linux-kernel - Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api

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Message-ID: <20180604230226.5saxrc75vreg4n2y@treble>
Date:   Mon, 4 Jun 2018 18:02:26 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api

On Mon, Jun 04, 2018 at 05:56:05PM -0500, Josh Poimboeuf wrote:
> On Mon, Jun 04, 2018 at 09:54:12AM -0500, Josh Poimboeuf wrote:
> > On Mon, Jun 04, 2018 at 04:05:11PM +0200, Jessica Yu wrote:
> > > +++ Josh Poimboeuf [04/06/18 08:16 -0500]:
> > > > On Mon, Jun 04, 2018 at 03:01:31PM +0200, Jessica Yu wrote:
> > > > > +++ 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().
> > > > 
> > > > That seems like it would work.  But wouldn't it be more robust if we
> > > > don't store the SHN_UNDEF symbols to start with?  Really it's only the
> > > > SHN_LIVEPATCH symbols that we need to keep, right?
> > > 
> > > Hm, didn't we have a reason for preserving the original symbol tables?
> > > It's not that we're storing the undef symbols on purpose, it's just
> > > that in the case of a livepatch module we keep *all* the symbols.
> > > IIRC, the original reasoning was that we needed to keep the original
> > > symtab indices in place, otherwise apply_relocate() (which uses the
> > > original symbol index encoded in the relocation) won't work.
> > > Selectively copying only SHN_LIVEPATCH symbols when building the
> > > module's symbol table will unfortunately mess the symbol indices up :/
> > > Under these restrictions, it just seemed easier to make kallsyms
> > > ignore undef symbols.
> > 
> > Ah, you're right.  I'll try out your patch.
> 
> I can confirm that your patch fixes the issue.  Thanks!

BTW you can add my

Reported-by: Josh Poimboeuf <jpoimboe@...hat.com>
Tested-by: Josh Poimboeuf <jpoimboe@...hat.com>

-- 
Josh