qcow2: Fix L1 table size after bdrv_snapshot_goto

When loading an internal snapshot whose L1 table is smaller than the current L1
table, the size of the current L1 would be shrunk to the snapshot's L1 size in
memory, but not on disk. This lead to incorrect refcount updates and eventuelly
to image corruption.

Instead of writing the new L1 size to disk, this simply retains the bigger L1
size that is currently in use and makes sure that the unused part is zeroed.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Tested-by: NPhilipp Hahn <hahn@univention.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block/qcow2-snapshot.c

@@ -317,7 +317,8 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
 {
     BDRVQcowState *s = bs->opaque;
     QCowSnapshot *sn;
-    int i, snapshot_index, l1_size2;
+    int i, snapshot_index;
+    int cur_l1_bytes, sn_l1_bytes;
 
     snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
     if (snapshot_index < 0)
@@ -330,14 +331,19 @@ int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
     if (qcow2_grow_l1_table(bs, sn->l1_size, true) < 0)
         goto fail;
 
-    s->l1_size = sn->l1_size;
-    l1_size2 = s->l1_size * sizeof(uint64_t);
+    cur_l1_bytes = s->l1_size * sizeof(uint64_t);
+    sn_l1_bytes = sn->l1_size * sizeof(uint64_t);
+
+    if (cur_l1_bytes > sn_l1_bytes) {
+        memset(s->l1_table + sn->l1_size, 0, cur_l1_bytes - sn_l1_bytes);
+    }
+
     /* copy the snapshot l1 table to the current l1 table */
     if (bdrv_pread(bs->file, sn->l1_table_offset,
-                   s->l1_table, l1_size2) != l1_size2)
+                   s->l1_table, sn_l1_bytes) < 0)
         goto fail;
     if (bdrv_pwrite_sync(bs->file, s->l1_table_offset,
-                    s->l1_table, l1_size2) < 0)
+                    s->l1_table, cur_l1_bytes) < 0)
         goto fail;
     for(i = 0;i < s->l1_size; i++) {
         be64_to_cpus(&s->l1_table[i]);