Re: [PATCH v9 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search

From: Alexander Lobakin <alexandr.lobakin@intel.com>
To: Miroslav Benes <mbenes@suse.cz>
Cc: "Alexander Lobakin" <alexandr.lobakin@intel.com>,
	"Fāng-ruì Sòng" <maskray@google.com>,
	"Borislav Petkov" <bp@alien8.de>,
	linux-hardening@vger.kernel.org, x86@kernel.org,
	"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>,
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	"Herbert Xu" <herbert@gondor.apana.org.au>,
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	"Thomas Gleixner" <tglx@linutronix.de>,
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	"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 v9 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search
Date: Mon,  3 Jan 2022 17:06:15 +0100	[thread overview]
Message-ID: <20220103160615.7904-1-alexandr.lobakin@intel.com> (raw)
In-Reply-To: <alpine.LSU.2.21.2201031447140.15051@pobox.suse.cz>

From: Miroslav Benes <mbenes@suse.cz>
Date: Mon, 3 Jan 2022 14:55:42 +0100 (CET)

> On Thu, 30 Dec 2021, Fāng-ruì Sòng wrote:
> 
> > On Thu, Dec 30, 2021 at 3:11 AM Borislav Petkov <bp@alien8.de> wrote:
> > >
> > > On Thu, Dec 23, 2021 at 01:21:56AM +0100, Alexander Lobakin wrote:
> > > > [PATCH v9 02/15] livepatch: use `-z unique-symbol` if available to nuke pos-based search
> 
> ...
> 
> > Apologies since I haven't read the patch series.
> > 
> > The option does not exist in ld.lld and I am a bit concerning about
> > its semantics: https://maskray.me/blog/2020-11-15-explain-gnu-linker-options#z-unique-symbol
> > 
> > I thought that someone forwarded my comments (originally posted months
> > on a feature request ago) here but seems not.
> > (I am a ld.lld maintainer.)
> 
> Do you mean 
> https://lore.kernel.org/all/20210123225928.z5hkmaw6qjs2gu5g@google.com/T/#u 
> ?
> 
> Unfortunately, it did not lead anywhere. I think that '-z unique-symbol' 
> option should work fine as long as the live patching is concerned. Maybe I 
> misunderstood but your concerns mentioned at the blog do not apply. The 
> stability is not an issue for us since we (KLP) always work with already 
> built and fixed kernel. And(at least) GCC already uses number suffices for 
> IPA clones and it has not been a problem anywhere.

LLD doesn't have such an option, so FG-KASLR + livepatching builds
wouldn't be available for LLVM with the current approach (or we'd
still need a stub that prints "FG-KASLR is not compatible with
sympos != 0").
Unfortunately, I discovered this a bit late, just after sending this
revision.

OTOH, there's no easy alternative. <file + function> pair looks
appealing, but is it even possible for now to implement in the
kernel without much refactoring?

>
> Am I wrong?
> 
> Thanks
> 
> Miroslav 

Thanks,
Al