--- /n/sources/plan9/sys/src/cmd/unix/u9fs/authp9any.c	Thu Sep  2 06:00:11 2004
+++ /sys/src/cmd/unix/u9fs/authp9any.c	Tue Sep 22 00:00:00 2026
@@ -376,7 +376,7 @@
 static int
 readstr(Fcall *rx, Fcall *tx, char *s, int len)
 {
-	if (rx->offset >= len)
+	if (rx->offset < 0 || rx->offset >= len)
 		return 0;
 	tx->count = len - rx->offset;
 	if (tx->count > rx->count)