linux-kernel - [PATCH] x86/entry: use STB_GLOBAL for register restoring thunk

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Message-Id: <20201223232126.759416-1-ndesaulniers@google.com>
Date:   Wed, 23 Dec 2020 15:21:26 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH] x86/entry: use STB_GLOBAL for register restoring thunk

Arnd found a randconfig that produces the warning:

arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at
offset 0x3e

when building with LLVM_IAS=1 (use Clang's integrated assembler). Josh
notes:

  With the LLVM assembler stripping the .text section symbol, objtool
  has no way to reference this code when it generates ORC unwinder
  entries, because this code is outside of any ELF function.

This behavior was implemented as an optimization in LLVM 5 years ago,
but it's not the first time this has caused issues for objtool.  A patch
has been authored against LLVM to revert the behavior, which may or may
not be accepted.  Until then use a global symbol for the thunk that way
objtool can generate proper unwind info here with LLVM_IAS=1.

Cc: Fangrui Song <maskray@...gle.com>
Reported-by: Arnd Bergmann <arnd@...db.de>
Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1209
Link: https://reviews.llvm.org/D93783
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 arch/x86/entry/thunk_64.S | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index ccd32877a3c4..878816034a73 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -31,7 +31,7 @@ SYM_FUNC_START_NOALIGN(\name)
 	.endif
 
 	call \func
-	jmp  .L_restore
+	jmp  __thunk_restore
 SYM_FUNC_END(\name)
 	_ASM_NOKPROBE(\name)
 	.endm
@@ -44,7 +44,7 @@ SYM_FUNC_END(\name)
 #endif
 
 #ifdef CONFIG_PREEMPTION
-SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
+SYM_CODE_START_NOALIGN(__thunk_restore)
 	popq %r11
 	popq %r10
 	popq %r9
@@ -56,6 +56,6 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
 	popq %rdi
 	popq %rbp
 	ret
-	_ASM_NOKPROBE(.L_restore)
-SYM_CODE_END(.L_restore)
+	_ASM_NOKPROBE(__thunk_restore)
+SYM_CODE_END(__thunk_restore)
 #endif
-- 
2.29.2.729.g45daf8777d-goog