There are some basic OpenBSD-isms like ucontext, stack mapping, mmap, fix trap context so rest of the %gs patching logic even knows where to work.
Rest is basically ~~two~~ three main issues
1. %gs/gsbase - we trap on %gs instructions because %gs=0 gsbase=0 so all of them illegally point at page zero. So we take the instruction, decode, turn it into %gs-less instruction, and stow it somewhere else. To do conversion we need a scratch register (now two, thanks MINGW32-GCC, you c*nt), we replace whatever gsbase should've been with our TEB pointer, and gogogo (with restore of the scratch register state {or two}). Old instruction (if wide enough) becomes jmp rel32 (I picked it because it is 5 bytes, and short jump branches are too... short). If it isn't, we are just constantly trap on that so we just create a hash between trap location and stub location. Those cases are thankfully rare.
Just this one patch gives us Notepad, Notepad++, Factorio, and many many other games
2. SIGUSR1. Basically Windows lets threads chat thru their thread API, and Electron apps (I really wanted Eve Online to work) and Chromium apps constantly chat and interrogate each other with that thread API. Wine decided to emulate some of the APIs with unix signals. Now the main issue here is that OpenBSD, does not allow one process to signal a thread in another process. So we catch it with the currently active thread, and reemit it now inside of the process as SIGUSR2. Read on SuspendThread, GetThreadContext, ReadProcessMemory, and why Mojo/Crashpad works how they work, very fun! Makes you really enjoy the security OpenBSD gives you. Another dev is making this code better and handle other SIGnals.
3. ReadProcessMemory / WriteProcessMemory (NtReadVirtualMemory / NtWriteVirtualMemory) - this is also for Electron/Chromium but mostly Steam. We only needed it majorly when we started pushing towards steam working. Steamwebhelper, steam overlay, and steam watching you play and tracking your game time, are completely against OpenBSD's security model. Wine processes are not descendants of wineserver, so wineserver cannot ptrace them unless you enable kern.global_ptrace=1. We were trying to keep kernel as stock as possible so...
2 and 3 are very much together and if you ever seen on windows how chrome looks in the Task Manager and how many chrome processes there are, well now you know my pain, haha