On Fri, Feb 02, 2018 at 03:30:40PM -0800, Guenter Roeck wrote: > 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. > Feedback, anyone ?
I understand that there is no love for the ABI differences between clang and gcc, but that doesn't help me. Even if the patch is unacceptable as-is, I would like to have some feedback to get an idea if I am on the right track, and what I would have to change to at least move into the right direction. Thanks, Guenter > Link: > https://github.com/llvm-mirror/clang/commit/0d816739a82da29748caf88570affb9715e18b69 > Cc: David Woodhouse <d...@amazon.co.uk> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Ingo Molnar <mi...@kernel.org> > Cc: gno...@lxorguk.ukuu.org.uk > Cc: Rik van Riel <r...@redhat.com> > Cc: Andi Kleen <a...@linux.intel.com> > Cc: Josh Poimboeuf <jpoim...@redhat.com> > Cc: thomas.lenda...@amd.com > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Linus Torvalds <torva...@linux-foundation.org> > Cc: Jiri Kosina <ji...@kernel.org> > Cc: Andy Lutomirski <l...@amacapital.net> > Cc: Dave Hansen <dave.han...@intel.com> > Cc: Kees Cook <keesc...@google.com> > Cc: Tim Chen <tim.c.c...@linux.intel.com> > Cc: Greg Kroah-Hartman <gre...@linux-foundation.org> > Cc: Paul Turner <p...@google.com> > Signed-off-by: Guenter Roeck <li...@roeck-us.net> > --- > 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 >
