arm64: ptrace: use HW_BREAKPOINT_EMPTY type for disabled breakpoints

If a debugger tries to zero a hardware debug control register, the
kernel will try to infer both the type and length of the breakpoint
in order to sanity-check against the requested regset type. This will
fail because the encoding will appear as a zero-length breakpoint.

This patch changes the control register setting so that disabled
breakpoints are treated as HW_BREAKPOINT_EMPTY and no further
sanity-checking is required.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/ptrace.c

@@ -234,28 +234,33 @@ static int ptrace_hbp_fill_attr_ctrl(unsigned int note_type,
 				     struct arch_hw_breakpoint_ctrl ctrl,
 				     struct perf_event_attr *attr)
 {
-	int err, len, type;
+	int err, len, type, disabled = !ctrl.enabled;
 
-	err = arch_bp_generic_fields(ctrl, &len, &type);
-	if (err)
-		return err;
-
-	switch (note_type) {
-	case NT_ARM_HW_BREAK:
-		if ((type & HW_BREAKPOINT_X) != type)
-			return -EINVAL;
-		break;
-	case NT_ARM_HW_WATCH:
-		if ((type & HW_BREAKPOINT_RW) != type)
+	if (disabled) {
+		len = 0;
+		type = HW_BREAKPOINT_EMPTY;
+	} else {
+		err = arch_bp_generic_fields(ctrl, &len, &type);
+		if (err)
+			return err;
+
+		switch (note_type) {
+		case NT_ARM_HW_BREAK:
+			if ((type & HW_BREAKPOINT_X) != type)
+				return -EINVAL;
+			break;
+		case NT_ARM_HW_WATCH:
+			if ((type & HW_BREAKPOINT_RW) != type)
+				return -EINVAL;
+			break;
+		default:
 			return -EINVAL;
-		break;
-	default:
-		return -EINVAL;
+		}
 	}
 
 	attr->bp_len	= len;
 	attr->bp_type	= type;
-	attr->disabled	= !ctrl.enabled;
+	attr->disabled	= disabled;
 
 	return 0;
 }