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

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Date:   Thu, 17 Feb 2022 08:45:44 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     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, 16 Feb 2022, Josh Poimboeuf wrote:

> On Wed, Feb 16, 2022 at 04:06:24PM +0100, Miroslav Benes wrote:
> > > > +	/*
> > > > +	 * If the LD's `-z unique-symbol` flag is available and enabled,
> > > > +	 * sympos checks are not relevant.
> > > > +	 */
> > > > +	if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL))
> > > > +		sympos = 0;
> > > > +
> > > 
> > > Similarly, I don't see a need for this.  If the patch is legit then
> > > sympos should already be zero.  If not, an error gets reported and the
> > > patch fails to load.
> > 
> > My concern was that if the patch is not legit (that is, sympos is > 0 for 
> > some reason), the error would be really cryptic and would not help the 
> > user at all. So zeroing sympos seems to be a good idea to me. There is no 
> > harm and the change is very small and compact.
> 
> But wouldn't a cryptic error be better than no error at all?  A bad
> sympos might be indicative of some larger issue, like the wrong symbol
> getting patched.

Maybe you are right. I do not feel confident enough to decide it. So 
either way would be fine, I guess.

Miroslav