linux-kernel - Re: [PREVIEW 10/10] linkage: add .cfi

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Message-ID: <20170217140717.sxvt7tkvsez2pl7e@treble>
Date:   Fri, 17 Feb 2017 08:07:17 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PREVIEW 10/10] linkage: add .cfi_{start/end}proc to
 ENTRY/ENDPROC

On Fri, Feb 17, 2017 at 02:36:15PM +0100, Jiri Slaby wrote:
> On 02/17/2017, 02:16 PM, Josh Poimboeuf wrote:
> > On Fri, Feb 17, 2017 at 11:47:57AM +0100, Jiri Slaby wrote:
> >> This is just a preview, not to merged now, only later with DWARF
> >> unwinder series. This is what the series will serve for (aside from
> >> cleanup and unification).
> >>
> >> I am aware of CFI_STARTPROC and CFI_ENDPROC left defined in other archs
> >> in spite of cfi annotations removal ages ago. For simplicity. I am using
> >> DW_ prefix here.
> > 
> > If objtool is going to be generating CFI instructions, why not have it
> > generate .cfi_startproc and .cfi_endproc too?
> 
> I tried that, but in many places it is very hard to recognize start
> and/or end of a function.

How so?  objtool already knows where the start and end of every function
is due to ENDPROC's use of the .type and .size macros.

> Having .cfi_startproc and .cfi_endproc in place makes it rather easy,
> actually reduced to "emit dwarf instructions for this code between
> here and there". Plus pre-prepared .eh_frame only to be extended.
> (.eh_frame_hdr has to be rehashed of course.)

Hm, but now objtool has to read *and* write CFI instead of just writing.
It would help to see the generation code.  Have you written it?

-- 
Josh