UBUNTU: SAUCE: (noup) cgroup namespaces: add a 'nsroot=' mountinfo field

BugLink: http://bugs.launchpad.net/bugs/1560489

[ note - this is a version of the patch I just sent to lkml ported to
  our xenial tree.  It's needed for things like docker and lxc to
  be certain of which cgroup tasks file is their own in certain nesting
  situations.  We currently work around it by blindly assuming that
  there are no legacy container managers running on cgroup-ns-enabled
  kernels ]

One practical problem I've found with cgroup namespaces is that there
is no way to disambiguate between a cgroupfs mount which was done in
a cgroup namespace, and a bind mount of a cgroupfs directory.  So
whether I do

unshare --cgroup -- bash -c "mount -t cgroup -o freezer f /mnt; cat /proc/self/mountinfo"

or whether I just

mount --bind /sys/fs/cgroup/freezer/$(awk -F: '/freezer/ { print $3 }' /proc/self/cgroup) /mnt

'mount root' field (field 3) in /proc/self/mountinfo will show the
same thing, the result of awk -F: '/freezer/ { print $3 }' /proc/self/cgroup.

This patch adds a 'nsroot=' field to cgroup mountinfo entries, so that
userspace can distinguish a mount made in a cgroup namespace from a bind
mount from a cgroup subdirectory.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

fs/kernfs/mount.c

@@ -36,7 +36,7 @@ static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
 	struct kernfs_syscall_ops *scops = root->syscall_ops;
 
 	if (scops && scops->show_options)
-		return scops->show_options(sf, root);
+		return scops->show_options(sf, dentry, root);
 	return 0;
 }
 

include/linux/kernfs.h

@@ -145,7 +145,8 @@ struct kernfs_node {
  */
 struct kernfs_syscall_ops {
 	int (*remount_fs)(struct kernfs_root *root, int *flags, char *data);
-	int (*show_options)(struct seq_file *sf, struct kernfs_root *root);
+	int (*show_options)(struct seq_file *sf, struct dentry *dentry,
+			    struct kernfs_root *root);
 
 	int (*mkdir)(struct kernfs_node *parent, const char *name,
 		     umode_t mode);

kernel/cgroup.c

@@ -1589,7 +1589,32 @@ static int rebind_subsystems(struct cgroup_root *dst_root,
 	return 0;
 }
 
-static int cgroup_show_options(struct seq_file *seq,
+static void cgroup_show_nsroot(struct seq_file *seq, struct dentry *dentry,
+			       struct kernfs_root *kf_root)
+{
+	struct kernfs_node *d_kn = dentry->d_fsdata;
+	char *nsroot;
+	int len, ret;
+
+	if (!kf_root)
+		return;
+	len = kernfs_path_from_node(d_kn, kf_root->kn, NULL, 0);
+	if (len <= 0)
+		return;
+	nsroot = kzalloc(len + 1, GFP_ATOMIC);
+	if (!nsroot)
+		return;
+	ret = kernfs_path_from_node(d_kn, kf_root->kn, nsroot, len + 1);
+	if (ret <= 0 || ret > len)
+		goto out;
+
+	seq_show_option(seq, "nsroot", nsroot);
+
+out:
+	kfree(nsroot);
+}
+
+static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry,
 			       struct kernfs_root *kf_root)
 {
 	struct cgroup_root *root = cgroup_root_from_kf(kf_root);
@@ -1615,6 +1640,8 @@ static int cgroup_show_options(struct seq_file *seq,
 		seq_puts(seq, ",clone_children");
 	if (strlen(root->name))
 		seq_show_option(seq, "name", root->name);
+	cgroup_show_nsroot(seq, dentry, kf_root);
+
 	return 0;
 }