sendfd is something we can add. Note that seccomp has limited visibility into recvmsg / sendmsg args because bpf can't dereference syscall arg pointers. I mention some of this in the caveats section. As for execpromises, it's documented in the Cosmopolitan Libc source code, but that got lost in translation when I copied the docs to the website. I just updated the site. I also appreciate the eyeballs. Something like this deserves critical examination. There's so much breadth to the modern system call interface and pledge. I started working on this about a month ago. With the support of folks like yourself, I think we'll have something really nice that will benefit Linux users! I'll definitely be doing more to make the C API as compatible with OpenBSD as possible.