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

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Date:   Sat, 20 May 2017 13:16:33 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. J. Lu" <hjl.tools@...il.com>, "H. Peter Anvin" <hpa@...or.com>,
        "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>
Subject: Re: [PATCH 7/7] DWARF: add the config option

On Fri, May 19, 2017 at 10:23 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> I personally like the idea of using real DWARF annotations in the
> entry code because it makes gdb work better (not kgdb -- real gdb
> attached to KVM).  I bet that we could get entry asm annotations into
> good shape if we really wanted to.  OTOH, getting DWARF to work well
> for inline asm is really nasty IIRC.

No. I will NAK *any* attempt to make our asm contain the crazy
shit-for-brains annotations.

Been there, done that, got the T-shirt, and then doused the T-shirt in
gasoline and put it on fire.

The amount of unreadable crap and bugs it requires is not worth the
pain. Not for *any* amount of gain, and the gain here is basically
zero.

> (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
>
> that will emit the right DWARF instructions regardless of whether
> there's a frame pointer or not?  .cfi_adjust_cfa_offset is not
> particularly helpful here because it's totally wrong if the CFA is
> currently being computed based on BP.)

Yeah, that's just a small example of the kind of crap people have to deal with.

Not going to happen. Assembler files are hard enough to write (and
read) as-is, anything that expects us to go back to the bad old days
with crazy shit cfi annotations is going to get violently NAK'ed and
vetoed forever.

The *only* acceptable model is automated tools (ie objtool). Don't
even bother to try to go any other way. Because I will not accept that
shit.

                 Linus