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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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

Christophe