Document issue with multiple threads about landlock-hs HOT 2 CLOSED

After some more research, it turns out the GHC threaded RTS and libpsx are not compatible because GHC creates a thread (the ticker) which explicitly disables all signals (see the use of sigfillset in rts/posix/ticker/Pthread.c). Since this also preempts delivery of SIGSYS as used by libpsx, we end up with a deadlock.

There's a work-around: wrapping sigfillset, and ensure SIGSYS is not set in the sigset_t, similar to how libpsx requires pthread_create to be wrapped. This could, obviously, cause weird runtime issues if SIGSYS is used for other purposes...

from landlock-hs.

It's possible to use libpsx with the GHC RTS. However, while working on a package exposing this functionality, I ran into hard-to-debug issues in the GitHub Actions CI environment, which I could not reproduce at all on my Fedora 36 host.

Turns out the version of libcap shipped with Ubuntu (up to at least Jammy 22.04) and current Debian, i.e., libcap 2.44, comes with an utterly buggy libpsx when using psx_syscall6: instead of passing the requested syscall number and arguments to syscall, it passes junk, due to a bug fixed in https://git.kernel.org/pub/scm/libs/libcap/libcap.git/commit/?id=e7e0e1b9e2cf3378d329174ed5b0c716b0539c72 which is included in libcap 2.46 and later.

Hence, this won't work unless

• we include a working version of libpsx in the build
• we somehow assert a working version of libpsx is linked in
• we write a libpsx equivalent as part of this library

from landlock-hs.