In FreeBSD before 11.1-STABLE, 11.1-RELEASE-p4, 11.0-RELEASE-p15, 10.4-STABLE, 10.4-RELEASE-p3, and 10.3-RELEASE-p24, not all information in the struct ptrace_lwpinfo is relevant for the state of any thread, and the kernel does not fill the irrelevant bytes or short strings. Since the structure filled by the kernel is allocated on the kernel stack and copied to userspace, a leak of information of the kernel stack of the thread is possible from the debugger. As a result, some bytes from the kernel stack of the thread using ptrace call can be observed in userspace.