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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Fangrui Song <maskray@google.com>, Arnd Bergmann <arnd@arndb.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH v2] x86/entry: use STB_GLOBAL for register restoring thunk
Date: Tue, 5 Jan 2021 19:58:10 -0600	[thread overview]
Message-ID: <20210106015810.5p6crnh7jqtmjtv4@treble> (raw)
In-Reply-To: <20210106004351.79130-1-ndesaulniers@google.com>

On Tue, Jan 05, 2021 at 04:43:51PM -0800, Nick Desaulniers wrote:
> 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.

On second thought, there's no need to make the symbol global.  Just
getting rid of the '.L' local label symbol prefix should be enough to
make an ELF symbol:

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


  reply	other threads:[~2021-01-06  1:59 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-23 23:21 [PATCH] x86/entry: use STB_GLOBAL for register restoring thunk Nick Desaulniers
2020-12-24  4:55 ` Josh Poimboeuf
2021-01-06  0:43   ` [PATCH v2] " Nick Desaulniers
2021-01-06  1:58     ` Josh Poimboeuf [this message]
2021-01-11 20:38       ` [PATCH v3] x86/entry: emit a symbol " Nick Desaulniers
2021-01-11 20:58         ` Fangrui Song
2021-01-11 22:09           ` Josh Poimboeuf
2021-01-11 22:16             ` Fāng-ruì Sòng
2021-01-11 22:12         ` Josh Poimboeuf
2021-01-12  0:38         ` Borislav Petkov
2021-01-12  0:41           ` Fāng-ruì Sòng
2021-01-12  1:00             ` Borislav Petkov
2021-01-12  1:13               ` Nick Desaulniers
2021-01-12  1:59                 ` Josh Poimboeuf
2021-01-12 11:54                   ` Borislav Petkov
2021-01-12 19:46                     ` [PATCH v4] " Nick Desaulniers
2021-01-12 21:01                       ` Mark Brown
2021-01-13 16:59                         ` Josh Poimboeuf
2021-01-13 17:46                           ` [PATCH] Documentation: asm-annotation: clarify .L local symbol names Nick Desaulniers
2021-01-13 19:56                             ` Mark Brown
2021-01-13 17:56                           ` [PATCH v4] x86/entry: emit a symbol for register restoring thunk Nick Desaulniers
2021-01-14 10:39                             ` Borislav Petkov
2021-01-14 11:36                               ` Peter Zijlstra
2021-01-14 13:28                                 ` [PATCH] x86/entry: Remove put_ret_addr_in_rdi THUNK macro argument Borislav Petkov
2021-01-14 15:53                                   ` Peter Zijlstra
2021-01-19 10:12                                 ` [tip: x86/entry] " tip-bot2 for Borislav Petkov
2021-01-14 15:14                               ` [PATCH v4] x86/entry: emit a symbol for register restoring thunk Josh Poimboeuf
2021-01-14 11:49                         ` [Linux-kernel-mentees] Fwd: " Lukas Bulwahn
2021-01-14 14:21                           ` Aditya
2021-01-14 14:35                             ` Lukas Bulwahn
2021-01-13 11:52                       ` [tip: x86/entry] x86/entry: Emit " tip-bot2 for Nick Desaulniers
2021-01-19 10:12                       ` tip-bot2 for Nick Desaulniers
2021-01-12 11:47                 ` [PATCH v3] x86/entry: emit " Borislav Petkov

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