bcachefs: Add checks for invalid snapshot IDs

Previously, we assumed that keys were consistent with the snapshots
btree - but that's not correct as fsck may not have been run or may not
be complete.

This adds checks and error handling when using the in-memory snapshots
table (that mirrors the snapshots btree).

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 6 insertions, 2 deletions

diff --git a/fs/bcachefs/snapshot.c b/fs/bcachefs/snapshot.c
index 9cd71e613dc9..4e074136c490 100644
--- a/fs/bcachefs/snapshot.c
+++ b/fs/bcachefs/snapshot.c
@@ -93,8 +93,10 @@ static int bch2_snapshot_tree_create(struct btree_trans *trans,
 
 static bool __bch2_snapshot_is_ancestor_early(struct snapshot_table *t, u32 id, u32 ancestor)
 {
-	while (id && id < ancestor)
-		id = __snapshot_t(t, id)->parent;
+	while (id && id < ancestor) {
+		const struct snapshot_t *s = __snapshot_t(t, id);
+		id = s ? s->parent : 0;
+	}
 	return id == ancestor;
 }
 
@@ -110,6 +112,8 @@ static bool bch2_snapshot_is_ancestor_early(struct bch_fs *c, u32 id, u32 ancest
 static inline u32 get_ancestor_below(struct snapshot_table *t, u32 id, u32 ancestor)
 {
 	const struct snapshot_t *s = __snapshot_t(t, id);
+	if (!s)
+		return 0;
 
 	if (s->skip[2] <= ancestor)
 		return s->skip[2];