_thunk_\reg) +ENTRY(\prefix\reg) CFI_STARTPROC JMP_NOSPEC %\reg CFI_ENDPROC -ENDPROC(__x86_indirect_thunk_\reg) +ENDPROC(\prefix\reg) +.endm + +.macro THUNK reg +_THUNK __x86_indirect_thunk_ \reg +.endm + +.macro CLANG_THUNK reg +_THUNK __llvm_external_retpoline_ \reg .endm /* @@ -27,8 +35,11 @@ ENDPROC(__x86_indirect_thunk_\reg) * the simple and nasty way... */ #define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym) -#define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg) +#define _EXPORT_THUNK(thunk, reg) __EXPORT_THUNK(thunk ## reg) +#define EXPORT_THUNK(reg) _EXPORT_THUNK(__x86_indirect_thunk_, reg) #define GENERATE_THUNK(reg) THUNK reg ; EXPORT_THUNK(reg) +#define EXPORT_CLANG_THUNK(reg) _EXPORT_THUNK(__llvm_external_retpoline_, reg) +#define GENERATE_CLANG_THUNK(reg) CLANG_THUNK reg ; EXPORT_CLANG_THUNK(reg) GENERATE_THUNK(_ASM_AX) GENERATE_THUNK(_ASM_BX) @@ -46,6 +57,11 @@ GENERATE_THUNK(r12) GENERATE_THUNK(r13) GENERATE_THUNK(r14) GENERATE_THUNK(r15) + +#ifdef __clang__ +GENERATE_CLANG_THUNK(r11) +#endif + #endif /* -- 2.7.4
