Re: [PATCH v6] livepatch: Clear relocation targets on a module removal

Song Liu
2022-12-09T19:24:53Z

On Fri, Dec 9, 2022 at 10:52 AM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 09/12/2022 à 19:30, Song Liu a écrit :
> > On Fri, Dec 9, 2022 at 4:55 AM Miroslav Benes <mbenes@suse.cz> wrote:
> >>
> >> Hi,
> >>
> >> first thank you for taking over and I also appologize for not replying
> >> much sooner.
> >>
> >> On Thu, 1 Sep 2022, Song Liu wrote:
> >>
> >>> From: Miroslav Benes <mbenes@suse.cz>
> >>>
> >>> Josh reported a bug:
> >>>
> >>> When the object to be patched is a module, and that module is
> >>> rmmod'ed and reloaded, it fails to load with:
> >>>
> >>> module: x86/modules: Skipping invalid relocation target, existing value is nonzero for type 2, loc 00000000ba0302e9, val ffffffffa03e293c
> >>> livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
> >>> livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'
> >>>
> >>> The livepatch module has a relocation which references a symbol
> >>> in the _previous_ loading of nfsd. When apply_relocate_add()
> >>> tries to replace the old relocation with a new one, it sees that
> >>> the previous one is nonzero and it errors out.
> >>>
> >>> On ppc64le, we have a similar issue:
> >>>
> >>> module_64: livepatch_nfsd: Expected nop after call, got e8410018 at e_show+0x60/0x548 [livepatch_nfsd]
> >>> livepatch: failed to initialize patch 'livepatch_nfsd' for module 'nfsd' (-8)
> >>> livepatch: patch 'livepatch_nfsd' failed for module 'nfsd', refusing to load module 'nfsd'
> >>>
> >>> He also proposed three different solutions. We could remove the error
> >>> check in apply_relocate_add() introduced by commit eda9cec4c9a1
> >>> ("x86/module: Detect and skip invalid relocations"). However the check
> >>> is useful for detecting corrupted modules.
> >>>
> >>> We could also deny the patched modules to be removed. If it proved to be
> >>> a major drawback for users, we could still implement a different
> >>> approach. The solution would also complicate the existing code a lot.
> >>>
> >>> We thus decided to reverse the relocation patching (clear all relocation
> >>> targets on x86_64). The solution is not
> >>> universal and is too much arch-specific, but it may prove to be simpler
> >>> in the end.
> >>>
> >>> Reported-by: Josh Poimboeuf <jpoimboe@redhat.com>
> >>> Signed-off-by: Miroslav Benes <mbenes@suse.cz>
> >>> Signed-off-by: Song Liu <song@kernel.org>
> >>
> >> Petr has commented on the code aspects. I will just add that s390x was not
> >> dealt with at the time because there was no live patching support for
> >> s390x back then if I remember correctly and my notes do not lie. The same
> >> applies to powerpc32. I think that both should be fixed as well with this
> >> patch. It might also help to clean up the ifdeffery in the patch a bit.
> >
> > I don't have test environments for s390 and powerpc, so I really don't know
> > whether I am doing something sane for them.
> >
> > Would you have time to finish these parts? (Or maybe the whole patch..)
>
> Setting up a powerpc test environment is fairly easy with QEMU.
>
> Some information below:
> - https://github.com/linuxppc/wiki/wiki
> - https://wiki.qemu.org/Documentation/Platforms/PowerPC

Thanks for these pointers! I will give it a try.

Song

PS: Sometimes I am just lazy, you know..