linux-kernel - [PATCH v3] x86/entry: emit a symbol for register restoring thunk

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Message-Id: <20210111203807.3547278-1-ndesaulniers@google.com>
Date:   Mon, 11 Jan 2021 12:38:06 -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 v3] x86/entry: emit a symbol 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.

Fangrui notes that this optimization 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].

We can omit the .L prefix on a label to emit an entry into the symbol
table for the label, with STB_LOCAL binding.  This enables objtool to
generate proper unwind info here with LLVM_IAS=1.

Cc: Fangrui Song <maskray@...gle.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1209
Link: https://reviews.llvm.org/D93783
Link: https://sourceware.org/binutils/docs/as/Symbol-Names.html
Link: https://sourceware.org/pipermail/binutils/2020-December/114671.html
Reported-by: Arnd Bergmann <arnd@...db.de>
Suggested-by: Josh Poimboeuf <jpoimboe@...hat.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Changes v2 -> v3:
* rework to use STB_LOCAL rather than STB_GLOBAL by dropping .L prefix,
  as per Josh.
* rename oneline to drop STB_GLOBAL in commit message.
* add link to GAS docs on .L prefix.
* drop Josh's ack since patch changed.

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..c9a9fbf1655f 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_LOCAL_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
-- 
2.30.0.284.gd98b1dd5eaa7-goog