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

From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: "Fāng-ruì Sòng" <maskray@google.com>,
	"Alexander Lobakin" <alexandr.lobakin@intel.com>,
	linux-hardening@vger.kernel.org, x86@kernel.org,
	"Borislav Petkov" <bp@alien8.de>,
	"Jesse Brandeburg" <jesse.brandeburg@intel.com>,
	"Kristen Carlson Accardi" <kristen@linux.intel.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Miklos Szeredi" <miklos@szeredi.hu>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Tony Luck" <tony.luck@intel.com>,
	"Bruce Schlobohm" <bruce.schlobohm@intel.com>,
	"Jessica Yu" <jeyu@kernel.org>,
	"kernel test robot" <lkp@intel.com>,
	"Evgenii Shatokhin" <eshatokhin@virtuozzo.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Michal Marek" <michal.lkml@markovi.net>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Herbert Xu" <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Will Deacon" <will@kernel.org>, "Ingo Molnar" <mingo@redhat.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Nathan Chancellor" <nathan@kernel.org>,
	"Masami Hiramatsu" <mhiramat@kernel.org>,
	"Marios Pomonis" <pomonis@google.com>,
	"Sami Tolvanen" <samitolvanen@google.com>,
	"H.J. Lu" <hjl.tools@gmail.com>,
	"Nicolas Pitre" <nico@fluxnic.net>,
	linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org,
	linux-arch@vger.kernel.org, live-patching@vger.kernel.org,
	llvm@lists.linux.dev
Subject: Re: [PATCH v10 02/15] livepatch: avoid position-based search if `-z unique-symbol` is available
Date: Wed, 16 Feb 2022 12:01:20 -0800	[thread overview]
Message-ID: <20220216200120.djt6ijjenmcxmko6@treble> (raw)
In-Reply-To: <alpine.LSU.2.21.2202161606430.1475@pobox.suse.cz>

On Wed, Feb 16, 2022 at 04:15:20PM +0100, Miroslav Benes wrote:
> > > I subscribe to llvm@lists.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