powerpc/hw_brk: Fix setting of length for exact mode breakpoints

The smallest match region for both the DABR and DAWR is 8 bytes, so the
kernel needs to filter matches when users want to look at regions smaller than
this.

Currently we set the length of PPC_BREAKPOINT_MODE_EXACT breakpoints to 8.
This is wrong as in exact mode we should only match on 1 address, hence the
length should be 1.

This ensures that the kernel will filter out any exact mode hardware breakpoint
matches on any addresses other than the requested one.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Reported-by: Edjunior Barbosa Machado <emachado@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/ptrace.c

@@ -1449,7 +1449,9 @@ static long ppc_set_hwdebug(struct task_struct *child,
 	 */
 	if (bp_info->addr_mode == PPC_BREAKPOINT_MODE_RANGE_INCLUSIVE) {
 		len = bp_info->addr2 - bp_info->addr;
-	} else if (bp_info->addr_mode != PPC_BREAKPOINT_MODE_EXACT) {
+	} else if (bp_info->addr_mode == PPC_BREAKPOINT_MODE_EXACT)
+		len = 1;
+	else {
 		ptrace_put_breakpoints(child);
 		return -EINVAL;
 	}