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

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Message-ID: <863bff71-0574-5093-ff45-2d68aedba906@zytor.com>
Date:   Mon, 22 May 2017 14:37:50 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "H.J. Lu" <hjl.tools@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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 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.

	-hpa