x/sys/unix: support Setuid/Setgid (...) syscalls on Linux

[mateusz834]

Go 1.16 added support for Setuid/Setgid (and other uid/gid related syscalls), instead of returning the EOPNOTSUPP error.
But the x/sys/unix still returns EOPNOTSUPP.

Proposal: uid/gid related syscalls might just call their counterparts in the syscall package (when golang 1.16 or higher (build tags)) and return EOPNOTSUPP (when golang 1.15 and lower)

[mateusz834]

Or it might make sense to just add a normal syscall invocation (not on all threads). It could be useful to have 2 different implementations. The x/sys/unix with the help of runtime.LockOSThread() would change uid only on one goroutine/thread. So it will allow to drop/gain privileges per thread.
Even unix.Setresuid, unix.Setreuid (...) are still working on per-thread basis, without returning EOPNOTSUPP.

[ianlancetaylor]

Thanks, I don't think this needs to be a proposal. We should just make the unix versions call the syscall versions on Linux.

[gopherbot]

Change https://go.dev/cl/428174 mentions this issue: unix: support all Setuid/Setgid and related syscalls