linux-kernel - Re: [PATCH v10 02/15] livepatch: avoid position-based search if `-z unique-symbol` is available

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Message-ID: <20220216200120.djt6ijjenmcxmko6@treble>
Date:   Wed, 16 Feb 2022 12:01:20 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Fāng-ruì Sòng <maskray@...gle.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        linux-hardening@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Bruce Schlobohm <bruce.schlobohm@...el.com>,
        Jessica Yu <jeyu@...nel.org>,
        kernel test robot <lkp@...el.com>,
        Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        Jonathan Corbet <corbet@....net>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Marios Pomonis <pomonis@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Nicolas Pitre <nico@...xnic.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, live-patching@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v10 02/15] livepatch: avoid position-based search if `-z
 unique-symbol` is available

On Wed, Feb 16, 2022 at 04:15:20PM +0100, Miroslav Benes wrote:
> > > I subscribe to llvm@...ts.linux.dev and happen to notice this message
> > > (can't keep up with the changes...)
> > > I am a bit concerned with this option and replied last time on
> > > https://lore.kernel.org/r/20220105032456.hs3od326sdl4zjv4@google.com
> > > 
> > > My full reasoning is on
> > > https://maskray.me/blog/2020-11-15-explain-gnu-linker-options#z-unique-symbol
> > 
> > Ah, right.  Also discussed here:
> > 
> >   https://lore.kernel.org/all/20210123225928.z5hkmaw6qjs2gu5g@google.com/T/#u
> >   https://lore.kernel.org/all/20210125172124.awabevkpvq4poqxf@treble/
> > 
> > I'm not qualified to comment on LTO/PGO stability issues, but it doesn't
> > sound good.  And we want to support livepatch for LTO kernels.
> 
> Hm, bear with me, because I am very likely missing something which is 
> clear to everyone else...
> 
> Is the stability really a problem for the live patching (and I am talking 
> about the live patching only here. It may be a problem elsewhere, but I am 
> just trying to understand.)? I understand that two different kernel builds 
> could have a different name mapping between the original symbols and their 
> unique renames. Not nice. But we can prepare two different live patches 
> for these two different kernels. Something one would like to avoid if 
> possible, but it is not impossible. Am I missing something?

Maybe Fāng-ruì can clarify, but my understanding was that the stability
issue affects the kernel in general (particularly if LTO or PGO is
enabled) and isn't necessarily specific to livepatch itself.

-- 
Josh