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From:   Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Date:   Sun, 17 Jul 2022 21:25:20 +0100
Message-ID: <CADVatmO1V8DiAgWMW9EFHwt+ghwfHhueigXh214tUa_=tVxiug@mail.gmail.com>
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Kees Cook <keescook@chromium.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-hardening@vger.kernel.org,
        Segher Boessenkool <segher@kernel.crashing.org>
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On Sun, Jul 17, 2022 at 3:44 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Sun, Jul 17, 2022 at 2:13 AM Sudip Mukherjee
> <sudipm.mukherjee@gmail.com> wrote:
> >
> > I was trying to check it. With gcc-11 the assembly code generated is
> > not using memset, but using __memset.
> > But with gcc-12, I can see the assembly code is using memset. One
> > example from the assembly:
>
> You could try making the 'args' array in 'struct prom_args' be marked
> 'volatile'.
>
> Ie something like this:
>
>   --- a/arch/powerpc/kernel/prom_init.c
>   +++ b/arch/powerpc/kernel/prom_init.c
>   @@ -115,6 +115,6 @@ struct prom_args {
>            __be32 service;
>            __be32 nargs;
>            __be32 nret;
>   -          __be32 args[10];
>   +        volatile __be32 args[10];
>    };
>
> because I think it's just the compilers turning the small loop over
> those fields into a "memset()".

That didn't work.
"Error: External symbol 'memset' referenced from prom_init.c" is still
there with this change.
And the generated assembly still has the memset for "struct prom_args".


-- 
Regards
Sudip