Hello, A very technical request regarding Cygwin internals: In mmap.c there is a function mmap_is_attached_or_noreserve(void *addr, size_t len) which is called from Cygwin's exception handler in the case of a STATUS_ACCESS_VIOLATION.
This is called in case an access violation occurs in memory that was allocated with Cygwin's mmap() with the MAP_NORESERVE flag, and allows us to commit the relevant pages when they are accessed. After a successful call of mmap_is_attached_or_noreserve(), the Cygwin exception handler returns with ExceptionContinueExecution. Unfortunately, if the application happens to have a Vectored Continue Handler registered which happens to do something in the case of STATUS_ACCESS_VIOLATION (see [1]) there is no obvious way to tell if we're handling this sort of case. Normally this isn't too much of a problem: E.g. we could just check the address that caused the access violation and see if its status is now MEM_COMMIT (i.e. Cygwin ran its exception handler and all is good). However, due to the bug described in [1], if an exception occurs in code running on a sigaltstack, the Cygwin exception handler isn't run. This makes for a tricky to handle use case: What if some code in a signal handler function tries to access uncommitted memory in a MAP_NORESERVE mmap? It's probably an unusual, undesirable case, and I haven't personally encountered it *yet*, but I could imagine some cases where it might happen. In order to handle such a case it might be nice if mmap_is_attached_or_noreserve were able to be called by user code, perhaps as a new cygwin_internal(...) call. I'd happily provide a patch, but I fear this might be an X/Y problem that I'm not seeing. Thanks, Madison [1] https://cygwin.com/ml/cygwin/2018-05/msg00333.html -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
