[PATCH 2/2] powerpc/module_64: Fix "expected nop" error on module re-patching

[Miroslav Benes]

On Tue, 24 Jan 2023, Josh Poimboeuf wrote:

> When a module with a livepatched function is unloaded and then reloaded,
> klp attempts to dynamically re-patch it.  On ppc64, that fails with the
> following error:
> 
>   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'
> 
> The error happens because the restore r2 instruction had already
> previously been written into the klp module's replacement function when
> the original function was patched the first time.  So the instruction
> wasn't a nop as expected.
> 
> When the restore r2 instruction has already been patched in, detect that
> and skip the warning and the instruction write.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe at kernel.org>

Reviewed-by: Miroslav Benes <mbenes at suse.cz>

M