librbd: prevent creation of v2 image ids that are too large

The librbd API is capped at 24 characters for expressing the
object prefix for data blocks (including trailing null byte).

Fixes: http://tracker.ceph.com/issues/16887
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 4de7c8d)

src/librbd/internal.cc

@@ -1029,6 +1029,14 @@ int validate_pool(IoCtx &io_ctx, CephContext *cct) {
     extra = rand() % 0xFFFFFFFF;
     bid_ss << std::hex << bid << std::hex << extra;
     id = bid_ss.str();
+
+    // ensure the image id won't overflow the fixed block name size
+    const size_t max_id_length = RBD_MAX_BLOCK_NAME_SIZE -
+                                 strlen(RBD_DATA_PREFIX) - 1;
+    if (id.length() > max_id_length) {
+      id = id.substr(id.length() - max_id_length);
+    }
+
     r = cls_client::set_id(&io_ctx, id_obj, id);
     if (r < 0) {
       lderr(cct) << "error setting image id: " << cpp_strerror(r) << dendl;