.. mode: -*- rst -*-

Memory protection
=================

:Tag: design.mps.prot
:Author: David Jones
:Date: 1997-04-02
:Status: complete design
:Revision: $Id$
:Copyright: See `Copyright and License`_.
:Index terms: pair: memory protection; design


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

_`.intro`: This is the design of the memory protection module.

_`.readership`: Any MPS developer; anyone porting the MPS to a new
platform.

_`.overview`: The memory protection module ensures that the mutator
sees a consistent view of memory during incremental collection, by
applying protection to areas of memory, ensuring that attempts to read
or write from those areas cause protection faults, and implementing
the means for the MPS to handle these faults.


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

_`.req.consistent`: Must ensure that the mutator sees a consistent
view of memory during incremental collection: in particular, the
mutator must never see objects in oldspace. (Otherwise there's no way
for the MPS to interface with uncooperative code.)

_`.req.prot.read`: Should allow collections to proceed incrementally,
by read-protecting pages that are not consistent from the mutator's
point of view. (This is the only way for the MPS to meet real-time
requirements on pause times.)

_`.req.prot.write`: Should allow the MPS to maintain remembered sets
for segments that it has scanned, by write-protecting pages in these
segments. (This improves performance by allowing the MPS to avoid
scanning these segments again.)

_`.req.fault.handle`: If the module implements protection, it must
also provide a mechanism for handling protection faults. (Otherwise
the MPS cannot take the correct action: that is, fixing references in
a read-protected segment, and discarding the remembered set from a
write-protected segment. See ``TraceSegAccess()``.)

_`.req.prot.exec`: The protection module should allow mutators to
write machine code into memory managed by the MPS and then execute
that code, for example, to implement just-in-time translation, or
other forms of dynamic compilation. Compare
design.mps.vm.req.prot.exec_.

.. _design.mps.vm.req.prot.exec: vm#.req.prot.exec


Design
------

_`.sol.sync`: If memory protection is not available, the only way to
meet `.req.consistent`_ is to ensure that no protection is required,
by running the collector until it has no more incremental work to do.
(This makes it impossible to meet real-time requirements on pause
times, but may be the best that can be done.)

_`.sol.fault.handle`: The protection module handles protection faults
by decoding the context of the fault (see
design.mps.prmc.req.fault.addr_ and design.mps.prmc.req.fault.access_)
and calling ``ArenaAccess()``.

.. _design.mps.prmc.req.fault.addr: prmc#.req.fault.addr
.. _design.mps.prmc.req.fault.access: prmc#.req.fault.access

_`.sol.prot.exec`: The protection module makes memory executable
whenever it is readable by the mutator, if this is supported by the
platform.


Interface
---------

``void ProtSetup(void)``

_`.if.setup`: Called exactly once (per process) as part of the
initialization of the first arena that is created. It must arrange for
the setup and initialization of any data structures or services that
are necessary in order to implement the memory protection module.

``Size ProtGranularity(void)``

_`.if.granularity`: Return the granularity of protection. The ``base``
and ``limit`` arguments to ``ProtSet()`` must be multiples of the
protection granularity.

``void ProtSet(Addr base, Addr limit, AccessSet mode)``

_`.if.set`: Set the protection of the range of memory between ``base``
(inclusive) and ``limit`` (exclusive) to *forbid* the specified modes.
The addresses ``base`` and ``limit`` are multiples of the protection
granularity. The ``mode`` parameter contains the ``AccessWRITE`` bit
if write accesses to the range are to be forbidden, and contains the
``AccessREAD`` bit if read accesses to the range are to be forbidden.

_`.if.set.read`: If the request is to forbid read accesses (that is,
``AccessREAD`` is set) then the implementation may also forbid write
accesses, but read accesses must not be forbidden unless
``AccessREAD`` is set.

_`.if.set.noop`: ``ProtSet()`` is permitted to be a no-op if
``ProtSync()`` is implemented.

``void ProtSync(Arena arena)``

_`.if.sync`: Ensure that the actual protection (as determined by the
operating system) of every segment in the arena matches the segment's
protection mode (``seg->pm``).

_`.if.sync.noop`: ``ProtSync()`` is permitted to be a no-op if
``ProtSet()`` is implemented.


Implementations
---------------

_`.impl.an`: Generic implementation in ``protan.c``.

_`.impl.an.set`: ``ProtSet()`` does nothing.

_`.impl.an.sync`: ``ProtSync()`` has no way of changing the protection
of a segment, so it simulates faults on all segments that are supposed
to be protected, by calling ``TraceSegAccess()``, until it determines
that no segments require protection any more. This forces the trace to
proceed until it is completed, preventing incremental collection.

_`.impl.an.sync.issue`: This relies on the pool actually removing the
protection, otherwise there is an infinite loop here. This is
therefore not compatible with implementations of the protection
mutator context module that support single-stepping of accesses (see design.mps.prmc.req.fault.step_).

.. _design.mps.prmc.req.fault.step: prmc#.req.fault.step

_`.impl.ix`: POSIX implementation. See design.mps.protix_.

.. _design.mps.protix: protix

_`.impl.w3`: Windows implementation.

_`.impl.xc`: macOS implementation.

_`.impl.xc.prot.exec`: The approach in `.sol.prot.exec`_ of always
making memory executable causes a difficulty on macOS on Apple
Silicon. On this platform, programs may enable `Hardened Runtime`_.
This feature rejects attempts to map or protect memory so that it is
simultaneously writable and executable. Moreover, the feature is
enabled by default (as of macOS 13 Ventura), so that if you install
Xcode and then use it to compile the following program, the executable
fails when run with "mmap: Permission denied". ::

    #include <stdio.h>
    #include <sys/mman.h>

    int main(void)
    {
      void *p = mmap(0, 1, PROT_WRITE | PROT_EXEC, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
      if (p == MAP_FAILED) perror("mmap");
      return 0;
    }

.. _Hardened Runtime: https://developer.apple.com/documentation/security/hardened_runtime

_`.impl.xc.prot.exec.detect`: The protection module detects Hardened
Runtime if the operating system is macOS, the CPU architecture is
ARM64, a call to ``mprotect()`` fails, the call requested writable and
executable access, and the error code is ``EACCES``.

_`.impl.xc.prot.exec.retry`: To avoid requiring developers who don't
need to allocate executable memory to figure out how to disable
Hardened Runtime, or enable the appropriate entitlement, the
protection module handles the ``EACCES`` error from ``mprotect()`` in
the Hardened Runtime case by retrying without the request for the
memory to be executable, and setting a global variable to prevent the
writable and executable combination being attempted again.


Document History
----------------

- 1997-04-02 David Jones. Incomplete document.

- 2002-06-07 RB_ Converted from MMInfo database design document.

- 2013-05-23 GDR_ Converted to reStructuredText.

- 2014-10-23 GDR_ Move mutator context interface to design.mps.prmc_.
  Bring design up to date.

  .. _design.mps.prmc: prmc

.. _RB: https://www.ravenbrook.com/consultants/rb/
.. _GDR: https://www.ravenbrook.com/consultants/gdr/


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---------------------

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