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

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Message-ID: <3980F538-919F-4A51-B7FE-4EC6E86AA259@zytor.com>
Date:   Sun, 07 May 2017 11:08:19 -0700
From:   hpa@...or.com
To:     Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        live-patching@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jiri Kosina <jikos@...nel.org>
Subject: Re: [PATCH 7/7] DWARF: add the config option

On May 7, 2017 10:59:16 AM PDT, Ingo Molnar <mingo@...nel.org> wrote:
>
>* Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
>> One instance of the structure would exist for each time the stack
>> pointer changes, e.g. for every function entry, push/pop, and rsp
>> add/subtract.  The data could be assembled and sorted offline,
>possibly
>> derived from DWARF, or more likely, generated by objtool.  After
>doing
>> some rough calculations, I think the section size would be comparable
>to
>> the sizes of the DWARF .eh_frame sections it would replace.
>
>That's something I've been thinking about as well: if objtool generates
>the 
>unwinder data structures then the kernel is not directly exposed to
>tooling bugs 
>anymore.
>
>A fair chunk of the fragility of DWARF comes from the fact that it's
>generated by 
>a tool chain that we cannot fix as part of the kernel project. If GCC
>generates 
>crap debuginfo, and GDB happens to work with it but the kernel not,
>we'll have to 
>work it around in the kernel. If GCC starts bloating debuginfo in the
>future we 
>are screwed as well, etc.
>
>If objtool generates debuginfo then it's _our_ responsibility to have
>sane 
>unwinder info and we obviously manage its structure and size as well.
>Win-win.
>
>The unwinder itself should still do sanity checks, etc. (like all good
>debugging 
>infrastructure code) - but the nature of the kernel's exposure to tool
>chain 
>details changes in a very fundamental way.
>
>So yes, I think this is a very good idea, assuming it works in
>practice! ;-)
>
>Thanks,
>
>	Ingo

Can objtool verify the unwinder at each address in the kernel, or is that an AI-complete problem?
-- 
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