linux-kernel - Re: Livepatch vs LTO

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Message-ID: <20190426191851.5oodjy2m2bobqoio@treble>
Date:   Fri, 26 Apr 2019 14:18:51 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     live-patching@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, mliska@...e.cz, hubicka@....cz
Subject: Re: Livepatch vs LTO

On Fri, Apr 26, 2019 at 11:00:48AM +0200, Miroslav Benes wrote:
> > - For binary-based patch generation (kpatch-build), we currently diff
> >   objects at a per-compilation-unit level.  That would have to be
> >   changed to work on vmlinux.o instead.
> > 
> > - Objtool would also have to be changed to work on vmlinux.o.  It's
> >   currently not optimized for large files, and the per-.o whitelisting
> >   would need to be fixed.  And there may be other issues lurking.
> > 
> > Also, thinking about objtool in this context has given me another idea,
> > which might allow us to get rid of the use of -flive-patching and
> > -fdump-ipa-clones altogether (which are both nasty and way too
> > compiler-dependent):
> > 
> > Since objtool is already reading every function in the kernel, it could
> > create a checksum associated with each function, based on all the
> > instructions (both within the function and any alternatives or other
> > special sections it relies on).  The function checksums could be written
> > to a file.
> > 
> > Then, when a patch file is applied and the kernel rebuilt, the checksum
> > files could be compared to determine exactly which functions have
> > changed at a binary level.
> > 
> > Thoughts?  Any reasons why that wouldn't work?
> 
> I think it could work. If nothing else, it would give us the information 
> we look for.

I'm gone next week but after that I'll start looking at implementing
this.

> > And, if we wanted to take the idea even further, objtool could have the
> > ability to write the changed functions to a new object file.  Voila, we
> > now pretty much have kpatch-build :-)  (Though whether this is better
> > than source-based patch generation is certainly an open question.)
> 
> ...worth of discussion.

When I get some free time I might prototype something to see what it
would look like.

-- 
Josh