By default signal handlers don't use a separate stack; they use the same stack as the process/thread that received the signal. A process/thread must call sigalt

By default signal handlers don't use a separate stack; they use the same stack as the process/thread that received the signal. A process/thread must call sigaltstack to install an alternative stack for signal handler execution. So to answer the previous poster's question: yes, AFAICT this bug could have effected userspace code unless there are some other nuances at play.