linux-kernel - Re: /proc/kallsyms shows undefined symbols for livepatch modules

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Message-ID: <20180604080528.4z6rbhapbhn7xf5e@d217.suse.de>
Date:   Mon, 4 Jun 2018 10:05:28 +0200
From:   Jessica Yu <jeyu@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: /proc/kallsyms shows undefined symbols for livepatch modules

+++ 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