file: let pick_file() tell caller it's done

Let pick_file() report back that the fd it was passed exceeded the
maximum fd in that fdtable. This allows us to simplify the caller of
this helper because it doesn't need to care anymore whether the passed
in max_fd is excessive. It can rely on pick_file() telling it that it's
past the last valid fd.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Giuseppe Scrivano <gscrivan@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/file.c

@@ -596,18 +596,32 @@ void fd_install(unsigned int fd, struct file *file)
 
 EXPORT_SYMBOL(fd_install);
 
+/**
+ * pick_file - return file associatd with fd
+ * @files: file struct to retrieve file from
+ * @fd: file descriptor to retrieve file for
+ *
+ * If this functions returns an EINVAL error pointer the fd was beyond the
+ * current maximum number of file descriptors for that fdtable.
+ *
+ * Returns: The file associated with @fd, on error returns an error pointer.
+ */
 static struct file *pick_file(struct files_struct *files, unsigned fd)
 {
-	struct file *file = NULL;
+	struct file *file;
 	struct fdtable *fdt;
 
 	spin_lock(&files->file_lock);
 	fdt = files_fdtable(files);
-	if (fd >= fdt->max_fds)
+	if (fd >= fdt->max_fds) {
+		file = ERR_PTR(-EINVAL);
 		goto out_unlock;
+	}
 	file = fdt->fd[fd];
-	if (!file)
+	if (!file) {
+		file = ERR_PTR(-EBADF);
 		goto out_unlock;
+	}
 	rcu_assign_pointer(fdt->fd[fd], NULL);
 	__put_unused_fd(files, fd);
 
@@ -622,7 +636,7 @@ int close_fd(unsigned fd)
 	struct file *file;
 
 	file = pick_file(files, fd);
-	if (!file)
+	if (IS_ERR(file))
 		return -EBADF;
 
 	return filp_close(file, files);
@@ -663,11 +677,16 @@ static inline void __range_close(struct files_struct *cur_fds, unsigned int fd,
 		struct file *file;
 
 		file = pick_file(cur_fds, fd++);
-		if (!file)
-			continue;
-
+		if (!IS_ERR(file)) {
+			/* found a valid file to close */
 			filp_close(file, cur_fds);
 			cond_resched();
+			continue;
+		}
+
+		/* beyond the last fd in that table */
+		if (PTR_ERR(file) == -EINVAL)
+			return;
 	}
 }