bcachefs: fix bch2_journal_keys_peek_prev_min() underflow

Signed-off-by: Kent Overstreet <[email protected]>

Author: Kent Overstreet

fs/bcachefs/btree_journal_iter.c

@@ -137,12 +137,15 @@ struct bkey_i *bch2_journal_keys_peek_prev_min(struct bch_fs *c, enum btree_id b
 	struct journal_key *k;
 
 	BUG_ON(*idx > keys->nr);
+
+	if (!keys->nr)
+		return NULL;
 search:
 	if (!*idx)
 		*idx = __bch2_journal_key_search(keys, btree_id, level, pos);
 
 	while (*idx < keys->nr &&
-	       __journal_key_cmp(btree_id, level, end_pos, idx_to_key(keys, *idx - 1)) >= 0) {
+	       __journal_key_cmp(btree_id, level, end_pos, idx_to_key(keys, *idx)) >= 0) {
 		(*idx)++;
 		iters++;
 		if (iters == 10) {
@@ -151,18 +154,23 @@ struct bkey_i *bch2_journal_keys_peek_prev_min(struct bch_fs *c, enum btree_id b
 		}
 	}
 
+	if (*idx == keys->nr)
+		--(*idx);
+
 	struct bkey_i *ret = NULL;
 	rcu_read_lock(); /* for overwritten_ranges */
 
-	while ((k = *idx < keys->nr ? idx_to_key(keys, *idx) : NULL)) {
+	while (true) {
+		k = idx_to_key(keys, *idx);
 		if (__journal_key_cmp(btree_id, level, end_pos, k) > 0)
 			break;
 
 		if (k->overwritten) {
 			if (k->overwritten_range)
-				*idx = rcu_dereference(k->overwritten_range)->start - 1;
-			else
-				*idx -= 1;
+				*idx = rcu_dereference(k->overwritten_range)->start;
+			if (!*idx)
+				break;
+			--(*idx);
 			continue;
 		}
 
@@ -171,6 +179,8 @@ struct bkey_i *bch2_journal_keys_peek_prev_min(struct bch_fs *c, enum btree_id b
 			break;
 		}
 
+		if (!*idx)
+			break;
 		--(*idx);
 		iters++;
 		if (iters == 10) {