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

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Message-ID: <alpine.LSU.2.21.2301271349490.7389@pobox.suse.cz>
Date:   Fri, 27 Jan 2023 13:50:04 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
cc:     Michael Ellerman <mpe@...erman.id.au>,
        live-patching@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Song Liu <song@...nel.org>
Subject: Re: [PATCH 2/2] powerpc/module_64: Fix "expected nop" error on module
 re-patching

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@...nel.org>

Reviewed-by: Miroslav Benes <mbenes@...e.cz>

M