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

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Date:   Mon, 14 Feb 2022 10:10:00 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Fāng-ruì Sòng <maskray@...gle.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>,
        Miroslav Benes <mbenes@...e.cz>,
        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 Mon, Feb 14, 2022 at 01:24:33PM +0100, Alexander Lobakin wrote:
> > One idea I mentioned before, it may be worth exploring changing the "F"
> > in FGKASLR to "File" instead of "Function".  In other words, only
> > shuffle at an object-file granularity.  Then, even with duplicates, the
> > <file+function> symbol pair doesn't change in the symbol table.  And as
> > a bonus, it should help FGKASLR i-cache performance, significantly.
> 
> Yeah, I keep that in mind. However, this wouldn't solve the
> duplicate static function names problem, right?
> Let's say you have a static function f() in file1 and f() in file2,
> then the layout each boot can be
> 
> .text.file1  or  .text.file2
> f()              f()
> .text.file2      .text.file1
> f()              f()
> 
> and position-based search won't work anyway, right?

Right, so we'd have to abandon position-based search in favor of
file+func based search.

It's not perfect because there are still a few file+func duplicates.
But it might be good enough.  We would presumably just refuse to patch a
duplicate.  Or we could remove them (and enforce their continued removal
with tooling-based warnings).

Another variant of this which I described here

  https://lore.kernel.org/all/20210125172124.awabevkpvq4poqxf@treble/

would be to keep it function-granular, but have kallsyms keep track of
what file each func belongs to.  Then livepatch could still do the
file+func based search.

-- 
Josh