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. > _______________________________________________ devel mailing list devel@rtems.org http://lists.rtems.org/mailman/listinfo/devel
