Discussion regarding high-level interface for GSoC memory protection project.

[Utkarsh Rai]

Got it, I will modify my proposal draft accordingly.

On Sat, Mar 28, 2020 at 7:12 PM Gedare Bloom <gedare at rtems.org> wrote:

> On Sat, Mar 28, 2020 at 4:59 AM Peter Dufault <dufault at hda.com> wrote:
> >
> >
> >
> > > On Mar 27, 2020, at 14:16 , Utkarsh Rai <utkarsh.rai60 at gmail.com>
> wrote:
> > >
> > >
> > > On Fri, Mar 27, 2020 at 9:31 PM Gedare Bloom <gedare at rtems.org> wrote:
> > > Hi Utkarsh,
> > >
> > > You can remove "Discussion regarding" from your subject lines. We know
> > > your emails are discussions regarding the subject.
> > >
> > > On Fri, Mar 27, 2020 at 5:26 AM Utkarsh Rai <utkarsh.rai60 at gmail.com>
> wrote:
> > > >
> > > > Hi,
> > > > My GSoC project proposal intended for providing thread-stack
> protection involves implementation on two levels, providing low-level
> hardware support for the target architecture and high-level
> architecture-independent APIs. As @Peter Dufault pointed to me in my draft
> the POSIX  compliant way of doing it would be through mmap, I would request
> your feedback on the details of the high-level implementation of
> thread-stack protection.
> > > > My idea is to obtain the stack attributes of the thread that is to
> be mapped by pthread_attr_getstack() and then get a file descriptor of the
> memory using posix_typed_mem_open() and finally mmap that to the stack of
> the required thread(With the specified permissions).
> > > >   Is this is a valid approach? If yes, I believe I would have to add
> the implementation of       posix_typed_mem_open() to my work plan as RTEMS
> does not support it as of now.
> > > >
> > >
> > > That's an interesting proposition. I guess you are suggesting to make
> > > thread stacks be "typed memory objects"? I don't know the
> > > ramifications of that, but it sounds like a really deep design and
> > > implementation challenge. It's not clear to me that "typed_mem_open"
> > > is proper to call on an existing typed object, but I'm not that
> > > familiar with the TYM interface. It could be something worth fleshing
> > > out though as a summer implementation project if there is plenty of
> > > work to do. It could be something for extension activities.
> > >
> > > I think however you could instead use shared memory objects, which
> > > already have some (limited) support, to accomplish the same ideas. You
> > > could give each thread's stack a "named" object in some filesystem,
> > > and other threads could shm_open() and mmap() the stack. I think that
> > > is the right way to go at least based on where we are in RTEMS now.
> > >
> > > You should also know and understand that the mmap() interface in RTEMS
> > > is quite shallow and restricted in its support. For file objects it
> > > basically only works to provide a copy of the file, because it works
> > > by copying the memory from the file to the destination. For shared
> > > memory objects it can provide rw access between two threads, but can't
> > > restrict access since we lack general protection mechanisms. If two
> > > threads want to share writeable stack regions then the current support
> > > could work, perhaps by using shared memory objects to set up the
> > > thread stacks. But two threads can't share read-only stack regions
> > > with the current implementation. That would be part of your work to
> > > figure out, in addition to perhaps improving and fixing up the
> > > existing mmap/shm support.
> > >
> > >   I had looked into that and therefore initially proposed a separate
> 'mem_share()' interface, but as was pointed out, it was not POSIX compliant.
> > >  So I guess, adding on to the existing mmap/shm support is the best
> way to move forward.
> > >
> > > Step back a minute and think about the requirements before you.
> > > Threads have stacks already. Sometimes they share them with each
> > > other. Now you want to isolate each thread's stack from other threads.
> > > But if they still want to share, then you should allow it. How?
> > >
> > > The suggestion is to allow threads to use mmap() to map other threads'
> > > stacks. Some questions for you to ponder: Since those stacks exist and
> > > have an address already, can you just fiddle with the protection
> > > regions and return a pointer directly to the stack to allow r/w access
> > > with sharing?
> > > I guess if a thread makes explicit calls to mmap for stack sharing and
> the access to other stacks is not granted, this can be implemented(At the
> hardware level it would mean that the page table attributes would be
> updated for the thread-stack that is to be mapped).
> > > What are the limitations on the solution (based on the
> > > number of protection regions supported in hardware)?
> > > As was mentioned in a separate thread we would have to go with the
> common minimum hardware support to support maximum targets.
> > > Gedare
> > >
> > > > Regards,
> > > > Utkarsh Rai.
> > > > ______________________________________________
> >
> > My rationale for suggesting Utkarsh examine the typed memory object
> interface, after five minutes of in-depth analysis, was that you could
> immediately get into the code specific to memory mapping stacks as the file
> descriptor would have to be one for a stack.  You could use
> "posix_typed_mem_get_info()" to assert that the FD is a stack object prior
> to doing your "mmap()".
> >
> > I've just spent another five minutes.  There are three functions that
> would need to be "derived" from existing functions with additional
> information associated with those typed memory file descriptors.
> >
> > • posix_mem_offset - find offset and length of a mapped typed memory
> block (ADVANCED REALTIME)
> > • posix_typed_mem_get_info - query typed memory information (ADVANCED
> REALTIME)
> > • posix_typed_mem_open - open a typed memory object (ADVANCED REALTIME)
> >
> > Maybe this is overkill.
> >
>
> For this specific use case I would try to make it work with SHM first,
> fixing up what we already have for mmap/shm support. But adding TYM is
> a noteworthy challenge that could be worked on in the same GSoC
> project. It just looks like a harder, more complicated road to follow.
>
> > Peter
> > -----------------
> > Peter Dufault
> > HD Associates, Inc.      Software and System Engineering
> >
> > This email is delivered through the public internet using protocols
> subject to interception and tampering.
> >
>
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