nbd/server: Use real permissions for NBD exports

NBD can't cope with device size changes, so resize must be forbidden,
but otherwise we can tolerate anything. Depending on whether the export
is writable or not, we only require consistent reads and writes.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Acked-by: NFam Zheng <famz@redhat.com>

nbd/server.c

@@ -891,10 +891,17 @@ NBDExport *nbd_export_new(BlockDriverState *bs, off_t dev_offset, off_t size,
 {
     BlockBackend *blk;
     NBDExport *exp = g_malloc0(sizeof(NBDExport));
+    uint64_t perm;
     int ret;
 
-    /* FIXME Use real permissions */
-    blk = blk_new(0, BLK_PERM_ALL);
+    /* Don't allow resize while the NBD server is running, otherwise we don't
+     * care what happens with the node. */
+    perm = BLK_PERM_CONSISTENT_READ;
+    if ((nbdflags & NBD_FLAG_READ_ONLY) == 0) {
+        perm |= BLK_PERM_WRITE;
+    }
+    blk = blk_new(perm, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED |
+                        BLK_PERM_WRITE | BLK_PERM_GRAPH_MOD);
     ret = blk_insert_bs(blk, bs, errp);
     if (ret < 0) {
         goto fail;