From: Peter Zijlstra <peterz@infradead.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Mark Brown <broonie@kernel.org>,
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>, Jonathan Corbet <corbet@lwn.net>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Nathan Chancellor <natechancellor@gmail.com>,
Miguel Ojeda <ojeda@kernel.org>,
Jiri Slaby <jirislaby@kernel.org>, Joe Perches <joe@perches.com>,
Linux Doc Mailing List <linux-doc@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v4] x86/entry: emit a symbol for register restoring thunk
Date: Thu, 14 Jan 2021 12:36:45 +0100 [thread overview]
Message-ID: <YAAszZJ2GcIYZmB5@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210114103928.GB12284@zn.tnic>
On Thu, Jan 14, 2021 at 11:39:28AM +0100, Borislav Petkov wrote:
> From: Nick Desaulniers <ndesaulniers@google.com>
> Date: Tue, 12 Jan 2021 11:46:24 -0800
> Subject: [PATCH] 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 (Clang's integrated assembler). Josh
> notes:
>
> With the LLVM assembler not generating section symbols, objtool has no
> way to reference this code when it generates ORC unwinder entries,
> because this code is outside of any ELF function.
>
> The limitation now being imposed by objtool is that all code must be
> contained in an ELF symbol. And .L symbols don't create such symbols.
>
> So basically, you can use an .L symbol *inside* a function or a code
> segment, you just can't use the .L symbol to contain the code using a
> SYM_*_START/END annotation pair.
>
> Fangrui notes that this optimization is helpful for reducing image size
> when compiling with -ffunction-sections and -fdata-sections. I have
> observed 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
> of not generating unused section symbols ([1]), so this will
> also become a problem for users of GNU binutils once they upgrade to 2.36.
>
> Omit the .L prefix on a label so that the assembler will 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 or
> GNU binutils 2.36+.
>
> [ bp: Massage commit message. ]
>
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Suggested-by: Borislav Petkov <bp@alien8.de>
> Suggested-by: Mark Brown <broonie@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
And while looking, I suppose we can delete the put_ret_addr_in_rdi crud,
but that's another patch.
next prev parent reply other threads:[~2021-01-14 11:37 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
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 [this message]
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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