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

["Fāng-ruì Sòng" <maskray@google.com>]

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

> > Check for its availability and always prefer when the livepatching
> > is on. It can be used unconditionally later on after broader testing
> > on a wide variety of machines, but for now let's stick to the actual
> > CONFIG_LIVEPATCH=y case, which is true for most of distro configs
> > anyways.
>
> Has anybody objected to just enabling it for *all* configs, not just for
> livepatch?
>
> I'd much prefer that: the less "special" livepatch is (and the distros
> which enable it), the better.  And I think having unique symbols would
> benefit some other components.
>
> > +++ b/kernel/livepatch/core.c
> > @@ -143,11 +143,13 @@ static int klp_find_callback(void *data, const char *name,
> >       args->count++;
> >
> >       /*
> > -      * Finish the search when the symbol is found for the desired position
> > -      * or the position is not defined for a non-unique symbol.
> > +      * Finish the search when unique symbol names are enabled
> > +      * or the symbol is found for the desired position or the
> > +      * position is not defined for a non-unique symbol.
> >        */
> > -     if ((args->pos && (args->count == args->pos)) ||
> > -         (!args->pos && (args->count > 1)))
> > +     if (IS_ENABLED(CONFIG_LD_HAS_Z_UNIQUE_SYMBOL) ||
> > +         (args->pos && args->count == args->pos) ||
> > +         (!args->pos && args->count > 1))
> >               return 1;
>
> There's no real need to do this.  The code already works as-is, even if
> there are no unique symbols.
>
> Even if there are no duplicates, there's little harm in going through
> all the symbols anyway, to check for errors just in case something
> unexpected happened with the linking (unexpected duplicate) or the patch
> creation (unexpected sympos).  It's not a hot path, so performance isn't
> really a concern.
>
> When the old linker versions eventually age out, we can then go strip
> out all the sympos stuff.
>
> > @@ -169,6 +171,13 @@ static int klp_find_object_symbol(const char *objname, const char *name,
> >       else
> >               kallsyms_on_each_symbol(klp_find_callback, &args);
> >
> > +     /*
> > +      * 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.
>
> --
> Josh
>
>


-- 
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