* Michael Kerrisk:

       Another  difference  for  the  raw clone() system call is that the
       stack argument may be NULL, in which case the child uses a  dupli‐
       cate  of the parent's stack.  (Copy-on-write semantics ensure that
       the child gets separate copies of stack pages when either  process
       modifies  the  stack.)   In  this case, for correct operation, the
       CLONE_VM option should not be specified.  (If the child shares the
       parent's  memory  because of the use of the CLONE_VM flag, then no
       copy-on-write duplication occurs and chaos is likely to result.)

I think sharing the stack also works with CLONE_VFORK with CLONE_VM, as
long as measures are taken to preserve the return address in a register.

Thanks,
Florian