3 files changed, 108 insertions, 0 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 847dd147b068..187c02ce534c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1711,8 +1711,34 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
 }
 #endif
 
+/*
+ * Poll support for process exit notification.
+ */
+static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts)
+{
+	struct task_struct *task;
+	struct pid *pid = file->private_data;
+	int poll_flags = 0;
+
+	poll_wait(file, &pid->wait_pidfd, pts);
+
+	rcu_read_lock();
+	task = pid_task(pid, PIDTYPE_PID);
+	/*
+	 * Inform pollers only when the whole thread group exits.
+	 * If the thread group leader exits before all other threads in the
+	 * group, then poll(2) should block, similar to the wait(2) family.
+	 */
+	if (!task || (task->exit_state && thread_group_empty(task)))
+		poll_flags = POLLIN | POLLRDNORM;
+	rcu_read_unlock();
+
+	return poll_flags;
+}
+
 const struct file_operations pidfd_fops = {
 	.release = pidfd_release,
+	.poll = pidfd_poll,
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo = pidfd_show_fdinfo,
 #endif
diff --git a/kernel/pid.c b/kernel/pid.c
index e5cad0c7d5dd..16263b526560 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -38,6 +38,8 @@
 #include <linux/syscalls.h>
 #include <linux/proc_ns.h>
 #include <linux/proc_fs.h>
+#include <linux/anon_inodes.h>
+#include <linux/sched/signal.h>
 #include <linux/sched/task.h>
 #include <linux/idr.h>
 
@@ -214,6 +216,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 	for (type = 0; type < PIDTYPE_MAX; ++type)
 		INIT_HLIST_HEAD(&pid->tasks[type]);
 
+	init_waitqueue_head(&pid->wait_pidfd);
+
 	upid = pid->numbers + ns->level;
 	spin_lock_irq(&pidmap_lock);
 	if (!(ns->pid_allocated & PIDNS_ADDING))
@@ -451,6 +455,73 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 	return idr_get_next(&ns->idr, &nr);
 }
 
+/**
+ * pidfd_create() - Create a new pid file descriptor.
+ *
+ * @pid:  struct pid that the pidfd will reference
+ *
+ * This creates a new pid file descriptor with the O_CLOEXEC flag set.
+ *
+ * Note, that this function can only be called after the fd table has
+ * been unshared to avoid leaking the pidfd to the new process.
+ *
+ * Return: On success, a cloexec pidfd is returned.
+ *         On error, a negative errno number will be returned.
+ */
+static int pidfd_create(struct pid *pid)
+{
+	int fd;
+
+	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
+			      O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		put_pid(pid);
+
+	return fd;
+}
+
+/**
+ * pidfd_open() - Open new pid file descriptor.
+ *
+ * @pid:   pid for which to retrieve a pidfd
+ * @flags: flags to pass
+ *
+ * This creates a new pid file descriptor with the O_CLOEXEC flag set for
+ * the process identified by @pid. Currently, the process identified by
+ * @pid must be a thread-group leader. This restriction currently exists
+ * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
+ * be used with CLONE_THREAD) and pidfd polling (only supports thread group
+ * leaders).
+ *
+ * Return: On success, a cloexec pidfd is returned.
+ *         On error, a negative errno number will be returned.
+ */
+SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
+{
+	int fd, ret;
+	struct pid *p;
+
+	if (flags)
+		return -EINVAL;
+
+	if (pid <= 0)
+		return -EINVAL;
+
+	p = find_get_pid(pid);
+	if (!p)
+		return -ESRCH;
+
+	ret = 0;
+	rcu_read_lock();
+	if (!pid_task(p, PIDTYPE_TGID))
+		ret = -EINVAL;
+	rcu_read_unlock();
+
+	fd = ret ?: pidfd_create(p);
+	put_pid(p);
+	return fd;
+}
+
 void __init pid_idr_init(void)
 {
 	/* Verify no one has done anything silly: */
diff --git a/kernel/signal.c b/kernel/signal.c
index 91cb8ca41954..dabe100d2091 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1881,6 +1881,14 @@ ret:
 	return ret;
 }
 
+static void do_notify_pidfd(struct task_struct *task)
+{
+	struct pid *pid;
+
+	pid = task_pid(task);
+	wake_up_all(&pid->wait_pidfd);
+}
+
 /*
  * Let a parent know about the death of a child.
  * For a stopped/continued status change, use do_notify_parent_cldstop instead.
@@ -1904,6 +1912,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 	BUG_ON(!tsk->ptrace &&
 	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
 
+	/* Wake up all pidfd waiters */
+	do_notify_pidfd(tsk);
+
 	if (sig != SIGCHLD) {
 		/*
 		 * This is only possible if parent == real_parent.