Re: [PATCH v10 2/2] livepatch,x86: Clear relocation targets on a module removal

[Song Liu]

On Wed, Jan 25, 2023 at 1:58 AM Christophe Leroy
<christophe.leroy@xxxxxxxxxx> wrote:
>
>
>
> Le 21/01/2023 à 01:49, Song Liu a écrit :
> > 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.
> >
> > 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.
>
> Would it be possible to not add empty clear_relocate_add() on all
> architecture while only x86 implements it ? Can we make it an empty
> static inline in generic livepatch.h for the architectures not
> implementing it, like we do for most mm functions ?
>
> Another solution would be to define an empty generic weak version of
> clear_relocate_add()

Sure, I will replace these with a week function.

Thanks,
Song