* szabolcs: > changing/disabling the alt stack is not valid while a handler is > executing on it. if we don't allow jumping out and back to an > alt stack (swapcontext) then there can be only one alt stack > live per thread and change/disable can do the shadow stack free. > > if jump back is allowed (linux even makes it race-free with > SS_AUTODISARM) then the life-time of alt stack is extended > beyond change/disable (jump back to an unregistered alt stack). > > to support jump back to an alt stack the requirements are > > 1) user has to manage an alt shadow stack together with the alt > stack (requies user code change, not just libc). > > 2) kernel has to push a restore token on the thread shadow stack > on signal entry (at least in case of alt shadow stack, and > deal with corner cases around shadow stack overflow). We need to have a story for stackful coroutine switching as well, not just for sigaltstack. I hope that we can use OpenJDK (Project Loom) and QEMU as guinea pigs. If we have something that works for both, hopefully that covers a broad range of scenarios. Userspace coordination can eventually be handled by glibc; we can deallocate alternate stacks on thread exit fairly easily (at least compared to the current stack 8-). Thanks, Florian
