From: Borislav Petkov <bp@alien8.de>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Fangrui Song <maskray@google.com>,
Arnd Bergmann <arnd@arndb.de>,
Josh Poimboeuf <jpoimboe@redhat.com>,
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 v3] x86/entry: emit a symbol for register restoring thunk
Date: Tue, 12 Jan 2021 01:38:39 +0100 [thread overview]
Message-ID: <20210112003839.GL25645@zn.tnic> (raw)
In-Reply-To: <20210111203807.3547278-1-ndesaulniers@google.com>
On Mon, Jan 11, 2021 at 12:38:06PM -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 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@google.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@arndb.de>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
> Changes v2 -> v3:
> * rework to use STB_LOCAL rather than STB_GLOBAL by dropping .L prefix,
> as per Josh.
Ok so I read a bit around those links above...
Are you trying to tell me here that we can't use .L-prefixed local
labels anymore because, well, clang's assembler is way too overzealous
when stripping symbols to save whopping KiBs of memory?!
Btw Josh made sense to me when asking for a flag or so to keep .text.
And I see --generate-unused-section-symbols=[yes|no] for binutils.
So why isn't there a patch using that switch on clang too instead of the
kernel having to dance yet again for some tool?
:-\
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2021-01-12 0:39 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
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 [this message]
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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