pid: add pidfd_get_task() helper

The number of system calls making use of pidfds is constantly
increasing. Some of those new system calls duplicate the code to turn a
pidfd into task_struct it refers to. Give them a simple helper for this.

Link: https://lore.kernel.org/r/20211004125050.1153693-2-christian.brauner@ubuntu.com
Link: https://lore.kernel.org/r/20211011133245.1703103-2-brauner@kernel.org
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Matthew Bobrowski <repnop@google.com>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Reviewed-by: Matthew Bobrowski <repnop@google.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

include/linux/pid.h

@@ -78,6 +78,7 @@ struct file;
 
 extern struct pid *pidfd_pid(const struct file *file);
 struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
+struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
 int pidfd_create(struct pid *pid, unsigned int flags);
 
 static inline struct pid *get_pid(struct pid *pid)

kernel/pid.c

@@ -539,6 +539,42 @@ struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
 	return pid;
 }
 
+/**
+ * pidfd_get_task() - Get the task associated with a pidfd
+ *
+ * @pidfd: pidfd for which to get the task
+ * @flags: flags associated with this pidfd
+ *
+ * Return the task associated with @pidfd. The function takes a reference on
+ * the returned task. The caller is responsible for releasing that reference.
+ *
+ * Currently, the process identified by @pidfd is always 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, the task_struct associated with the pidfd.
+ *	   On error, a negative errno number will be returned.
+ */
+struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags)
+{
+	unsigned int f_flags;
+	struct pid *pid;
+	struct task_struct *task;
+
+	pid = pidfd_get_pid(pidfd, &f_flags);
+	if (IS_ERR(pid))
+		return ERR_CAST(pid);
+
+	task = get_pid_task(pid, PIDTYPE_TGID);
+	put_pid(pid);
+	if (!task)
+		return ERR_PTR(-ESRCH);
+
+	*flags = f_flags;
+	return task;
+}
+
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *