powerpc/mm/subpage: Clear RWX bit to indicate no access

Subpage protection used to depend on the _PAGE_USER bit to implement no
access mode. This patch switches that to use _PAGE_RWX. We clear Read,
Write and Execute access from the pte instead of clearing _PAGE_USER
now. This was done so that we can switch to _PAGE_PRIVILEGED in a later
patch.

subpage_protection() returns pte bits that need to be cleared. Instead
of updating the interface to handle no-access in a separate way, it
appears simpler to clear RWX acecss to indicate no access.

We still don't insert hash ptes for no access implied by !_PAGE_RWX.
Hence we should not get PROT_FAULT with change.

Signed-off-by: Aneesh Kumar K.V <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

arch/powerpc/mm/hash_utils_64.c

···
  * Userspace sets the subpage permissions using the subpage_prot system call.
  *
  * Result is 0: full permissions, _PAGE_RW: read-only,
- * _PAGE_USER or _PAGE_USER|_PAGE_RW: no access.
+ * _PAGE_RWX: no access.
  */
 static int subpage_protection(struct mm_struct *mm, unsigned long ea)
 {
···
 	/* extract 2-bit bitfield for this 4k subpage */
 	spp >>= 30 - 2 * ((ea >> 12) & 0xf);
 
-	/* turn 0,1,2,3 into combination of _PAGE_USER and _PAGE_RW */
-	spp = ((spp & 2) ? _PAGE_USER : 0) | ((spp & 1) ? _PAGE_RW : 0);
+	/*
+	 * 0 -> full premission
+	 * 1 -> Read only
+	 * 2 -> no access.
+	 * We return the flag that need to be cleared.
+	 */
+	spp = ((spp & 2) ? _PAGE_RWX : 0) | ((spp & 1) ? _PAGE_WRITE : 0);
 	return spp;
 }