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

[Alexander Lobakin]

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx 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@xxxxxxxxxx
> > 
> > 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@xxxxxxxxxx/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