> vfork and clone do not work at all with sysenter due to user stack issues.
I actually meant CLONE_VFORK, that if used without CLONE_VM can go
through sysenter (if CLONE_VFORK is not used TIF_NEED_RESCHED is set
so there is no problem).

I also think that vfork() could be used with AT_SYSINFO by switching
stacks around the call (with care about recursive vfork and signals
calling vfork inside the vfork stack).