linux-kernel - Re: [RFC PATCH 05/10] objtool, x86: add facility for asm code to provide CFI hints

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Message-ID: <CALCETrXsLqsqYWFcjQBw-F_booEAfLKhG+EEZtBfrDcntEgjvg@mail.gmail.com>
Date:   Thu, 1 Jun 2017 06:57:24 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>, Jiri Slaby <jslaby@...e.cz>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 05/10] objtool, x86: add facility for asm code to
 provide CFI hints

On Wed, May 31, 2017 at 10:44 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> Some asm (and inline asm) code does special things to the stack which
> objtool can't understand.  (Nor can GCC or GNU assembler, for that
> matter.)  In such cases we need a facility for the code to provide
> annotations, so the unwinder can unwind through it.
>
> This provides such a facility, in the form of CFI hints.  They're
> similar to the GNU assembler .cfi* directives, but they give more
> information, and are needed in far fewer places, because objtool can
> fill in the blanks by following branches and adjusting the stack pointer
> for pushes and pops.

Two minor suggestions:

Could you prefix these with something other than "CFI_"?  For those of
use who have read the binutils manual, using "CFI_" sounds awfully
like .cfi_, and people might expect the semantics to be the same.

> +#define CFI_HINT(cfa_reg, cfa_offset, type)                    \
> +       "999: \n\t"                                             \

Have you checked if 999 is used elsewhere?  My personal preference is to use:

.Ldescriptive_text_\@:

instead of a hopefully-unique number.  I never researched the history,
but I suspect that the convention of using large numbers came from
early binutils versions that didn't have \@, but we use \@ fairly
extensively in the kernel these days, so it would seem that we no
longer support binutils versions that old.