Memory Protection project interface details (GSoC 2020)

[Utkarsh Rai]

Hello,
This is to ensure that all the interested parties are on the same page
before I start my project and can give their invaluable feedback.
My GSoC project, providing user-configurable thread stack protection,
requires adding architecture-specific low-level support as well as
high-level API support. I will be starting my project with ARMv7-A (on BBB)
based MMU since RTEMS already has quite mature support for it. As already
mentioned in my proposal I will be focusing more on the High-level
interface and let it drive whatever further low-level support is needed.
Once the application uses MMU for thread stack address generation each
thread will be automatically protected as the page tables other than that
of the executing thread would be made dormant. When the user has to share
thread stacks they will have to obtain the stack attributes of the threads
to be shared by pthread_attr_getstack() and then get a file descriptor of
the memory to be mapped by a call to shm_open() and finally map this to the
stack of the other thread through
mmap(), this is the POSIX compliant way I could think of. Now at the low
level, it means mapping the page table of the thread to be shared into the
address space of the executing thread. This is an area where the low-level
support has to be provided. At the high-level, this means providing support
to mmap and shared-memory interface as mmap provides support for a file by
simply
copying the memory from the file to the destination. For shared memory
objects it can
provide read/write access but cannot provide restriction of write/read
access. One of the areas that I have to look into more detail is thread
context-switch, as after every context switch the TLBs need to be flushed
and reinitialized lest we get an invalid address for the executing thread.
Since context-switch is low-level architecture-specific, this also has to
be provided with more support.
Kindly provide your feedback if I have missed something or I have a wrong
idea about it.

Regards,
Utkarsh Rai.
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