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Date:   Mon, 14 Feb 2022 13:24:33 +0100
From:   Alexander Lobakin <alexandr.lobakin@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        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

From: Josh Poimboeuf <jpoimboe@...hat.com>
Date: Fri, 11 Feb 2022 10:35:29 -0800

> On Fri, Feb 11, 2022 at 10:05:02AM -0800, Fāng-ruì Sòng wrote:
> > On Fri, Feb 11, 2022 at 9:41 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > >
> > > On Wed, Feb 09, 2022 at 07:57:39PM +0100, Alexander Lobakin wrote:
> > > > Position-based search, which means that if there are several symbols
> > > > with the same name, the user needs to additionally provide the
> > > > "index" of a desired symbol, is fragile. For example, it breaks
> > > > when two symbols with the same name are located in different
> > > > sections.
> > > >
> > > > Since a while, LD has a flag `-z unique-symbol` which appends
> > > > numeric suffixes to the functions with the same name (in symtab
> > > > and strtab). It can be used to effectively prevent from having
> > > > any ambiguity when referring to a symbol by its name.
> > >
> > > In the patch description can you also give the version of binutils (and
> > > possibly other linkers) which have the flag?
> > 
> > GNU ld>=2.36 supports -z unique-symbol. ld.lld doesn't support -z unique-symbol.
> > 
> > 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.
> 
> Also I realized that this flag would have a negative effect on
> kpatch-build, as it currently does its analysis on .o files.  So it
> would have to figure out how to properly detect function renames, to
> avoid patching the wrong function for example.
> 
> And if LLD doesn't plan to support the flag then it will be a headache
> for livepatch (and the kernel in general) to deal with the divergent
> configs.

I'm always down with replacing any of the parts, I'm just not
familiar with any other ways of approaching this without huge diffs.
I've read Fāng-ruì's blogpost previously and there's a possible
replacement described there, but I dunno how to approach it.
And them Miroslav just told me that unique-symbol should work just
fine and I can go with it.
So I asked here prevously and ask once again for any hints regarding
some other ways :p

> 
> 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?

> 
> -- 
> Josh

Thanks,
Al

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