diff --git a/block.c b/block.c
index 8416376c9b..6dbcb7e083 100644
--- a/block.c
+++ b/block.c
@@ -6809,8 +6809,11 @@ void bdrv_refresh_filename(BlockDriverState *bs)
         pstrcpy(bs->filename, sizeof(bs->filename), bs->exact_filename);
     } else {
         QString *json = qobject_to_json(QOBJECT(bs->full_open_options));
-        snprintf(bs->filename, sizeof(bs->filename), "json:%s",
-                 qstring_get_str(json));
+        if (snprintf(bs->filename, sizeof(bs->filename), "json:%s",
+                     qstring_get_str(json)) >= sizeof(bs->filename)) {
+            /* Give user a hint if we truncated things. */
+            strcpy(bs->filename + sizeof(bs->filename) - 4, "...");
+        }
         qobject_unref(json);
     }
 }
diff --git a/block/nbd.c b/block/nbd.c
index 4ac23c8f62..eed160c5cd 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1984,6 +1984,7 @@ static void nbd_refresh_filename(BlockDriverState *bs)
 {
     BDRVNBDState *s = bs->opaque;
     const char *host = NULL, *port = NULL, *path = NULL;
+    size_t len = 0;
 
     if (s->saddr->type == SOCKET_ADDRESS_TYPE_INET) {
         const InetSocketAddress *inet = &s->saddr->u.inet;
@@ -1996,17 +1997,21 @@ static void nbd_refresh_filename(BlockDriverState *bs)
     } /* else can't represent as pseudo-filename */
 
     if (path && s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd+unix:///%s?socket=%s", s->export, path);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd+unix:///%s?socket=%s", s->export, path);
     } else if (path && !s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd+unix://?socket=%s", path);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd+unix://?socket=%s", path);
     } else if (host && s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd://%s:%s/%s", host, port, s->export);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd://%s:%s/%s", host, port, s->export);
     } else if (host && !s->export) {
-        snprintf(bs->exact_filename, sizeof(bs->exact_filename),
-                 "nbd://%s:%s", host, port);
+        len = snprintf(bs->exact_filename, sizeof(bs->exact_filename),
+                       "nbd://%s:%s", host, port);
+    }
+    if (len > sizeof(bs->exact_filename)) {
+        /* Name is too long to represent exactly, so leave it empty. */
+        bs->exact_filename[0] = '\0';
     }
 }