linux-kernel - Re: [PATCH v3 0/9] klp-convert livepatch build tooling

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Message-ID: <20190424192334.pdtbzpuz2dclsxsg@treble>
Date:   Wed, 24 Apr 2019 14:23:34 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joao Moreira <jmoreira@...e.de>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        linux-kbuild@...r.kernel.org, Jessica Yu <jeyu@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michael Matz <matz@...e.de>, Nicolai Stange <nstange@...e.de>,
        Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH v3 0/9] klp-convert livepatch build tooling

On Wed, Apr 24, 2019 at 04:13:59PM -0300, Joao Moreira wrote:
> > > Future Work
> > > -----------
> > > 
> > > I don't see an easy way to support multiple homonym <object, name>
> > > symbols with unique <position> values in the current livepatch module
> > > Elf format.  The only solutions that come to mind right now include
> > > renaming homonym symbols somehow to retain the relocation->symbol
> > > relationship when separate object files are combined.  Perhaps an
> > > intermediate linker step could make annotated symbols unique in some way
> > > to achieve this.  /thinking out loud
> > 
> > I'd set this aside for now and we can return to it later. I think it could
> > be quite rare in practice.
> > 
> > I was thinking about renaming the symbol too. We can extend the symbol
> > naming convention we have now and deal with it in klp_resolve_symbols(),
> > but maybe Josh will come up with something clever and cleaner.
> 
> I think this could work well, but (sorry if I understood Joe's idea wrongly)
> not as a linker step. Instead of modifying the linker, I think we could
> create another tool and plug it into the kbuild pipeline prior to the
> livepatch module linking. This way, we would parse the .o elf files, check
> for homonyms and rename them based on a convention that is later understood
> by klp-convert, as suggested.
> 
> If I am not missing something, this would fix the case where we have
> homonyms pointing to the same or different positions, without additional
> user intervention other then adding the SYMPOS annotations.
> 
> If you consider this to be useful I can start experiencing.

I tend to agree with Miroslav that it's probably ok to ignore this issue
for now, as long as klp-convert can detect it and spit out an error.  I
assume the patch author could work around it with a kallsyms hack.  If
we encounter it much in the real world then we could figure out a
solution at that point.

-- 
Josh