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    SubjectRe: [PATCH v3] livepatch: Clear relocation targets on a module removal
    On Sat, Jul 30, 2022 at 3:32 PM Song Liu <song@kernel.org> wrote:
    >
    > On Tue, Jul 26, 2022 at 8:54 PM Song Liu <song@kernel.org> wrote:
    > >
    > > On Tue, Jul 26, 2022 at 4:33 PM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
    > > >
    > > > On Thu, Jul 21, 2022 at 10:51:47AM -0700, 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>
    > > > >
    > > > > ---
    > > > >
    > > > > Changes from v2:
    > > > > 1. Rewrite x86 changes to match current code style.
    > > > > 2. Remove powerpc changes as there is no test coverage in v3.
    > > > > 3. Only keep 1/3 of v2.
    > > >
    > > > 1) All the copy/paste is ugly and IMO guaranteed to eventually introduce
    > > > bugs when somebody forgets to update the copy. Wouldn't it be more
    > > > robust to reuse the existing apply_relocate_add() code by making it
    > > > more generic somehow, like with a new 'clear' bool arg which sets
    > > > 'val' to zero?
    > >
    > > Agreed. I can give it a try.
    >
    > I finished this part, though it is not really clean (added if else for
    > each "case:").
    >
    > >
    > > >
    > > > 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?
    >
    > >
    > > > 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?

    Thanks,
    Song

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