Lots of crazy long lines in the patch.  Remember that you should only
go past 80 lines if it clearly improves readability, and I don't
think it does anywhere in here.

Even with the whole series applied this structure is only used in
namespace.c, so it might be worth moving there.

This refactoring seems worth a little prep patch.

 
+static int build_attr_flags(unsigned int attr_flags, unsigned int *flags)
+{
+	unsigned int aflags = 0;
+
+	if (attr_flags & ~(MOUNT_ATTR_RDONLY |
+			   MOUNT_ATTR_NOSUID |
+			   MOUNT_ATTR_NODEV |
+			   MOUNT_ATTR_NOEXEC |
+			   MOUNT_ATTR__ATIME |
+			   MOUNT_ATTR_NODIRATIME))
+		return -EINVAL;
+
+	if (attr_flags & MOUNT_ATTR_RDONLY)
+		aflags |= MNT_READONLY;
+	if (attr_flags & MOUNT_ATTR_NOSUID)
+		aflags |= MNT_NOSUID;
+	if (attr_flags & MOUNT_ATTR_NODEV)
+		aflags |= MNT_NODEV;
+	if (attr_flags & MOUNT_ATTR_NOEXEC)
+		aflags |= MNT_NOEXEC;
+	if (attr_flags & MOUNT_ATTR_NODIRATIME)
+		aflags |= MNT_NODIRATIME;
+
+	*flags = aflags;
+	return 0;
+}

Same for adding this helper.

+	*kattr = (struct mount_kattr){

Missing whitespace before the {.

+	switch (attr->propagation) {
+	case MAKE_PROPAGATION_UNCHANGED:
+		kattr->propagation = 0;
+		break;
+	case MAKE_PROPAGATION_UNBINDABLE:
+		kattr->propagation = MS_UNBINDABLE;
+		break;
+	case MAKE_PROPAGATION_PRIVATE:
+		kattr->propagation = MS_PRIVATE;
+		break;
+	case MAKE_PROPAGATION_DEPENDENT:
+		kattr->propagation = MS_SLAVE;
+		break;
+	case MAKE_PROPAGATION_SHARED:
+		kattr->propagation = MS_SHARED;
+		break;
+	default:

Any reason to not just reuse the MS_* flags in the new API?  Yes, your
new names are more descriptive, but having different names for the same
thing is also rather confusing.

+	if (upper_32_bits(attr->attr_set))
+		return -EINVAL;
+	if (build_attr_flags(lower_32_bits(attr->attr_set), &kattr->attr_set))
+		return -EINVAL;
+
+	if (upper_32_bits(attr->attr_clr))
+		return -EINVAL;
+	if (build_attr_flags(lower_32_bits(attr->attr_clr), &kattr->attr_clr))
+		return -EINVAL;

What is so magic about the upper and lower 32 bits?

+		return -EINVAL;
+	else if ((attr->attr_clr & MOUNT_ATTR__ATIME) &&
+		 ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME))
+		return -EINVAL;

No need for the else here.

That being said I'd reword the thing to be a little more obvious:

	if (attr->attr_clr & MOUNT_ATTR__ATIME) {
		if ((attr->attr_clr & MOUNT_ATTR__ATIME) != MOUNT_ATTR__ATIME)
			return -EINVAL;

		... code doing the update of the atime flags here
	} else {
		if (attr->attr_set & MOUNT_ATTR__ATIME)
			return -EINVAL;
	}

+/* Change propagation through mount_setattr(). */
+enum propagation_type {
+	MAKE_PROPAGATION_UNCHANGED	= 0, /* Don't change mount propagation (default). */
+	MAKE_PROPAGATION_UNBINDABLE	= 1, /* Make unbindable. */
+	MAKE_PROPAGATION_PRIVATE	= 2, /* Do not receive or send mount events. */
+	MAKE_PROPAGATION_DEPENDENT	= 3, /* Only receive mount events. */
+	MAKE_PROPAGATION_SHARED		= 4, /* Send and receive mount events. */
+};

FYI, in uapis using defines instead of enums is usually the better
choice, as that allows userspace to probe for later added defines.

But if we use MS_* here that would be void anyway.

+/* List of all mount_attr versions. */
+#define MOUNT_ATTR_SIZE_VER0	24 /* sizeof first published struct */
+#define MOUNT_ATTR_SIZE_LATEST	MOUNT_ATTR_SIZE_VER0

The _LATEST things is pretty dangerous as there basically is no safe
and correct way for userspace to use it.