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

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Message-ID: <20210112115421.GB13086@zn.tnic>
Date:   Tue, 12 Jan 2021 12:54:21 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Fāng-ruì Sòng <maskray@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v3] x86/entry: emit a symbol for register restoring thunk

On Mon, Jan 11, 2021 at 07:59:52PM -0600, Josh Poimboeuf wrote:
> Right.  In the vast majority of cases, .L symbols are totally fine.
> 
> The limitation now being imposed by objtool (due to these assembler
> changes) 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.
> 
> It only affects a tiny fraction of all .L usage.  Just a handful of code
> sites I think.

@Nick, this belongs into the commit message too pls.

Also,

Documentation/asm-annotations.rst
include/linux/linkage.h

would need some of that blurb added explaining to users *why* they
should not use .L local symbols as SYM_* macro arguments.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette