Re: /proc/kallsyms shows undefined symbols for livepatch modules

[Jessica Yu]

+++ 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.

I think I see how this can cause bugs, since if we're searching for
seq_printf we want the real seq_printf and not the undef symbol from
the livepatch module. But I thought we renamed these symbols with some
.klp prefix? Or is the livepatch bug you're running into something else?

Thanks,

Jessica