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 09:36:02 -0800
From:   Song Liu <song@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>,
        live-patching@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 2/2] powerpc/module_64: Fix "expected nop" error on module re-patching

On Wed, Jan 25, 2023 at 8:46 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Tue, Jan 24, 2023 at 10:09:56PM -0800, Song Liu wrote:
> > > @@ -514,9 +515,18 @@ static int restore_r2(const char *name, u32 *instruction, struct module *me)
> > >         if (!instr_is_relative_link_branch(ppc_inst(*prev_insn)))
> > >                 return 0;
> > >
> > > -       if (*instruction != PPC_RAW_NOP()) {
> > > +       /*
> > > +        * For livepatch, the restore r2 instruction might have already been
> > > +        * written previously, if the referenced symbol is in a previously
> > > +        * unloaded module which is now being loaded again.  In that case, skip
> > > +        * the warning and the instruction write.
> > > +        */
> > > +       if (insn_val == PPC_INST_LD_TOC)
> > > +               return 0;
> >
> > Do we need "sym->st_shndx == SHN_LIVEPATCH" here?
>
> My original patch had that check, but I dropped it for simplicity.
>
> In the non-livepatch case, the condition should never be true, but it
> doesn't hurt to check it anyway.

While this is the only place we use PPC_INST_LD_TOC, there is another
place we use "PPC_RAW_STD(_R2, _R1, R2_STACK_OFFSET)", which
is identical to PPC_INST_LD_TOC. So I am not quite sure whether this
happens for non-livepatch.

Thanks,
Song