block/vdi: Don't assume that blocks are larger than VdiHeader

Given that the block size is read from the header of the VDI file, a
wide variety of sizes might be seen. Rather than re-using a block
sized memory region when writing the VDI header, allocate an
appropriately sized buffer.

Signed-off-by: David Edmondson <david.edmondson@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Max Reitz <mreitz@redhat.com>
Message-id: 20210325112941.365238-3-pbonzini@redhat.com
Message-Id: <20210309144015.557477-3-david.edmondson@oracle.com>
Acked-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/vdi.c

@@ -696,18 +696,20 @@ vdi_co_pwritev(BlockDriverState *bs, uint64_t offset, uint64_t bytes,
 
     if (block) {
         /* One or more new blocks were allocated. */
-        VdiHeader *header = (VdiHeader *) block;
+        VdiHeader *header;
         uint8_t *base;
         uint64_t offset;
         uint32_t n_sectors;
 
+        g_free(block);
+        header = g_malloc(sizeof(*header));
+
         logout("now writing modified header\n");
         assert(VDI_IS_ALLOCATED(bmap_first));
         *header = s->header;
         vdi_header_to_le(header);
-        ret = bdrv_pwrite(bs->file, 0, block, sizeof(VdiHeader));
-        g_free(block);
-        block = NULL;
+        ret = bdrv_pwrite(bs->file, 0, header, sizeof(*header));
+        g_free(header);
 
         if (ret < 0) {
             return ret;