bcachefs: Fix NULL ptr dereference in btree_node_iter_and_journal_peek

Add NULL check for key returned from bch2_btree_and_journal_iter_peek in
btree_node_iter_and_journal_peek to avoid NULL ptr dereference in
bch2_bkey_buf_reassemble.

When key returned from bch2_btree_and_journal_iter_peek is NULL it means
that btree topology needs repair. Print topology error message with
position at which node wasn't found, its parent node information and
btree_id with level.

Return error code returned by bch2_topology_error to ensure that topology
error is handled properly by recovery.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=005ef9aa519f30d97657
Fixes: 5222a46 ("bcachefs: BTREE_ITER_WITH_JOURNAL")
Suggested-by: Alan Huang <[email protected]>
Suggested-by: Kent Overstreet <[email protected]>
Signed-off-by: Piotr Zalewski <[email protected]>
Signed-off-by: Kent Overstreet <[email protected]>

Author: JungerBoyo

fs/bcachefs/btree_iter.c

@@ -882,13 +882,26 @@ static noinline int btree_node_iter_and_journal_peek(struct btree_trans *trans,
 	__bch2_btree_and_journal_iter_init_node_iter(trans, &jiter, l->b, l->iter, path->pos);
 
 	k = bch2_btree_and_journal_iter_peek(&jiter);
+	if (!k.k) {
+		struct printbuf buf = PRINTBUF;
+
+		prt_str(&buf, "node not found at pos ");
+		bch2_bpos_to_text(&buf, path->pos);
+		prt_str(&buf, " at btree ");
+		bch2_btree_pos_to_text(&buf, c, l->b);
+
+		ret = bch2_fs_topology_error(c, "%s", buf.buf);
+		printbuf_exit(&buf);
+		goto err;
+	}
 
 	bch2_bkey_buf_reassemble(out, c, k);
 
 	if ((flags & BTREE_ITER_prefetch) &&
 	    c->opts.btree_node_prefetch)
 		ret = btree_path_prefetch_j(trans, path, &jiter);
 
+err:
 	bch2_btree_and_journal_iter_exit(&jiter);
 	return ret;
 }