[PATCH 7/8] cgroup: mount cgroupns-root when inside non-init cgroupns

[serge@hallyn.com]

From: Aditya Kali <adityakali@google.com>

This patch enables cgroup mounting inside userns when a process
as appropriate privileges. The cgroup filesystem mounted is
rooted at the cgroupns-root. Thus, in a container-setup, only
the hierarchy under the cgroupns-root is exposed inside the container.
This allows container management tools to run inside the containers
without depending on any global state.
In order to support this, a new kernfs api is added to lookup the
dentry for the cgroupns-root.

Signed-off-by: Aditya Kali <adityakali@google.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
---
 fs/kernfs/mount.c      |   48 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/kernfs.h |    2 ++
 kernel/cgroup.c        |   32 +++++++++++++++++++++++++++++++-
 3 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 8eaf417..64613864 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -62,6 +62,54 @@ struct kernfs_root *kernfs_root_from_sb(struct super_block *sb)
 	return NULL;
 }
 
+/**
+ * kernfs_obtain_root - get a dentry for the given kernfs_node
+ * @sb: the kernfs super_block
+ * @kn: kernfs_node for which a dentry is needed
+ *
+ * This can used used by callers which want to mount only a part of the kernfs
+ * as root of the filesystem.
+ */
+struct dentry *kernfs_obtain_root(struct super_block *sb,
+				  struct kernfs_node *kn)
+{
+	struct dentry *dentry;
+	struct inode *inode;
+
+	BUG_ON(sb->s_op != &kernfs_sops);
+
+	/* inode for the given kernfs_node should already exist. */
+	inode = ilookup(sb, kn->ino);
+	if (!inode) {
+		pr_debug("kernfs: could not get inode for '");
+		pr_cont_kernfs_path(kn);
+		pr_cont("'.\n");
+		return ERR_PTR(-EINVAL);
+	}
+
+	/* instantiate and link root dentry */
+	dentry = d_obtain_root(inode);
+	if (!dentry) {
+		pr_debug("kernfs: could not get dentry for '");
+		pr_cont_kernfs_path(kn);
+		pr_cont("'.\n");
+		return ERR_PTR(-ENOMEM);
+	}
+
+	/* If this is a new dentry, set it up. We need kernfs_mutex because this
+	 * may be called by callers other than kernfs_fill_super. */
+	mutex_lock(&kernfs_mutex);
+	if (!dentry->d_fsdata) {
+		kernfs_get(kn);
+		dentry->d_fsdata = kn;
+	} else {
+		WARN_ON(dentry->d_fsdata != kn);
+	}
+	mutex_unlock(&kernfs_mutex);
+
+	return dentry;
+}
+
 static int kernfs_fill_super(struct super_block *sb, unsigned long magic)
 {
 	struct kernfs_super_info *info = kernfs_info(sb);
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index d025ebd..1903777 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -284,6 +284,8 @@ struct kernfs_node *kernfs_node_from_dentry(struct dentry *dentry);
 struct kernfs_root *kernfs_root_from_sb(struct super_block *sb);
 struct inode *kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn);
 
+struct dentry *kernfs_obtain_root(struct super_block *sb,
+				  struct kernfs_node *kn);
 struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
 				       unsigned int flags, void *priv);
 void kernfs_destroy_root(struct kernfs_root *root);
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1d696de..0a3e893 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1980,6 +1980,14 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 	int ret;
 	int i;
 	bool new_sb;
+	struct cgroup_namespace *ns =
+		get_cgroup_ns(current->nsproxy->cgroup_ns);
+
+	/* Check if the caller has permission to mount. */
+	if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN)) {
+		put_cgroup_ns(ns);
+		return ERR_PTR(-EPERM);
+	}
 
 	/*
 	 * The first time anyone tries to mount a cgroup, enable the list
@@ -2112,11 +2120,31 @@ out_free:
 	kfree(opts.release_agent);
 	kfree(opts.name);
 
-	if (ret)
+	if (ret) {
+		put_cgroup_ns(ns);
 		return ERR_PTR(ret);
+	}
 
 	dentry = kernfs_mount(fs_type, flags, root->kf_root,
 				CGROUP_SUPER_MAGIC, &new_sb);
+
+	if (!IS_ERR(dentry)) {
+		/* In non-init cgroup namespace, instead of root cgroup's
+		 * dentry, we return the dentry corresponding to the
+		 * cgroupns->root_cgrp.
+		 */
+		if (ns != &init_cgroup_ns) {
+			struct dentry *nsdentry;
+			struct cgroup *cgrp;
+
+			cgrp = cset_cgroup_from_root(ns->root_cgrps, root);
+			nsdentry = kernfs_obtain_root(dentry->d_sb,
+				cgrp->kn);
+			dput(dentry);
+			dentry = nsdentry;
+		}
+	}
+
 	if (IS_ERR(dentry) || !new_sb)
 		cgroup_put(&root->cgrp);
 
@@ -2129,6 +2157,7 @@ out_free:
 		deactivate_super(pinned_sb);
 	}
 
+	put_cgroup_ns(ns);
 	return dentry;
 }
 
@@ -2157,6 +2186,7 @@ static struct file_system_type cgroup_fs_type = {
 	.name = "cgroup",
 	.mount = cgroup_mount,
 	.kill_sb = cgroup_kill_sb,
+	.fs_flags = FS_USERNS_MOUNT,
 };
 
 char * __must_check cgroup_path_ns(struct cgroup_namespace *ns,
-- 
1.7.9.5