bug#60890: least-authority-wrapper and make-forkexec-constructor composition problem

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi,

I'm creating a bug to keep track of a problem that was uncovered when
attempting to migrate the jami-service-type service to use the
least-authority-wrapper [0], to avoid forgetting about it.

It was found that using something like:

--8<---------------cut here---------------start------------->8---
(make-forkexec-constructor
  (least-authority
    (list (file-append coreutils "/bin/true"))
    (mappings (delq 'user %namespaces))
  #:user  "nobody"
  #:group "nobody"))
--8<---------------cut here---------------end--------------->8---

Would fail with EPERM, because in order to be able to drop the user
namespace, the CAP_SYS_ADMIN capability is required, but in the above
case, make-forkexec-constructor has already changed the user to
"nobody", which lacks such capability.

The solution proposed by Ludovic in would be to [1]:

> [...] add #:user and #:group to ‘least-authority-wrapper’ and
> have it call setuid/setgid.  ‘make-forkexec-constructor’ doesn’t need to
> be modified, but the user simply won’t pass #:user and #:group to it.

[0]  https://issues.guix.gnu.org/54786#16
[1]  https://issues.guix.gnu.org/54786#17

-- 
Thanks,
Maxim