[PATCH v2] bcachefs: Fix NULL ptr dereference in btree_node_iter_and_journal_peek

[Piotr Zalewski]

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 error message with position at
which node wasn't found and its parent node information. Call
bch2_topology_error and return error code returned by it to ensure that
topology error is handled properly.

Reported-by: syzbot+005ef9aa519f30d97657@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=005ef9aa519f30d97657
Fixes: 5222a4607cd8 ("bcachefs: BTREE_ITER_WITH_JOURNAL")
Suggested-by: Alan Huang <mmpgouride@xxxxxxxxx>
Suggested-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Signed-off-by: Piotr Zalewski <pZ010001011111@xxxxxxxxx>
---

Notes:
    changes in v2:
        - make commit message more verbose.
        - set topology error, print error message and return
          appropriate error code.

    link to v1: https://lore.kernel.org/linux-bcachefs/20241023072024.98915-3-pZ010001011111@xxxxxxxxx/

 fs/bcachefs/btree_iter.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/bcachefs/btree_iter.c b/fs/bcachefs/btree_iter.c
index 15ac72b1af51..40c824779b15 100644
--- a/fs/bcachefs/btree_iter.c
+++ b/fs/bcachefs/btree_iter.c
@@ -880,6 +880,18 @@ 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, " within parent node ");
+		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(&l->b->key));
+
+		ret = bch2_fs_topology_error(c, "%s", buf.buf);
+		printbuf_exit(&buf);
+		goto err;
+	}
 
 	bch2_bkey_buf_reassemble(out, c, k);
 
@@ -887,6 +899,7 @@ static noinline int btree_node_iter_and_journal_peek(struct btree_trans *trans,
 	    c->opts.btree_node_prefetch)
 		ret = btree_path_prefetch_j(trans, path, &jiter);
 
+err:
 	bch2_btree_and_journal_iter_exit(&jiter);
 	return ret;
 }
-- 
2.47.0