On Mon, Dec 06, 2021 at 05:59:41AM +0000, slembcke wrote:
> So this is a fairly esoteric question, and I expect the answer might
> be just as esoteric.
>
> I have a little toy fiber/stackless coroutine library that I made a
> few years ago and have been using in some of my hobby projects.
> (https://github.com/slembcke/Tina) The recent 7.0 release rekindled my
> interest in OpenBSD, so I tried building my current game project for
> it, and I was quite pleased that it mostly just needed a few ifdefs
> changed. Well, except for the fiber library... There were two issues,
> the first was trivial to fix. I just had to switch to mmap() +
> MAP_STACK to allocate the stack buffers since OpenBSD requires it. The
> second I found a workaround for easily enough, but I'm really curious
> why it happens. If I mmap() a buffer, I can mark the final page as
> PROT_NONE as a guard page. Then if I set %rsp to the guard page's
> address and push a value, it subtracts and then writes the value just
> below the guard page as expected on Linux/FreeBSD/Mac. However, on
> OpenBSD this fails with a segfault. Either starting 16 bytes lower, or
> replacing the push instruction with a sub and a mov to emulate it
> works fine. So there is an easy workaround, but my question is why? I
> assumed pagefaults were implemented in hardware by the MMU as an
> interrupt or something during reads and writes. Is this a subtle bug
> in OpenBSD, and if it is, how does it even know since the write
> doesn't even affect the protected page?! (Protected memory is black
> magic already! Hah!) I would assume that whatever is happening here
> would have to be handled when initializing the stack for processes or
> threads too.
>
> - Scott
>
We prefer to see code instead of stories.
>From your story it is not clear if yo are changing the protection
flags of a general buffer or a MAP_STACK region. I t is the latter,
there are extra restrictions on the stack pointer that are enforced in
the kernel on behalf of the user process. You should check
/var/log/messages
-Otto
