On Wed, 2 Oct 2019, Josh Poimboeuf wrote:

On Thu, Sep 05, 2019 at 02:45:12PM +0200, Miroslav Benes wrote:

quoted

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, or return back nops on powerpc). 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 <redacted>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>

Since we decided to fix late module patching at LPC, the commit message
and clear_relocate_add() should both probably clarify that these
functions are hacks which are relatively temporary, until we fix the
root cause.

It was the plan, but thanks for pointing it out explicitly. I could 
forget.
 

But this patch gives me a bad feeling :-/  Not that I have a better
idea.

I know what you are talking about.

Has anybody seen this problem in the real world?  If not, maybe we'd be
better off just pretending the problem doesn't exist for now.

I don't think so. You reported the issue originally and I guess it 
happened during the testing. Then there is a report from Huawei, but it 
suggests testing environment too. Reloading modules seems artificial to 
me.

So I agree, we can pretend the issue does not exist and wait for the real 
solution.

Miroslav