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

[Miroslav Benes <mbenes@suse.cz>]

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On Fri, 11 Feb 2022, Josh Poimboeuf wrote:

> 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@redhat.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@lists.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.

Hm, bear with me, because I am very likely missing something which is 
clear to everyone else...

Is the stability really a problem for the live patching (and I am talking 
about the live patching only here. It may be a problem elsewhere, but I am 
just trying to understand.)? I understand that two different kernel builds 
could have a different name mapping between the original symbols and their 
unique renames. Not nice. But we can prepare two different live patches 
for these two different kernels. Something one would like to avoid if 
possible, but it is not impossible. Am I missing something?
 
> 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.

Yes, that is unfortunate. And not only for kpatch-build.

> 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.

True.

The position-based approach clearly shows its limits. I like <file+func> 
approach based on kallsyms tracking, that you proposed elsewhere in the 
thread, more.

Miroslav