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

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Date:   Thu, 1 Jun 2017 10:02:14 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
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>,
        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 Thu, Jun 01, 2017 at 07:40:47AM -0700, Andy Lutomirski wrote:
> On Thu, Jun 1, 2017 at 7:16 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Thu, Jun 01, 2017 at 06:57:24AM -0700, Andy Lutomirski wrote:
> >> 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.
> >
> > The intention was that even if this undwarf thing doesn't work out, the
> > CFI_ macros could still be used by objtool to generate proper DWARF.
> > Would prefixing them with CFI_HINT_ be better?  Or UNWIND_HINT_?
> 
> This has nothing to do with the data format or implementation.   I
> just think that "CFI_" suggests that they're semantically equivalent
> to binutils' .cfi directives.  If they're not, then maybe UNWIND_HINT
> is better.

Ok, I'll go with the UNWIND_HINT_ prefix.

-- 
Josh