diff --git a/sys/amd64/linux/linux_ptrace.c b/sys/amd64/linux/linux_ptrace.c
--- a/sys/amd64/linux/linux_ptrace.c
+++ b/sys/amd64/linux/linux_ptrace.c
@@ -540,7 +540,7 @@
 		return (error);
 	}
 
-	iov.iov_len -= len;
+	iov.iov_len = len;
 	error = copyout(&iov, (void *)data, sizeof(iov));
 	if (error != 0) {
 		linux_msg(td, "iov copyout error %d", error);