Sanitize the type of struct user.u_ar0

struct user.u_ar0 is defined to contain a pointer offset on all
architectures in which it is defined (all architectures which define an
a.out format except SPARC.) However, it has a pointer type in the headers,
which is pointless -- <asm/user.h> is not exported to userspace, and it
just makes the code messy.

Redefine the field as "unsigned long" (which is the same size as a pointer
on all Linux architectures) and change the setting code to user offsetof()
instead of hand-coded arithmetic.

Cc: Linux Arch Mailing List <linux-arch@vger.kernel.org>
Cc: Bryan Wu <bryan.wu@analog.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Lennert Buytenhek <kernel@wantstofly.org>
Cc: Håvard Skinnemoen <hskinnemoen@atmel.com>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_aout.c

@@ -115,7 +115,7 @@ static int aout_core_dump(long signr, struct pt_regs *regs, struct file *file, u
 	current->flags |= PF_DUMPCORE;
        	strncpy(dump.u_comm, current->comm, sizeof(dump.u_comm));
 #ifndef __sparc__
-	dump.u_ar0 = (void *)(((unsigned long)(&dump.regs)) - ((unsigned long)(&dump)));
+	dump.u_ar0 = offsetof(struct user, regs);
 #endif
 	dump.signal = signr;
 	dump_thread(regs, &dump);