linux-kernel - Re: [PATCH 7/7] DWARF: add the config option

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Date:   Mon, 22 May 2017 17:11:16 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     "H.J. Lu" <hjl.tools@...il.com>, Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        live-patching@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/7] DWARF: add the config option

On Mon, May 22, 2017 at 02:37:50PM -0700, H. Peter Anvin wrote:
> On 05/22/17 14:07, H. Peter Anvin wrote:
> > On 05/20/17 13:01, H.J. Lu wrote:
> >> On Sat, May 20, 2017 at 9:20 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >>
> >>>>
> >>>> (H.J., could we get a binutils feature that allows is to do:
> >>>>
> >>>> pushq %whatever
> >>>> .cfi_adjust_sp -8
> >>>> ...
> >>>> popq %whatever
> >>>> .cfi_adjust_sp 8
> >>>>
> >>
> >> Np.  Compiler needs to generate this.
> >>
> > 
> > For actual assembly we have such a feature, it is called macros.
> > 
> > push/pop is the easy stuff; macros take care of that, but the real pain
> > is dealing with the flow of control.
> > 
> 
> My biggest beef with the CFI directives that gas uses is that there is
> that .cfi_remember_state/.cfi_restore_state doesn't have a way to
> specify more than one state.  That makes it really hard to get sanity
> around control flow changes, especially with code that is intentionally
> out of line.
> 
> That, and some of the CFI directives seem to be a bit ill-defined in
> their definition (are they even applicable to anything other than
> DWARF?)  They almost seem to be referencing some external specification,
> but the only thing I'm finding is the DWARF documentation which is
> written in very different terms.
> 
> The best description of what a personality routine is I found in an
> article by Ian Lance Taylor.  It doesn't seem to be applicable to C as
> far as I can tell.

So my understanding is that there's stock DWARF (.debug_frame) and then
there's souped-up DWARF (.eh_frame), which is basically DWARF with a few
extensions.

The remember/restore state thing is stock DWARF (DW_CFA_remember_state
and DW_CFA_restore_state).  The personality routine thing is in the
.eh_frame extension which is documented here:

  http://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html

-- 
Josh