Messages in this thread | | | Date | Tue, 5 Jun 2018 09:39:27 +0200 | From | Jessica Yu <> | Subject | Re: [PATCH] module: exclude SHN_UNDEF symbols from kallsyms api |
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+++ Josh Poimboeuf [04/06/18 18:02 -0500]: >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@redhat.com> >Tested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Thanks for testing! Will clean up the patch and formally repost it. It'll be queued up in modules-next after the merge window.
Thanks,
Jessica
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