x86/retpoline: Fix retpoline unwind

Currently objtool cannot understand retpolines, and thus cannot
generate ORC unwind information for them. This means that we cannot
unwind from the middle of a retpoline.

The recent ANNOTATE_INTRA_FUNCTION_CALL and UNWIND_HINT_RET_OFFSET
support in objtool enables it to understand the basic retpoline
construct. A further problem is that the ORC unwind information is
alternative invariant; IOW. every alternative should have the same
ORC, retpolines obviously violate this. This means we need to
out-of-line them.

Since all GCC generated code already uses out-of-line retpolines, this
should not affect performance much, if anything.

This will enable objtool to generate valid ORC data for the
out-of-line copies, which means we can correctly and reliably unwind
through a retpoline.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200428191700.210835357@infradead.org
This commit is contained in:
Peter Zijlstra
2020-04-16 14:34:26 +02:00
parent 34fdce6981
commit cc1ac9c792
3 changed files with 38 additions and 51 deletions

View File

@@ -7,15 +7,31 @@
#include <asm/alternative-asm.h>
#include <asm/export.h>
#include <asm/nospec-branch.h>
#include <asm/unwind_hints.h>
#include <asm/frame.h>
.macro THUNK reg
.section .text.__x86.indirect_thunk
.align 32
SYM_FUNC_START(__x86_indirect_thunk_\reg)
CFI_STARTPROC
JMP_NOSPEC %\reg
CFI_ENDPROC
JMP_NOSPEC \reg
SYM_FUNC_END(__x86_indirect_thunk_\reg)
SYM_FUNC_START_NOALIGN(__x86_retpoline_\reg)
ANNOTATE_INTRA_FUNCTION_CALL
call .Ldo_rop_\@
.Lspec_trap_\@:
UNWIND_HINT_EMPTY
pause
lfence
jmp .Lspec_trap_\@
.Ldo_rop_\@:
mov %\reg, (%_ASM_SP)
UNWIND_HINT_RET_OFFSET
ret
SYM_FUNC_END(__x86_retpoline_\reg)
.endm
/*
@@ -32,6 +48,7 @@ SYM_FUNC_END(__x86_indirect_thunk_\reg)
#define __EXPORT_THUNK(sym) _ASM_NOKPROBE(sym); EXPORT_SYMBOL(sym)
#define EXPORT_THUNK(reg) __EXPORT_THUNK(__x86_indirect_thunk_ ## reg)
#define EXPORT_RETPOLINE(reg) __EXPORT_THUNK(__x86_retpoline_ ## reg)
#undef GEN
#define GEN(reg) THUNK reg
@@ -41,3 +58,6 @@ SYM_FUNC_END(__x86_indirect_thunk_\reg)
#define GEN(reg) EXPORT_THUNK(reg)
#include <asm/GEN-for-each-reg.h>
#undef GEN
#define GEN(reg) EXPORT_RETPOLINE(reg)
#include <asm/GEN-for-each-reg.h>