bcachefs: Move snapshot table size to struct snapshot_table

We need to add bounds checking for snapshot table accesses - it turns
out there are cases where we do need to use the snapshots table before
fsck checks have completed (and indeed, fsck may not have been run).
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -810,7 +810,6 @@ struct bch_fs {
 
 	/* snapshot.c: */
 	struct snapshot_table __rcu *snapshots;
-	size_t			snapshot_table_size;
 	struct mutex		snapshot_table_lock;
 	struct rw_semaphore	snapshot_create_lock;
 

fs/bcachefs/snapshot.c

@@ -151,36 +151,39 @@ bool __bch2_snapshot_is_ancestor(struct bch_fs *c, u32 id, u32 ancestor)
 static noinline struct snapshot_t *__snapshot_t_mut(struct bch_fs *c, u32 id)
 {
 	size_t idx = U32_MAX - id;
-	size_t new_size;
 	struct snapshot_table *new, *old;
 
-	new_size = max(16UL, roundup_pow_of_two(idx + 1));
+	size_t new_bytes = kmalloc_size_roundup(struct_size(new, s, idx + 1));
+	size_t new_size = (new_bytes - sizeof(*new)) / sizeof(new->s[0]);
 
-	new = kvzalloc(struct_size(new, s, new_size), GFP_KERNEL);
+	new = kvzalloc(new_bytes, GFP_KERNEL);
 	if (!new)
 		return NULL;
 
+	new->nr = new_size;
+
 	old = rcu_dereference_protected(c->snapshots, true);
 	if (old)
-		memcpy(new->s,
-		       rcu_dereference_protected(c->snapshots, true)->s,
-		       sizeof(new->s[0]) * c->snapshot_table_size);
+		memcpy(new->s, old->s, sizeof(old->s[0]) * old->nr);
 
 	rcu_assign_pointer(c->snapshots, new);
-	c->snapshot_table_size = new_size;
-	kvfree_rcu_mightsleep(old);
+	kvfree_rcu(old, rcu);
 
-	return &rcu_dereference_protected(c->snapshots, true)->s[idx];
+	return &rcu_dereference_protected(c->snapshots,
+				lockdep_is_held(&c->snapshot_table_lock))->s[idx];
 }
 
 static inline struct snapshot_t *snapshot_t_mut(struct bch_fs *c, u32 id)
 {
 	size_t idx = U32_MAX - id;
+	struct snapshot_table *table =
+		rcu_dereference_protected(c->snapshots,
+				lockdep_is_held(&c->snapshot_table_lock));
 
 	lockdep_assert_held(&c->snapshot_table_lock);
 
-	if (likely(idx < c->snapshot_table_size))
-		return &rcu_dereference_protected(c->snapshots, true)->s[idx];
+	if (likely(table && idx < table->nr))
+		return &table->s[idx];
 
 	return __snapshot_t_mut(c, id);
 }

fs/bcachefs/snapshot.h

@@ -33,7 +33,11 @@ int bch2_mark_snapshot(struct btree_trans *, enum btree_id, unsigned,
 
 static inline struct snapshot_t *__snapshot_t(struct snapshot_table *t, u32 id)
 {
-	return &t->s[U32_MAX - id];
+	u32 idx = U32_MAX - id;
+
+	return likely(t && idx < t->nr)
+		? &t->s[idx]
+		: NULL;
 }
 
 static inline const struct snapshot_t *snapshot_t(struct bch_fs *c, u32 id)

fs/bcachefs/subvolume_types.h

@@ -20,6 +20,8 @@ struct snapshot_t {
 };
 
 struct snapshot_table {
+	struct rcu_head		rcu;
+	size_t			nr;
 #ifndef RUST_BINDGEN
 	DECLARE_FLEX_ARRAY(struct snapshot_t, s);
 #else