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.
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.
Yes, this only works for user space stacks, but that is sufficient since with ORC kernel stacks are solved (IMO) and it avoids all the issues with trying to mlock debuginfo of all processes that you mentioned. The NMI handler would still unwind the kernel stack.
> Why not implement your approach and mail it to LKML :-)
because this would still be an in-kernel dwarf unwinder and I would expect an instant reject, and because I am lazy and/or don’t care enough about this problem or linux to work on it. Even if people could be persuaded, I don’t have the interest or temperance to debate this with LKML.