Joachim Durchholz wrote: > A concrete example: the > first thing that Windows does when accepting userland data structures > is... to copy them; this were unnecessary if the structures were immutable.
It is necessary also because the userland structures are in a different privilege domain, and the memory backing them is potentially mapped by other processes, or accessible by other threads of this process, that can be running in parallel while the current thread is in the kernel. Also, it tends to be easier and more secure to treat userland structures as if they were in a different address space (even if for a particular OS, the kernel would be able to access user mode addresses directly). The kernel cannot trust that pointers in userland structures are valid, for instance. So, in order to avoid a copy when accepting a userland structure: - when servicing a kernel call for a given process, the OS must use kernel-mode page table mappings that are a superset of the user-mode mappings for that process at the point of the call. - the memory backing the structure must be immutable in all processes that could map it, and the kernel must trust the means by which it is made immutable. -- David Hopwood <[EMAIL PROTECTED]> -- http://mail.python.org/mailman/listinfo/python-list
