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

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Message-ID: <20170507214836.62odebycv42fzngm@treble>
Date:   Sun, 7 May 2017 16:48:36 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     hpa@...or.com
Cc:     Ingo Molnar <mingo@...nel.org>,
        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 Sun, May 07, 2017 at 11:08:19AM -0700, hpa@...or.com wrote:
> 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?

It can't verify the *unwinder*, but it can verify the data which is fed
to the unwinder (either DWARF or the structs I proposed above).  For
each function, it follows every possible code path, and it can keep
track of the stack pointer while doing so.

-- 
Josh