file: Implement task_lookup_fd_rcu

As a companion to lookup_fd_rcu implement task_lookup_fd_rcu for
querying an arbitrary process about a specific file.
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200818103713.aw46m7vprsy4vlve@wittgenstein
Link: https://lkml.kernel.org/r/20201120231441.29911-11-ebiederm@xmission.comSigned-off-by: Eric W. Biederman <ebiederm@xmission.com>

fs/file.c

@@ -865,6 +865,21 @@ struct file *fget_task(struct task_struct *task, unsigned int fd)
 	return file;
 }
 
+struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd)
+{
+	/* Must be called with rcu_read_lock held */
+	struct files_struct *files;
+	struct file *file = NULL;
+
+	task_lock(task);
+	files = task->files;
+	if (files)
+		file = files_lookup_fd_rcu(files, fd);
+	task_unlock(task);
+
+	return file;
+}
+
 /*
  * Lightweight file lookup - no refcnt increment if fd table isn't shared.
  *

include/linux/fdtable.h

@@ -110,6 +110,8 @@ static inline struct file *lookup_fd_rcu(unsigned int fd)
 	return files_lookup_fd_rcu(current->files, fd);
 }
 
+struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd);
+
 struct task_struct;
 
 struct files_struct *get_files_struct(struct task_struct *);