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 07:40:47 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, 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 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.