[PATCH v2] x86/entry: use STB_GLOBAL for register restoring thunk

[Nick Desaulniers]

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.

Fangrui notes that this is helpful for reducing images size when
compiling with -ffunction-sections and -fdata-sections. I have observerd
on the order of tens of thousands of symbols for the kernel images built
with those flags. A patch has been authored against GNU binutils to
match this behavior, with a new flag
--generate-unused-section-symbols=[yes|no].

Use a global symbol for the thunk that way
objtool can generate proper unwind info here with LLVM_IAS=1.

Cc: Fangrui Song <maskray@xxxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/1209
Link: https://reviews.llvm.org/D93783
Link: https://sourceware.org/pipermail/binutils/2020-December/114671.html
Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Acked-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Changes v1 -> v2:
* Pick up Josh's Ack.
* Add commit message info about -ffunction-sections/-fdata-sections, and
  link to binutils patch.

 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