VMDK: add field BDRVVmdkState.desc_offset

There are several occurrence of magic number 0x200 as the descriptor
offset within mono sparse image file. This is not the case for images
with separate descriptor file. So a field is added to BDRVVmdkState to
hold the correct value.
Signed-off-by: NFam Zheng <famcool@gmail.com>
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vmdk.c

@@ -81,6 +81,7 @@ typedef struct VmdkExtent {
 } VmdkExtent;
 
 typedef struct BDRVVmdkState {
+    int desc_offset;
     uint32_t parent_cid;
     int num_extents;
     /* Extent array with num_extents entries, ascend ordered by address */
@@ -175,10 +176,11 @@ static uint32_t vmdk_read_cid(BlockDriverState *bs, int parent)
     uint32_t cid;
     const char *p_name, *cid_str;
     size_t cid_str_size;
+    BDRVVmdkState *s = bs->opaque;
 
-    /* the descriptor offset = 0x200 */
-    if (bdrv_pread(bs->file, 0x200, desc, DESC_SIZE) != DESC_SIZE)
+    if (bdrv_pread(bs->file, s->desc_offset, desc, DESC_SIZE) != DESC_SIZE) {
         return 0;
+    }
 
     if (parent) {
         cid_str = "parentCID";
@@ -200,10 +202,12 @@ static int vmdk_write_cid(BlockDriverState *bs, uint32_t cid)
 {
     char desc[DESC_SIZE], tmp_desc[DESC_SIZE];
     char *p_name, *tmp_str;
+    BDRVVmdkState *s = bs->opaque;
 
-    /* the descriptor offset = 0x200 */
-    if (bdrv_pread(bs->file, 0x200, desc, DESC_SIZE) != DESC_SIZE)
-        return -1;
+    memset(desc, 0, sizeof(desc));
+    if (bdrv_pread(bs->file, s->desc_offset, desc, DESC_SIZE) != DESC_SIZE) {
+        return -EIO;
+    }
 
     tmp_str = strstr(desc,"parentCID");
     pstrcpy(tmp_desc, sizeof(tmp_desc), tmp_str);
@@ -213,8 +217,9 @@ static int vmdk_write_cid(BlockDriverState *bs, uint32_t cid)
         pstrcat(desc, sizeof(desc), tmp_desc);
     }
 
-    if (bdrv_pwrite_sync(bs->file, 0x200, desc, DESC_SIZE) < 0)
-        return -1;
+    if (bdrv_pwrite_sync(bs->file, s->desc_offset, desc, DESC_SIZE) < 0) {
+        return -EIO;
+    }
     return 0;
 }
 
@@ -402,10 +407,11 @@ static int vmdk_parent_open(BlockDriverState *bs)
 {
     char *p_name;
     char desc[DESC_SIZE];
+    BDRVVmdkState *s = bs->opaque;
 
-    /* the descriptor offset = 0x200 */
-    if (bdrv_pread(bs->file, 0x200, desc, DESC_SIZE) != DESC_SIZE)
+    if (bdrv_pread(bs->file, s->desc_offset, desc, DESC_SIZE) != DESC_SIZE) {
         return -1;
+    }
 
     if ((p_name = strstr(desc,"parentFileNameHint")) != NULL) {
         char *end_name;
@@ -506,8 +512,10 @@ static int vmdk_open_vmdk3(BlockDriverState *bs, int flags)
     int ret;
     uint32_t magic;
     VMDK3Header header;
+    BDRVVmdkState *s = bs->opaque;
     VmdkExtent *extent;
 
+    s->desc_offset = 0x200;
     ret = bdrv_pread(bs->file, sizeof(magic), &header, sizeof(header));
     if (ret < 0) {
         goto fail;
@@ -539,6 +547,7 @@ static int vmdk_open_vmdk4(BlockDriverState *bs, int flags)
     BDRVVmdkState *s = bs->opaque;
     VmdkExtent *extent;
 
+    s->desc_offset = 0x200;
     ret = bdrv_pread(bs->file, sizeof(magic), &header, sizeof(header));
     if (ret < 0) {
         goto fail;