In persistent_prepare(), the "enough-room-left" check only verifies that the
start of the next-free chunk is within the cow device. If the cow device
is not an even multiple of the chunk-size, the snapshot code will eventually
try to write off the end of the cow device.

Check that the entire next-free chunk is within the cow device. 
[Kevin Corry]

--- diff/drivers/md/dm-exception-store.c	2003-02-25 14:44:36.000000000 +0000
+++ source/drivers/md/dm-exception-store.c	2003-03-27 09:40:03.000000000 +0000
@@ -449,7 +449,7 @@
 	sector_t size = get_dev_size(store->snap->cow->dev);
 
 	/* Is there enough room ? */
-	if (size <= (ps->next_free * store->snap->chunk_size))
+	if (size < ((ps->next_free + 1) * store->snap->chunk_size))
 		return -ENOSPC;
 
 	e->new_chunk = ps->next_free;