[gentoo-dev] [pre-GLEP] User and group management via dedicated packages

[Michał Górny]

Hi,

Here's a reiteration (or well, rewrite) of mjo's earlier work [1].  I've
made it into more GLEP-ish form, removed inline code samples which
belong in implementation part and did some changes.

The main change from the earlier proposal is that we are extremely
careful not to break stuff.  Packages only create users/groups if they
don't exist, they do not modify existing accounts.  Users and groups are
not removed at all.  The text included below.


[1] https://wiki.gentoo.org/wiki/User:Mjo/GLEP:User_packages

---
GLEP: 9999
Title: User and group management via dedicated packages
Author: Michał Górny <mgorny@g.o>,
        Michael Orlitzky <mjo@g.o>
Type: Standards Track
Status: Draft
Version: 1
Created: 2019-05-29
Last-Modified: 2019-05-29
Post-History: 
Content-Type: text/x-rst
Requires: 
Replaces: 27
---

Abstract
========

A new approach for user/group management is proposed.  Regular packages
in dedicated categories are used to represent and create user and group
accounts.  Dependencies are used to request users and group from within
regular packages, and to track their usage.


Motivation
==========

User management in Gentoo is currently ad-hoc.  Users and groups are
created through calling system tools directly in packages needing them.
There is no systematic way of tracking which packages need specific
users or groups, and determining which ones are obsolete.  Coordinating
properties of users and groups used by multiple packages must be done
manually by developers.

GLEP 27 originally attempted to address the problem.  Posted in 2004,
it never had reached the reference implementation state, and became
obsolete.  [#GLEP27]_

A good system user and group management proposal should address:

1. Tracking usage of users and groups, and determining which ones
   are obsolete.

2. Sharing users and groups reliably between different packages.

3. Maintaining fixed UIDs/GIDs that are consistent between different
   systems.

4. Providing local overrides for user/group properties.

5. Ensuring that users and groups are not created unnecessarily
   at build time.

At the same time, the proposal should avoid unnecessary complexity
to avoid sharing the fate of GLEP 27.  This proposal aims to address
those points without requiring a new EAPI or any changes in the package
manager.


Specification
=============

Logical structure
-----------------

In this proposal, system users and groups are represented by regular
packages.  Those packages logically represent the ownership of
the respective users and group, and technically implement their
creation.

User packages are placed in ``user`` category.  Each user package
defines the properties of the particular user, and should be named after
the user it creates.  It must depend at build and run time on the groups
user belongs to.

Group packages are placed in ``group`` category.  Each group package
defines the properties of the particular group, and should be named
after the group it creates.

All user and group packages must define preferred fixed UIDs/GIDs,
and they must be unique within the repository.  The packages should
indicate whether the value needs to be strictly enforced, or whether
another UID/GID can be used when the user exists already or requested
UID/GID is taken.

Packages needing a specific user or group use dependencies to pull
the required user/group packages.  If the user is needed at build time,
a build time dependency (``DEPEND``) must be used.  If the user is
needed at install time, a run time dependency (``RDEPEND``) must be
used.  If the user is only needed after the package is installed,
``PDEPEND`` must be used.


Maintaining users/groups
------------------------

The primary technical function of user and group packages is to create
the users and groups.  This is done via invoking the respective system
tools at ``pkg_preinst`` phase.  This is done only if the user/group
does not exist on the system already.

Normally, the packages should not modify existing users, to preserve
local modifications.  If such a modification is necessary, the package
should verify the existing values of changed properties, and update
them only if they match the previous value.

The package must not remove users/groups.  Any cleanup actions must
be done with explicit user approval, and therefore should be addressed
by separate tooling.


Home directory ownership
------------------------

If the user in question uses a regular home directory (i.e. not
``/dev/null``), the user package should maintain the directory
via ``keepdir`` command.  This allows for clean removal of the home
directory if it is no longer needed.  The package manager will also
apply correct permissions if the directory does not exist yet.

Note that since the user is not created until ``pkg_preinst``,
the permissions to home directory should not be applied earlier than
that.


User/group name/identifier collision detection
----------------------------------------------

The user/group packages can install additional files in subdirectories
of ``/var/lib`` indicating their respective names and identifiers.
This ensures that if two packages ever happen to collide, the package
manager's collision detection mechanism will trigger.


Rationale
=========

Satisfied goals
---------------

Tracking of user/group usage is done through dependencies.  As long
as any installed package depends on a specific user/group package,
the respective user/group is assumed to be used.  If no package
requiring the specific user/group is left, the package manager
automatically prunes the package clearly indicating it is no longer
used.

Each user and group has a single respective package creating it.
If multiple packages need it, they depend on the same package.  This
ensures that all properties are kept in a single location, and do not
need to be synced.

Having a single location with all predefined user/group ranges makes it
possible to maintain fixed UID/GID definitions.  This GLEP makes
allocating them obligatory, and provides for detecting collisions.
While this isn't enforced for existing users, it provides a way forward
for new installations.

Local overrides can be trivially implemented via local repository,
through overriding the respective user/group ebuilds.  The proposal also
respects direct sysadmin modifications.

Avoiding unnecessary user/group creation at build time is implemented
via correct dependency types.  While this was possible with the status
quo, the dependency model should be more natural to developers and cause
less mistakes.


Category names
--------------

The original proposal used ``sys-user`` and ``sys-group`` as category
names.  This was changed in order to avoid mixing them with regular
packages in ``sys-*`` categories.

The new names intentionally are single component to distinguish them
from regular packages.


Naming rules
------------

It has been pointed out that the package naming rules are more
restrictive than user/group naming rules.  This is why the proposal
allows for package names to be different from user/group names.
However, this is strongly discouraged and should only be used when
really necessary.


User/group updates
------------------

If sysadmin needs to change the properties (e.g. home directory) of some
user/group, the obvious course of action is to modify the system
databases directly.  The GLEP aims to respect that and disallow altering
existing user/groups unconditionally.  If any updates need to be done,
the packages need to verify previous values first.


User/group removal
------------------

The original proposal attempted to remove user/groups automatically
when the respective package was unmerged.  This required verifying that
no files are owned by the user/group in question which was both
expensive in terms of I/O, and fragile.

This GLEP follows the best practice of leaving obsolete user/groups
accounts intact.  This guarantees that no files with stale ownership are
left (e.g. on unmounted filesystems) and that the same UID/GID is not
reused for another user/group.


Backwards Compatibility
=======================

This GLEP preserves backwards compatibility with the existing method
of user/group management.  Both methods can coexist as long as necessary
for the transition period, and the same user/group can be governed
by both in parallel.

However, some of the advantages will only be reliable once the old
method is phased out, and only on new installations.  This particularly
applies to fixed UIDs/GIDs.


Reference Implementation
========================

TODO


References
==========

.. [#GLEP27] GLEP 27: Portage Management of UIDs/GIDs
   (https://www.gentoo.org/glep/glep-0027.html)


Copyright
=========
This work is licensed under the Creative Commons Attribution-ShareAlike 3.0
Unported License. To view a copy of this license, visit
http://creativecommons.org/licenses/by-sa/3.0/.
-- 
Best regards,
Michał Górny