This patch adds the per-directory "tasks" file for containerfs mounts;
this allows the user to determine which tasks are members of a
container by reading a container's "tasks", and to move a task into a
container by writing its pid to its "tasks".

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

---
 include/linux/container.h |    2 
 kernel/container.c        |  344 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 346 insertions(+)

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
@@ -128,6 +128,8 @@ int container_is_removed(const struct co
 
 int container_path(const struct container *cont, char *buf, int buflen);
 
+int container_task_count(const struct container *cont);
+
 /* Return true if the container is a descendant of the current container */
 int container_is_descendant(const struct container *cont);
 
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
@@ -676,6 +676,111 @@ static inline void get_first_subsys(cons
 		*subsys_id = test_ss->subsys_id;
 }
 
+/*
+ * Attach task 'tsk' to container 'cont'
+ *
+ * Call holding container_mutex.  May take task_lock of
+ * the task 'pid' during call.
+ */
+
+static int attach_task(struct container *cont, struct task_struct *tsk)
+{
+	int retval = 0;
+	struct container_subsys *ss;
+	struct container *oldcont;
+	struct css_group *cg = &tsk->containers;
+	struct containerfs_root *root = cont->root;
+	int i;
+
+	int subsys_id;
+	get_first_subsys(cont, NULL, &subsys_id);
+
+	/* Nothing to do if the task is already in that container */
+	oldcont = task_container(tsk, subsys_id);
+	if (cont == oldcont)
+		return 0;
+
+	for_each_subsys(root, ss) {
+		if (ss->can_attach) {
+			retval = ss->can_attach(ss, cont, tsk);
+			if (retval) {
+				return retval;
+			}
+		}
+	}
+
+	task_lock(tsk);
+	if (tsk->flags & PF_EXITING) {
+		task_unlock(tsk);
+		return -ESRCH;
+	}
+	/* Update the css_group pointers for the subsystems in this
+	 * hierarchy */
+	for (i = 0; i < CONTAINER_SUBSYS_COUNT; i++) {
+		if (root->subsys_bits & (1ull << i)) {
+			/* Subsystem is in this hierarchy. So we want
+			 * the subsystem state from the new
+			 * container. Transfer the refcount from the
+			 * old to the new */
+			atomic_inc(&cont->count);
+			atomic_dec(&cg->subsys[i]->container->count);
+			rcu_assign_pointer(cg->subsys[i], cont->subsys[i]);
+		}
+	}
+	task_unlock(tsk);
+
+	for_each_subsys(root, ss) {
+		if (ss->attach) {
+			ss->attach(ss, cont, oldcont, tsk);
+		}
+	}
+
+	synchronize_rcu();
+	return 0;
+}
+
+/*
+ * Attach task with pid 'pid' to container 'cont'. Call with
+ * container_mutex, may take task_lock of task
+ *
+ */
+
+static int attach_task_by_pid(struct container *cont, char *pidbuf)
+{
+	pid_t pid;
+	struct task_struct *tsk;
+	int ret;
+
+	if (sscanf(pidbuf, "%d", &pid) != 1)
+		return -EIO;
+
+	if (pid) {
+		read_lock(&tasklist_lock);
+
+		tsk = find_task_by_pid(pid);
+		if (!tsk || tsk->flags & PF_EXITING) {
+			read_unlock(&tasklist_lock);
+			return -ESRCH;
+		}
+
+		get_task_struct(tsk);
+		read_unlock(&tasklist_lock);
+
+		if ((current->euid) && (current->euid != tsk->uid)
+		    && (current->euid != tsk->suid)) {
+			put_task_struct(tsk);
+			return -EACCES;
+		}
+	} else {
+		tsk = current;
+		get_task_struct(tsk);
+	}
+
+	ret = attach_task(cont, tsk);
+	put_task_struct(tsk);
+	return ret;
+}
+
 /* The various types of files and directories in a container file system */
 
 typedef enum {
@@ -684,6 +789,54 @@ typedef enum {
 	FILE_TASKLIST,
 } container_filetype_t;
 
+static ssize_t container_common_file_write(struct container *cont,
+					   struct cftype *cft,
+					   struct file *file,
+					   const char __user *userbuf,
+					   size_t nbytes, loff_t *unused_ppos)
+{
+	container_filetype_t type = cft->private;
+	char *buffer;
+	int retval = 0;
+
+	if (nbytes >= PATH_MAX)
+		return -E2BIG;
+
+	/* +1 for nul-terminator */
+	if ((buffer = kmalloc(nbytes + 1, GFP_KERNEL)) == 0)
+		return -ENOMEM;
+
+	if (copy_from_user(buffer, userbuf, nbytes)) {
+		retval = -EFAULT;
+		goto out1;
+	}
+	buffer[nbytes] = 0;	/* nul-terminate */
+
+	mutex_lock(&container_mutex);
+
+	if (container_is_removed(cont)) {
+		retval = -ENODEV;
+		goto out2;
+	}
+
+	switch (type) {
+	case FILE_TASKLIST:
+		retval = attach_task_by_pid(cont, buffer);
+		break;
+	default:
+		retval = -EINVAL;
+		goto out2;
+	}
+
+	if (retval == 0)
+		retval = nbytes;
+out2:
+	mutex_unlock(&container_mutex);
+out1:
+	kfree(buffer);
+	return retval;
+}
+
 static ssize_t container_file_write(struct file *file, const char __user *buf,
 						size_t nbytes, loff_t *ppos)
 {
@@ -872,6 +1025,194 @@ int container_add_files(struct container
 	return 0;
 }
 
+/* Count the number of tasks in a container. Could be made more
+ * time-efficient but less space-efficient with more linked lists
+ * running through each container and the css_group structures
+ * that referenced it. */
+
+int container_task_count(const struct container *cont) {
+	int count = 0;
+	struct task_struct *g, *p;
+	struct container_subsys_state *css;
+	int subsys_id;
+	get_first_subsys(cont, &css, &subsys_id);
+
+	read_lock(&tasklist_lock);
+	do_each_thread(g, p) {
+		if (task_subsys_state(p, subsys_id) == css)
+			count ++;
+	} while_each_thread(g, p);
+	read_unlock(&tasklist_lock);
+	return count;
+}
+
+/*
+ * Stuff for reading the 'tasks' file.
+ *
+ * Reading this file can return large amounts of data if a container has
+ * *lots* of attached tasks. So it may need several calls to read(),
+ * but we cannot guarantee that the information we produce is correct
+ * unless we produce it entirely atomically.
+ *
+ * Upon tasks file open(), a struct ctr_struct is allocated, that
+ * will have a pointer to an array (also allocated here).  The struct
+ * ctr_struct * is stored in file->private_data.  Its resources will
+ * be freed by release() when the file is closed.  The array is used
+ * to sprintf the PIDs and then used by read().
+ */
+
+/* containers_tasks_read array */
+
+struct ctr_struct {
+	char *buf;
+	int bufsz;
+};
+
+/*
+ * Load into 'pidarray' up to 'npids' of the tasks using container
+ * 'cont'.  Return actual number of pids loaded.  No need to
+ * task_lock(p) when reading out p->container, since we're in an RCU
+ * read section, so the css_group can't go away, and is
+ * immutable after creation.
+ */
+static int pid_array_load(pid_t *pidarray, int npids, struct container *cont)
+{
+	int n = 0;
+	struct task_struct *g, *p;
+	struct container_subsys_state *css;
+	int subsys_id;
+	get_first_subsys(cont, &css, &subsys_id);
+	rcu_read_lock();
+	read_lock(&tasklist_lock);
+
+	do_each_thread(g, p) {
+		if (task_subsys_state(p, subsys_id) == css) {
+			pidarray[n++] = pid_nr(task_pid(p));
+			if (unlikely(n == npids))
+				goto array_full;
+		}
+	} while_each_thread(g, p);
+
+array_full:
+	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
+	return n;
+}
+
+static int cmppid(const void *a, const void *b)
+{
+	return *(pid_t *)a - *(pid_t *)b;
+}
+
+/*
+ * Convert array 'a' of 'npids' pid_t's to a string of newline separated
+ * decimal pids in 'buf'.  Don't write more than 'sz' chars, but return
+ * count 'cnt' of how many chars would be written if buf were large enough.
+ */
+static int pid_array_to_buf(char *buf, int sz, pid_t *a, int npids)
+{
+	int cnt = 0;
+	int i;
+
+	for (i = 0; i < npids; i++)
+		cnt += snprintf(buf + cnt, max(sz - cnt, 0), "%d\n", a[i]);
+	return cnt;
+}
+
+/*
+ * Handle an open on 'tasks' file.  Prepare a buffer listing the
+ * process id's of tasks currently attached to the container being opened.
+ *
+ * Does not require any specific container mutexes, and does not take any.
+ */
+static int container_tasks_open(struct inode *unused, struct file *file)
+{
+	struct container *cont = __d_cont(file->f_dentry->d_parent);
+	struct ctr_struct *ctr;
+	pid_t *pidarray;
+	int npids;
+	char c;
+
+	if (!(file->f_mode & FMODE_READ))
+		return 0;
+
+	ctr = kmalloc(sizeof(*ctr), GFP_KERNEL);
+	if (!ctr)
+		goto err0;
+
+	/*
+	 * If container gets more users after we read count, we won't have
+	 * enough space - tough.  This race is indistinguishable to the
+	 * caller from the case that the additional container users didn't
+	 * show up until sometime later on.
+	 */
+	npids = container_task_count(cont);
+	pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
+	if (!pidarray)
+		goto err1;
+
+	npids = pid_array_load(pidarray, npids, cont);
+	sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);
+
+	/* Call pid_array_to_buf() twice, first just to get bufsz */
+	ctr->bufsz = pid_array_to_buf(&c, sizeof(c), pidarray, npids) + 1;
+	ctr->buf = kmalloc(ctr->bufsz, GFP_KERNEL);
+	if (!ctr->buf)
+		goto err2;
+	ctr->bufsz = pid_array_to_buf(ctr->buf, ctr->bufsz, pidarray, npids);
+
+	kfree(pidarray);
+	file->private_data = ctr;
+	return 0;
+
+err2:
+	kfree(pidarray);
+err1:
+	kfree(ctr);
+err0:
+	return -ENOMEM;
+}
+
+static ssize_t container_tasks_read(struct container *cont,
+				    struct cftype *cft,
+				    struct file *file, char __user *buf,
+				    size_t nbytes, loff_t *ppos)
+{
+	struct ctr_struct *ctr = file->private_data;
+
+	if (*ppos + nbytes > ctr->bufsz)
+		nbytes = ctr->bufsz - *ppos;
+	if (copy_to_user(buf, ctr->buf + *ppos, nbytes))
+		return -EFAULT;
+	*ppos += nbytes;
+	return nbytes;
+}
+
+static int container_tasks_release(struct inode *unused_inode, struct file *file)
+{
+	struct ctr_struct *ctr;
+
+	if (file->f_mode & FMODE_READ) {
+		ctr = file->private_data;
+		kfree(ctr->buf);
+		kfree(ctr);
+	}
+	return 0;
+}
+
+/*
+ * for the common functions, 'private' gives the type of file
+ */
+
+static struct cftype cft_tasks = {
+	.name = "tasks",
+	.open = container_tasks_open,
+	.read = container_tasks_read,
+	.write = container_common_file_write,
+	.release = container_tasks_release,
+	.private = FILE_TASKLIST,
+};
+
 static int container_populate_dir(struct container *cont)
 {
 	int err;
@@ -880,6 +1221,9 @@ static int container_populate_dir(struct
 	/* First clear out any existing files */
 	container_clear_directory(cont->dentry);
 
+	if ((err = container_add_file(cont, &cft_tasks)) < 0)
+		return err;
+
 	for_each_subsys(cont->root, ss) {
 		if (ss->populate && (err = ss->populate(ss, cont)) < 0)
 			return err;

--