qcow2: Fix error path in qcow2_snapshot_load_tmp

If the bdrv_read() of the snapshot's L1 table fails, return the right
error code and make sure that the old L1 table is still loaded and we
don't break the BlockDriverState completely.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

block/qcow2-snapshot.c

@@ -573,32 +573,42 @@ int qcow2_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_tab)
 
 int qcow2_snapshot_load_tmp(BlockDriverState *bs, const char *snapshot_name)
 {
-    int i, snapshot_index, l1_size2;
+    int i, snapshot_index;
     BDRVQcowState *s = bs->opaque;
     QCowSnapshot *sn;
+    uint64_t *new_l1_table;
+    int new_l1_bytes;
+    int ret;
 
+    assert(bs->read_only);
+
+    /* Search the snapshot */
     snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_name);
     if (snapshot_index < 0) {
         return -ENOENT;
     }
-
     sn = &s->snapshots[snapshot_index];
-    s->l1_size = sn->l1_size;
-    l1_size2 = s->l1_size * sizeof(uint64_t);
-    if (s->l1_table != NULL) {
-        g_free(s->l1_table);
-    }
 
-    s->l1_table_offset = sn->l1_table_offset;
-    s->l1_table = g_malloc0(align_offset(l1_size2, 512));
+    /* Allocate and read in the snapshot's L1 table */
+    new_l1_bytes = s->l1_size * sizeof(uint64_t);
+    new_l1_table = g_malloc0(align_offset(new_l1_bytes, 512));
 
-    if (bdrv_pread(bs->file, sn->l1_table_offset,
-                   s->l1_table, l1_size2) != l1_size2) {
-        return -1;
+    ret = bdrv_pread(bs->file, sn->l1_table_offset, new_l1_table, new_l1_bytes);
+    if (ret < 0) {
+        g_free(new_l1_table);
+        return ret;
     }
 
+    /* Switch the L1 table */
+    g_free(s->l1_table);
+
+    s->l1_size = sn->l1_size;
+    s->l1_table_offset = sn->l1_table_offset;
+    s->l1_table = new_l1_table;
+
     for(i = 0;i < s->l1_size; i++) {
         be64_to_cpus(&s->l1_table[i]);
     }
+
     return 0;
 }