.. index::
   pair: SunOS 4; protection interface design
   pair: SunOS 4 protection interface; design

.. _design-protsu:


SunOS 4 protection module
=========================

.. mps:prefix:: design.mps.protsu
   pair: SunOS 4; protection interface design
   pair: SunOS 4 protection interface; design


.. warning::

    As of 2013-05-26, the MPS is no longer supported on SunOS, so
    this document is only of historical interest.


Introduction
------------

:mps:tag:`readership` Any MPS developer.

:mps:tag:`intro` This is the design of the SunOS 4 implementation of the
protection module. It is intended to be used only in SunOS 4 (os.su).
It makes use of various services provided by SunOS 4.


Requirements
------------

:mps:tag:`req.general` Required to implement the general protection
interface defined in design.mps.prot.if.*.


Overview
--------

Uses :c:func:`mprotect()`.


Misc
----

:mps:tag:`improve.sig-stack` Currently we do not handle signals on a
separate signal stack. If we handled signals on our own stack then we
could guarantee not to run out of stack while we were handling the
signal. This would be useful (it may even be required). We would have
to use ``sigvec(2)`` rather than ``signal(3)`` (set the :c:macro:`SV_ONSTACK`
flag and use ``sigstack(2)``). This has drawbacks as the signal stack
is not grown automatically, so we would have to to frig the stacks
back if we wanted to pass on the signal to some other handler as that
handler may require arbitrary amounts of stack.

:mps:tag:`improve.sigvec` Note 1 of :c:func:`ProtSetup()` notes that we can't
honour the ``sigvec(2)`` entries of the next handler in the chain.
What if when we want to pass on the signal instead of calling the
handler we call :c:func:`sigvec()` with the old entry and use kill to send
the signal to ourselves and then restore our handler using sigvec
again.


Data structures
---------------

:mps:tag:`data.signext` This is static. Because that is the only
communications channel available to signal handlers. [write a little
more here]


Functions
---------

:mps:tag:`fun.setup` :c:func:`ProtSetup()`. The setup involves installing a signal
handler for the signal :c:macro:`SIGSEGV` to catch and handle protection
faults (this handler is the function :c:func:`sigHandle()`). The previous
handler is recorded (in the variable ``sigNext``, see
:mps:ref:`.data.signext`) so that it can be reached from :c:func:`sigHandle()` if it
fails to handle the fault.

The problem with this approach is that we can't honor the wishes of the 
``sigvec(2)`` entry for the previous handler (in terms of masks in particular).

Obviously it would be okay to always chain the previous signal handler
onto ``sigNext``, however in the case where the previous handler is
the one we've just installed (that is, ``sigHandle``) then it is not
necessary to chain the handler, so we don't.

:mps:tag:`fun.set` :c:func:`ProtSet()`

:mps:tag:`fun.set.convert` The requested protection (which is expressed in
the mode parameter, see design.mps.prot.if.set) is translated into an
operating system protection. If read accesses are to be forbidden then
all accesses are forbidden, this is done by setting the protection of
the page to :c:macro:`PROT_NONE`. If write access are to be forbidden (and
not read accesses) then write accesses are forbidden and read accesses
are allowed, this is done by setting the protection of the page to
``PROT_READ | PROT_EXEC``. Otherwise (all access are okay), the
protection is set to ``PROT_READ | PROT_WRITE | PROT_EXEC``.

:mps:tag:`fun.set.assume.mprotect` We assume that the call to :c:func:`mprotect()`
always succeeds. This is because we should always call the function
with valid arguments (aligned, references to mapped pages, and with an
access that is compatible with the access of the underlying object).

:mps:tag:`fun.sync` :c:func:`ProtSync()`. This does nothing in this implementation
as ProtSet sets the protection without any delay.