[zznzz]

It is not required to unwind the user space stack in the NMI handler. It can be done later before returning to user space in a context that can handle faults.

[irogers]

Allowing processes to sniff each others stacks has some fairly obvious security issues.

[zznzz]

I don’t understand your concern - what about this would involve one process sniffing another process’s memory? The kernel would still be doing the unwinding, just not in the NMI handler.