bcachefs: install fd later to avoid race with close

Calling fd_install() makes a file reachable for userland, including the
possibility to close the file descriptor, which leads to calling its
'release' hook. If that happens before the code had a chance to bump the
reference of the newly created task struct, the release callback will
call put_task_struct() too early, leading to the premature destruction
of the kernel thread.

Avoid that race by calling fd_install() later, after all the setup is
done.

Fixes: 1c6fdbd8 ("bcachefs: Initial commit")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/thread_with_file.c

@@ -53,9 +53,9 @@ int bch2_run_thread_with_file(struct thread_with_file *thr,
 	if (ret)
 		goto err;
 
-	fd_install(fd, file);
 	get_task_struct(thr->task);
 	wake_up_process(thr->task);
+	fd_install(fd, file);
 	return fd;
 err:
 	if (fd >= 0)