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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] x86/entry: use STB_GLOBAL for register restoring thunk
Date: Wed, 23 Dec 2020 22:55:02 -0600	[thread overview]
Message-ID: <20201224045502.zkm34cc5srdgpddb@treble> (raw)
In-Reply-To: <20201223232126.759416-1-ndesaulniers@google.com>

On Wed, Dec 23, 2020 at 03:21:26PM -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.
> 
> 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.

As Fangrui pointed out, the section symbol stripping is useful for when
there are a ton of sections like '-ffunction-sections' and
'-fdata-sections'.  Maybe add that justification to the patch
description.

We can try to support it, though I suspect other tools may also end up
getting surprised.

> Cc: Fangrui Song <maskray@google.com>
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1209
> Link: https://reviews.llvm.org/D93783
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Code looks familiar ;-)

Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>

-- 
Josh


  reply	other threads:[~2020-12-24  4:56 UTC|newest]

Thread overview: 30+ 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 [this message]
2021-01-06  0:43   ` [PATCH v2] " Nick Desaulniers
2021-01-06  1:58     ` Josh Poimboeuf
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
2021-01-13 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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