bcachefs: Annotate bch_replicas_entry_{v0,v1} with __counted_by()

Add the __counted_by compiler attribute to the flexible array members
devs to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Increment nr_devs before adding a new device to the devs array and
adjust the array indexes accordingly. Add a helper macro for adding a
new device.

In bch2_journal_read(), explicitly set nr_devs to 0.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets.c

@@ -741,7 +741,7 @@ static int __trigger_extent(struct btree_trans *trans,
 				return ret;
 		} else if (!p.has_ec) {
 			*replicas_sectors       += disk_sectors;
-			acc_replicas_key.replicas.devs[acc_replicas_key.replicas.nr_devs++] = p.ptr.dev;
+			replicas_entry_add_dev(&acc_replicas_key.replicas, p.ptr.dev);
 		} else {
 			ret = bch2_trigger_stripe_ptr(trans, k, p, data_type, disk_sectors, flags);
 			if (ret)

fs/bcachefs/journal_io.c

@@ -1353,6 +1353,7 @@ int bch2_journal_read(struct bch_fs *c,
 	genradix_for_each(&c->journal_entries, radix_iter, _i) {
 		struct bch_replicas_padded replicas = {
 			.e.data_type = BCH_DATA_journal,
+			.e.nr_devs = 0,
 			.e.nr_required = 1,
 		};
 
@@ -1379,7 +1380,7 @@ int bch2_journal_read(struct bch_fs *c,
 			goto err;
 
 		darray_for_each(i->ptrs, ptr)
-			replicas.e.devs[replicas.e.nr_devs++] = ptr->dev;
+			replicas_entry_add_dev(&replicas.e, ptr->dev);
 
 		bch2_replicas_entry_sort(&replicas.e);
 

fs/bcachefs/replicas.c

@@ -123,7 +123,7 @@ static void extent_to_replicas(struct bkey_s_c k,
 			continue;
 
 		if (!p.has_ec)
-			r->devs[r->nr_devs++] = p.ptr.dev;
+			replicas_entry_add_dev(r, p.ptr.dev);
 		else
 			r->nr_required = 0;
 	}
@@ -140,7 +140,7 @@ static void stripe_to_replicas(struct bkey_s_c k,
 	for (ptr = s.v->ptrs;
 	     ptr < s.v->ptrs + s.v->nr_blocks;
 	     ptr++)
-		r->devs[r->nr_devs++] = ptr->dev;
+		replicas_entry_add_dev(r, ptr->dev);
 }
 
 void bch2_bkey_to_replicas(struct bch_replicas_entry_v1 *e,
@@ -181,7 +181,7 @@ void bch2_devlist_to_replicas(struct bch_replicas_entry_v1 *e,
 	e->nr_required	= 1;
 
 	darray_for_each(devs, i)
-		e->devs[e->nr_devs++] = *i;
+		replicas_entry_add_dev(e, *i);
 
 	bch2_replicas_entry_sort(e);
 }

fs/bcachefs/replicas_format.h

@@ -5,7 +5,7 @@
 struct bch_replicas_entry_v0 {
 	__u8			data_type;
 	__u8			nr_devs;
-	__u8			devs[];
+	__u8			devs[] __counted_by(nr_devs);
 } __packed;
 
 struct bch_sb_field_replicas_v0 {
@@ -17,7 +17,7 @@ struct bch_replicas_entry_v1 {
 	__u8			data_type;
 	__u8			nr_devs;
 	__u8			nr_required;
-	__u8			devs[];
+	__u8			devs[] __counted_by(nr_devs);
 } __packed;
 
 struct bch_sb_field_replicas {
@@ -28,4 +28,9 @@ struct bch_sb_field_replicas {
 #define replicas_entry_bytes(_i)					\
 	(offsetof(typeof(*(_i)), devs) + (_i)->nr_devs)
 
+#define replicas_entry_add_dev(e, d) ({					\
+	(e)->nr_devs++;							\
+	(e)->devs[(e)->nr_devs - 1] = (d);				\
+})
+
 #endif /* _BCACHEFS_REPLICAS_FORMAT_H */