Got it, I will modify my proposal draft accordingly. On Sat, Mar 28, 2020 at 7:12 PM Gedare Bloom <ged...@rtems.org> wrote:
> On Sat, Mar 28, 2020 at 4:59 AM Peter Dufault <dufa...@hda.com> wrote: > > > > > > > > > On Mar 27, 2020, at 14:16 , Utkarsh Rai <utkarsh.ra...@gmail.com> > wrote: > > > > > > > > > On Fri, Mar 27, 2020 at 9:31 PM Gedare Bloom <ged...@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.ra...@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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