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

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Message-ID: <CAPhsuW7FLVQ4CeBp0crTh5TQk30E113=ZMz_iHh5xK-QGNcT+g@mail.gmail.com>
Date:   Mon, 1 Aug 2022 14:19:32 -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 3:25 AM Petr Mladek <pmladek@...e.com> wrote:
>
> On Sat 2022-07-30 20:20:22, Song Liu wrote:
> > On Sat, Jul 30, 2022 at 3:32 PM Song Liu <song@...nel.org> 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'
> > > > > >
> > > > > 3) A selftest would be a good idea.
> > > >
> > >
> > > I found it is pretty tricky to run the selftests inside a qemu VM. How about
> > > we test it with modules in samples/livepatch? Specifically, we can add a
> > > script try to reload livepatch-shadow-mod.ko.
> >
> > Actually, livepatch-shadow-mod.ko doesn't have the reload problem before
> > the fix. Is this expected?
>
> Good question. I am afraid that there is no easy way to prepare
> the selftest at the moment.
>
> There are two situations when a symbol from the livepatched module is
> relocated:
>
>
> 1. The livepatch might access a symbol exported by the module via
>    EXPORT_SYMBOL(). In this case, it is "normal" external symbol
>    and it gets relocated by the module loader.
>
>    But EXPORT_SYMBOL() will create an explicit dependency between the
>    livepatch and livepatched module. As a result, the livepatch
>    module could be loaded only when the livepatched module is loaded.
>    And the livepatched module could not be removed when the livepatch
>    module is loaded.
>
>    In this case, the problem will not exist. Well, the developers
>    of the livepatch module will probably want to avoid this
>    dependency.
>
>
> 2. The livepatch module might access a non-exported symbol from another
>    module using the special elf section for klp relocation, see
>    section, see Documentation/livepatch/module-elf-format.rst
>
>    These symbols are relocated in klp_apply_section_relocs().
>
>    The problem is that upstream does not have a support to
>    create this elf section. There is a patchset for this, see
>    https://lore.kernel.org/all/20220216163940.228309-1-joe.lawrence@redhat.com/
>    It requires some more review.
>
>
> Resume: I think that we could not prepare the selftest without
>         upstreaming klp-convert tool.

Thanks for the explanation! I suspected the same issue, but couldn't
connect all the logic.

I guess the selftests can wait until the klp-convert tool.

Thanks,
Song