- Use unambiguous variable names (w/o package name conflict).
- Fail on invalid input such as the empty string or `:`.
- Do not change group without user, i.e. fail on `:group`.
- Parse input using mnemonic APIs.
- Do not juggle between integer types.
- Unset supplementary groups.
- Use set[ug]id(2) to follow the idiom of OpenBSD base programs.
(cannot use setres[ug]id(2) as macOS does not have them.)
Includes/Supersedes #1202.
Fixes#927.
I only tested on OpenBSD (so far), but other systems should just work.
Changing the real and effective user/group IDs and the saved
set-user/group-ID is not enough to get rid of intial access permissions.
The list of groups must be cleared also, otherwise a process changing
from, e.g. `root:root` to `nobody:nobody` retains rights to access
`:wheel` files (assuming `root` is a member of the `wheel` group).
For example:
```
# id
uid=0(root) gid=0(wheel) groups=0(wheel), 2(kmem), 3(sys), 4(tty), 5(operator), 20(staff), 31(guest)
# ./yggdrasil -autoconf -logto /dev/null -user nobody &
[1] 4337
# ps -o command,user,group,supgrp -U nobody
COMMAND USER GROUP SUPGRP
./yggdrasil -aut nobody nobody wheel,kmem,sys,tty,operator,staff,guest
```
Fix that so the process runs as mere
```
COMMAND USER GROUP SUPGRP
./yggdrasil -aut nobody nobody nobody
```
Fixes#927.
