Re: mainline build failure of powerpc allmodconfig for prom_init_check

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
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
Subject: Re: mainline build failure of powerpc allmodconfig for prom_init_check
Date: Sun, 17 Jul 2022 07:44:22 -0700	[thread overview]
Message-ID: <CAHk-=whc3Uvhrmrr27xp5=oOhSDjXc5s1ZxC3B7xMYV6oj4WRQ@mail.gmail.com> (raw)
In-Reply-To: <CADVatmO9XzFnX+N0TuOtr0FYyxKr1oe5RAhCEJjmnvjteT5QNw@mail.gmail.com>

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()".

              Linus