Re: [PATCH 05/34] fs: introduce MOUNT_ATTR_IDMAP

On Thu, Oct 29, 2020 at 01:32:23AM +0100, Christian Brauner wrote:

Introduce a new mount bind mount property to allow idmapping mounts. The
MOUNT_ATTR_IDMAP flag can be set via the new mount_setattr() syscall
together with a file descriptor referring to a user namespace.

Shouldn't this go to the end of the series once all the infrastructure
is in place?

+config IDMAP_MOUNTS
+	bool "Support id mappings per mount"
+	default n

n is the default default.

But why do we need a config option here anyway?

+#ifdef CONFIG_IDMAP_MOUNTS
+		if (kattr->attr_set & MNT_IDMAPPED) {
+			struct user_namespace *user_ns;
+			struct vfsmount *vmnt;

All the code here looks like it should go into a helper.

+				struct user_namespace *user_ns = READ_ONCE(m->mnt.mnt_user_ns);
+				WRITE_ONCE(m->mnt.mnt_user_ns, get_user_ns(kattr->userns));

More unreadable long lines.

+	if (attr->attr_set & MOUNT_ATTR_IDMAP) {
+		struct ns_common *ns;
+		struct user_namespace *user_ns;
+		struct file *file;
+
+		file = fget(attr->userns);

The code here looks like another candidate for a self contained helper.

+
+static inline struct user_namespace *mnt_user_ns(const struct vfsmount *mnt)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return READ_ONCE(mnt->mnt_user_ns);
+#else
+	return &init_user_ns;
+#endif

How is the READ_ONCE on a pointer going to work?