bcachefs: Fix a buffer overrun in bch2_fs_usage_read()

We were copying the size of a struct bch_fs_usage_online to a struct
bch_fs_usage, which is 8 bytes smaller.

This adds some new helpers so we can do this correctly, and get rid of
some magic +1s too.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets.c

@@ -137,17 +137,17 @@ u64 bch2_fs_usage_read_one(struct bch_fs *c, u64 *v)
 struct bch_fs_usage_online *bch2_fs_usage_read(struct bch_fs *c)
 {
 	struct bch_fs_usage_online *ret;
-	unsigned seq, i, v, u64s = fs_usage_u64s(c) + 1;
+	unsigned nr_replicas = READ_ONCE(c->replicas.nr);
+	unsigned seq, i;
 retry:
-	ret = kmalloc(u64s * sizeof(u64), GFP_NOFS);
+	ret = kmalloc(__fs_usage_online_u64s(nr_replicas) * sizeof(u64), GFP_NOFS);
 	if (unlikely(!ret))
 		return NULL;
 
 	percpu_down_read(&c->mark_lock);
 
-	v = fs_usage_u64s(c) + 1;
-	if (unlikely(u64s != v)) {
-		u64s = v;
+	if (nr_replicas != c->replicas.nr) {
+		nr_replicas = c->replicas.nr;
 		percpu_up_read(&c->mark_lock);
 		kfree(ret);
 		goto retry;
@@ -157,10 +157,12 @@ struct bch_fs_usage_online *bch2_fs_usage_read(struct bch_fs *c)
 
 	do {
 		seq = read_seqcount_begin(&c->usage_lock);
-		unsafe_memcpy(&ret->u, c->usage_base, u64s * sizeof(u64),
+		unsafe_memcpy(&ret->u, c->usage_base,
+			      __fs_usage_u64s(nr_replicas) * sizeof(u64),
 			      "embedded variable length struct");
 		for (i = 0; i < ARRAY_SIZE(c->usage); i++)
-			acc_u64s_percpu((u64 *) &ret->u, (u64 __percpu *) c->usage[i], u64s);
+			acc_u64s_percpu((u64 *) &ret->u, (u64 __percpu *) c->usage[i],
+					__fs_usage_u64s(nr_replicas));
 	} while (read_seqcount_retry(&c->usage_lock, seq));
 
 	return ret;

fs/bcachefs/buckets.h

@@ -207,10 +207,24 @@ static inline u64 dev_buckets_available(struct bch_dev *ca,
 
 /* Filesystem usage: */
 
+static inline unsigned __fs_usage_u64s(unsigned nr_replicas)
+{
+	return sizeof(struct bch_fs_usage) / sizeof(u64) + nr_replicas;
+}
+
 static inline unsigned fs_usage_u64s(struct bch_fs *c)
 {
-	return sizeof(struct bch_fs_usage) / sizeof(u64) +
-		READ_ONCE(c->replicas.nr);
+	return __fs_usage_u64s(READ_ONCE(c->replicas.nr));
+}
+
+static inline unsigned __fs_usage_online_u64s(unsigned nr_replicas)
+{
+	return sizeof(struct bch_fs_usage_online) / sizeof(u64) + nr_replicas;
+}
+
+static inline unsigned fs_usage_online_u64s(struct bch_fs *c)
+{
+	return __fs_usage_online_u64s(READ_ONCE(c->replicas.nr));
 }
 
 static inline unsigned dev_usage_u64s(void)