linux-kernel - Re: mainline build failure of powerpc allmodconfig for prom_init

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Date:   Sun, 17 Jul 2022 21:25:20 +0100
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Kees Cook <keescook@...omium.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org,
        Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check

On Sun, Jul 17, 2022 at 3:44 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Sun, Jul 17, 2022 at 2:13 AM Sudip Mukherjee
> <sudipm.mukherjee@...il.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