[PATCH 5/9] Containers (V9): Add container_clone() interface

[menage@google.com]

This patch adds support for container_clone(), a speculative interface
to creating new containers intended to be used for systems such as
namespace unsharing.

Signed-off-by: Paul Menage <menage@google.com>

---
 include/linux/container.h |    2 
 kernel/container.c        |  121 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 123 insertions(+)

Index: container-2.6.21-rc7-mm1/kernel/container.c
===================================================================
--- container-2.6.21-rc7-mm1.orig/kernel/container.c
+++ container-2.6.21-rc7-mm1/kernel/container.c
@@ -1619,3 +1619,124 @@ void container_exit(struct task_struct *
 	tsk->containers = init_task.containers;
 	task_unlock(tsk);
 }
+
+static atomic_t namecnt;
+static void get_unused_name(char *buf) {
+	sprintf(buf, "node%d", atomic_inc_return(&namecnt));
+}
+
+/**
+ * container_clone - duplicate the current container in the hierarchy
+ * that the given subsystem is attached to, and move this task into
+ * the new child
+ */
+int container_clone(struct task_struct *tsk, struct container_subsys *subsys)
+{
+	struct dentry *dentry;
+	int ret = 0;
+	char nodename[32];
+	struct container *parent, *child;
+	struct inode *inode;
+	struct css_group *cg;
+	struct containerfs_root *root;
+
+	/* We shouldn't be called by an unregistered subsystem */
+	BUG_ON(!subsys->active);
+
+	/* First figure out what hierarchy and container we're dealing
+	 * with, and pin them so we can drop container_mutex */
+	mutex_lock(&container_mutex);
+ again:
+	root = subsys->root;
+	if (root == &rootnode) {
+		printk(KERN_INFO
+		       "Not cloning container for unused subsystem %s\n",
+		       subsys->name);
+		mutex_unlock(&container_mutex);
+		return 0;
+	}
+	cg = &tsk->containers;
+	parent = task_container(tsk, subsys->subsys_id);
+	/* Pin the hierarchy */
+	atomic_inc(&parent->root->sb->s_active);
+
+	mutex_unlock(&container_mutex);
+
+	/* Now do the VFS work to create a container */
+	get_unused_name(nodename);
+	inode = parent->dentry->d_inode;
+
+	/* Hold the parent directory mutex across this operation to
+	 * stop anyone else deleting the new container */
+	mutex_lock(&inode->i_mutex);
+	dentry = container_get_dentry(parent->dentry, nodename);
+	if (IS_ERR(dentry)) {
+		printk(KERN_INFO
+		       "Couldn't allocate dentry for %s: %ld\n", nodename,
+		       PTR_ERR(dentry));
+		ret = PTR_ERR(dentry);
+		goto out_release;
+	}
+
+	/* Create the container directory, which also creates the container */
+	ret = vfs_mkdir(inode, dentry, S_IFDIR | 0755);
+	child = __d_cont(dentry);
+	dput(dentry);
+	if (ret) {
+		printk(KERN_INFO
+		       "Failed to create container %s: %d\n", nodename,
+		       ret);
+		goto out_release;
+	}
+
+	if (!child) {
+		printk(KERN_INFO
+		       "Couldn't find new container %s\n", nodename);
+		ret = -ENOMEM;
+		goto out_release;
+	}
+
+	/* The container now exists. Retake container_mutex and check
+	 * that we're still in the same state that we thought we
+	 * were. */
+	mutex_lock(&container_mutex);
+	if ((root != subsys->root) ||
+	    (parent != task_container(tsk, subsys->subsys_id))) {
+		/* Aargh, we raced ... */
+		mutex_unlock(&inode->i_mutex);
+
+		deactivate_super(parent->root->sb);
+		/* The container is still accessible in the VFS, but
+		 * we're not going to try to rmdir() it at this
+		 * point. */
+		printk(KERN_INFO
+		       "Race in container_clone() - leaking container %s\n",
+		       nodename);
+		goto again;
+	}
+
+	/* All seems fine. Finish by moving the task into the new container */
+	ret = attach_task(child, tsk);
+	mutex_unlock(&container_mutex);
+
+ out_release:
+	mutex_unlock(&inode->i_mutex);
+	deactivate_super(parent->root->sb);
+	return ret;
+}
+
+/* See if "cont" is a descendant of the current task's container in
+ * the appropriate hierarchy */
+
+int container_is_descendant(const struct container *cont) {
+	int ret;
+	struct container *target;
+	int subsys_id;
+	get_first_subsys(cont, NULL, &subsys_id);
+	target = task_container(current, subsys_id);
+	while (cont != target && cont!= cont->top_container) {
+		cont = cont->parent;
+	}
+	ret = (cont == target);
+	return ret;
+}
Index: container-2.6.21-rc7-mm1/include/linux/container.h
===================================================================
--- container-2.6.21-rc7-mm1.orig/include/linux/container.h
+++ container-2.6.21-rc7-mm1/include/linux/container.h
@@ -189,6 +189,8 @@ static inline struct container* task_con
 
 int container_path(const struct container *cont, char *buf, int buflen);
 
+int container_clone(struct task_struct *tsk, struct container_subsys *ss);
+
 #else /* !CONFIG_CONTAINERS */
 
 static inline int container_init_early(void) { return 0; }

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