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

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Date:   Wed, 25 Jan 2023 14:31:31 +0100
From:   Petr Mladek <pmladek@...e.com>
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 2023-01-24 19:38:05, 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>

It seems that the function does what it says. And it seems to be the
only location where an instruction is checked before it is modified.
I am fine with this approach.

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr