powerpc: More stack randomisation for 64bit binaries

At the moment we randomise the stack by 8MB on 32bit and 64bit tasks. Since we
have a lot more address space to play with on 64bit, lets do what x86 does and
increase that randomisation to 1GB:

before:
# for i in seq `1 10` ; do sleep 1 & cat /proc/${!}/maps | grep stack; done
fffffebc000-fffffed1000 rw-p ffffffeb000 00:00 0       [stack]
ffffff5a000-ffffff6f000 rw-p ffffffeb000 00:00 0       [stack]
fffffdb2000-fffffdc7000 rw-p ffffffeb000 00:00 0       [stack]
fffffd3e000-fffffd53000 rw-p ffffffeb000 00:00 0       [stack]
fffffad9000-fffffaee000 rw-p ffffffeb000 00:00 0       [stack]

after:
# for i in seq `1 10` ; do sleep 1 & cat /proc/${!}/maps | grep stack; done
ffff5c27000-ffff5c3c000 rw-p ffffffeb000 00:00 0       [stack]
fffebe5e000-fffebe73000 rw-p ffffffeb000 00:00 0       [stack]
fffcb298000-fffcb2ad000 rw-p ffffffeb000 00:00 0       [stack]
fffc719d000-fffc71b2000 rw-p ffffffeb000 00:00 0       [stack]
fffe01af000-fffe01c4000 rw-p ffffffeb000 00:00 0       [stack]
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/elf.h

@@ -271,6 +271,11 @@ extern int arch_setup_additional_pages(struct linux_binprm *bprm,
 				       int uses_interp);
 #define VDSO_AUX_ENT(a,b) NEW_AUX_ENT(a,b);
 
+/* 1GB for 64bit, 8MB for 32bit */
+#define STACK_RND_MASK (is_32bit_task() ? \
+	(0x7ff >> (PAGE_SHIFT - 12)) : \
+	(0x3ffff >> (PAGE_SHIFT - 12)))
+
 #endif /* __KERNEL__ */
 
 /*