linux-kernel - Re: [PATCH v3] livepatch: Clear relocation targets on a module removal

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Message-ID: <CAPhsuW6M=wOk-72F20p-Vbzka_--k8P3DAziJJh8oi1spEmP-g@mail.gmail.com>
Date:   Mon, 1 Aug 2022 09:24:16 -0700
From:   Song Liu <song@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Josh Poimboeuf <jpoimboe@...nel.org>,
        live-patching@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        X86 ML <x86@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3] livepatch: Clear relocation targets on a module removal

On Mon, Aug 1, 2022 at 3:31 AM Petr Mladek <pmladek@...e.com> wrote:
>
> On Sat 2022-07-30 15:32:58, Song Liu wrote:
> > On Tue, Jul 26, 2022 at 8:54 PM Song Liu <song@...nel.org> wrote:
> > >
> > > On Tue, Jul 26, 2022 at 4:33 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> > > >
> > > > On Thu, Jul 21, 2022 at 10:51:47AM -0700, Song Liu wrote:
> > > > > From: Miroslav Benes <mbenes@...e.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'
> > > > >
> > > > 2) We can't only fix x86, powerpc also needs a fix.
> > >
> > > I have very little experience with powerpc. Would someone be willing to
> > > help with powerpc part of this?
> >
> > I guess folks are all busy. Any suggestions on how to test powerpc changes?
>
> You might also send the patch and just mention that you were not able
> to test it on powerpc. There are people with access to powerpc that
> might check it.

Let me give it a try.

>
> Another question is how to actually test it. I wonder how you do
> it for x86.

I tested it with a patch on an in-tree module. I was using kpatch-build and
testing it on 5.18 kernels. This is tricky at the moment for 5.19 because of
this issue:

https://github.com/dynup/kpatch/issues/1277

>
> Finally, also s390 supports livepatching. I guess that the problem is
> there as well. Is is straightforward to write the revert code, please?

v2 of the work doesn't have any real logic for s390. I wonder whether
it is needed.

Thanks,
Song

v2: https://lore.kernel.org/all/20190905124514.8944-1-mbenes@suse.cz/T/#u