[PATCH] module: exclude SHN_UNDEF symbols from kallsyms api

[Jessica Yu]

+++ Jessica Yu [04/06/18 11:54 +0200]:
> +++ Jessica Yu [04/06/18 10:05 +0200]:
> > +++ Josh Poimboeuf [02/06/18 12:32 -0500]:
> > > Hi Jessica,
> > >
> > > I found a bug:
> > >
> > > [root@f25 ~]# modprobe livepatch-sample
> > > [root@f25 ~]# grep ' u ' /proc/kallsyms
> > > ffffffff81161080 u klp_enable_patch	[livepatch_sample]
> > > ffffffff81a01800 u __fentry__	[livepatch_sample]
> > > ffffffff81161250 u klp_unregister_patch	[livepatch_sample]
> > > ffffffff81161870 u klp_register_patch	[livepatch_sample]
> > > ffffffff8131f0b0 u seq_printf	[livepatch_sample]
> > >
> > > Notice that livepatch modules' undefined symbols are showing up in
> > > /proc/kallsyms.  This can confuse klp_find_object_symbol() which can
> > > cause subtle bugs in livepatch.
> > >
> > > I stared at the module kallsyms code for a bit, but I don't see the bug.
> > > Maybe it has something to do with how we save the symbol table in
> > > copy_module_elf().  Any ideas?
> >
> > Hi Josh!
> >
> > This is because we preserve the entire symbol table for livepatch
> > modules, including the SHN_UNDEF symbols. IIRC, this is so that we can
> > still apply relocations properly with apply_relocate_add() after a
> > to-be-patched object is loaded. Normally we don't save these SHN_UNDEF
> > symbols for modules so they do not appear in /proc/kallsyms.
>
> Hm, if having the full symtab in kallsyms is causing trouble, one
> possibility would be to just have the module kallsyms code simply
> skip/ignore undef symbols. That's what we technically do for normal
> modules anyway (we normally cut undef syms out of the symtab). Haven't
> tested this idea but does that sound like it'd help?

See if the following patch (untested) helps. It does not fix the
/proc/kallsyms lookup, that requires a separate patch. But it should
exclude the undef symbols from module_kallsyms_on_each_symbol() and
thus also from klp_find_object_symbol().