bcachefs: Fix null ptr deref in twf from BCH_IOCTL_FSCK_OFFLINE

We need to initialize the stdio redirects before they're used.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/chardev.c

@@ -232,13 +232,15 @@ static long bch2_ioctl_fsck_offline(struct bch_ioctl_fsck_offline __user *user_a
 	/* We need request_key() to be called before we punt to kthread: */
 	opt_set(thr->opts, nostart, true);
 
+	bch2_thread_with_stdio_init(&thr->thr, &bch2_offline_fsck_ops);
+
 	thr->c = bch2_fs_open(devs.data, arg.nr_devs, thr->opts);
 
 	if (!IS_ERR(thr->c) &&
 	    thr->c->opts.errors == BCH_ON_ERROR_panic)
 		thr->c->opts.errors = BCH_ON_ERROR_ro;
 
-	ret = bch2_run_thread_with_stdio(&thr->thr, &bch2_offline_fsck_ops);
+	ret = __bch2_run_thread_with_stdio(&thr->thr);
 out:
 	darray_for_each(devs, i)
 		kfree(*i);

fs/bcachefs/thread_with_file.c

@@ -294,16 +294,27 @@ static int thread_with_stdio_fn(void *arg)
 	return 0;
 }
 
-int bch2_run_thread_with_stdio(struct thread_with_stdio *thr,
-			       const struct thread_with_stdio_ops *ops)
+void bch2_thread_with_stdio_init(struct thread_with_stdio *thr,
+				 const struct thread_with_stdio_ops *ops)
 {
 	stdio_buf_init(&thr->stdio.input);
 	stdio_buf_init(&thr->stdio.output);
 	thr->ops = ops;
+}
 
+int __bch2_run_thread_with_stdio(struct thread_with_stdio *thr)
+{
 	return bch2_run_thread_with_file(&thr->thr, &thread_with_stdio_fops, thread_with_stdio_fn);
 }
 
+int bch2_run_thread_with_stdio(struct thread_with_stdio *thr,
+			       const struct thread_with_stdio_ops *ops)
+{
+	bch2_thread_with_stdio_init(thr, ops);
+
+	return __bch2_run_thread_with_stdio(thr);
+}
+
 int bch2_run_thread_with_stdout(struct thread_with_stdio *thr,
 				const struct thread_with_stdio_ops *ops)
 {

fs/bcachefs/thread_with_file.h

@@ -63,6 +63,9 @@ struct thread_with_stdio {
 	const struct thread_with_stdio_ops	*ops;
 };
 
+void bch2_thread_with_stdio_init(struct thread_with_stdio *,
+				 const struct thread_with_stdio_ops *);
+int __bch2_run_thread_with_stdio(struct thread_with_stdio *);
 int bch2_run_thread_with_stdio(struct thread_with_stdio *,
 			       const struct thread_with_stdio_ops *);
 int bch2_run_thread_with_stdout(struct thread_with_stdio *,