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

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Message-ID: <CAHk-=whc3Uvhrmrr27xp5=oOhSDjXc5s1ZxC3B7xMYV6oj4WRQ@mail.gmail.com>
Date:   Sun, 17 Jul 2022 07:44:22 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sudip Mukherjee <sudipm.mukherjee@...il.com>
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
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check

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()".

              Linus