Default Component ACL Rules

Joomla Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30550 (submitted for both Joomla 2.5 and 4)

Background / Motivation

There is currently no easy way to specify the default action permission rules
for a component that has component-specific custom permission rules. If
component developers want default ACL rules for their custom ACL rules, they
must create code that modifies the root asset. This is extra work for
component developers and a potential source of errors and problems.

Proposed Fix

The proposed fix addresses this problem by extending the syntax of component
'access.xml' file that lists the component-specific ACL rules. The goal is to
allow the component developer to specify the groups that should be allowed to
perform the ACL action as part of the XML definition for each ACL rule. This
would require updates to Joomla (the platform) to interpret the additions to
the 'access.xml' and add the appropriate permissions to the system.

The proposed function provides new functions to install or remove the default
permissions specified in the 'access.xml' file. Normally this would be done
in by the installer by having the component installer call the new function
during installation. In the following description, assume the new function is
called by the installer.

These additions will be backwards compatible since the additional XML
attributes will be ignored by versions of Joomla that have not implemented
this fix.

Since there are many different ways that a particular component can implement
ACL rules, a few guiding principles are necessary:

1. If the default for a custom ACL rule is not specified, the default root
   ACL rules will not be modified. This is the current behavior.
2. If the group with the specified permssions in the default rules does not
   exist in the Joomla installation, no root permission will be set and an
   information message will be generated. Some Joomla installations have
   been known to delete all common groups (such as Public, Registered) and
   create new groups. So there is no guarantee that normally expected groups
   will exist.
3. The component developer will specify a component (usually com_content)
   and an action to check and the installer will find a group with that
   permission. The component developer may give a suggestion for which
   group to check first. If the group is found and has the required
   permissions, then it will be used. Otherwise the installer will search
   for a suitable group with the required permissions. All groups will be
   checked and the one with the least authority will be used (eg, if both
   Author and Publisher have the required permission, Author will be chosen
   since Publisher has more authority).
4. This new feature would only be applied to any custom component-specific
   rule in a components 'access.xml'. This approach cannot be used to
   modify the core rules in the root rule.

Example 'access.xml' syntax additions

The following syntax change is proposed. In the following examples, the
component for the 'access.xml' file is 'com_example':

Notice the new 'default' attribute at the end. The groups will be searched to
find the least authoritative group that has the 'core.delete' permission for
com_content (for instance, delete example items they created). This will mean
that all derived groups also have this permission.

Note that this could be a comma-separated list of group names.

The component developer could also specify a hint on which group to check
first:

In this case, the installer would first check to see if the 'Author' group has
the required permission. If it does, it would be used. If not, no warning
message would be given and the search for a group with that permission would
continue.

Note that the test component could be something other than 'com_content':

Which would find a group that has permission to do 'core.delete' for the
'com_xyz123' component. This could be used to synchronize default permissions
between two related components.

Testing the new functionality

A test component 'com_permtest' is available on github:

https://github.com/jmcameron/component_permissions_test

This includes a zipped up version of the component that is ready to install.

comp_permtest.zip

This test component should work on both Joomla 2.5.x and 3.x. Obviously the
functions will not do anything if the fixes to Joomla described here are not
implemented.

To try these fixes:

• Install the fixes on your Joomla 2.5 or 3.x site (by doing 'git pull' or
  using a patch file)
• Install the component in the regular way and then go to the
  'component-permissions-test' component under the 'Components' menu in the
  back end.
• Read the instructions there on how to test it.

Note that these fixes include unit tests for the new capabilities (on the
Joomal 3.x version). See the bottom of the this document.

Implementation Notes

The functionality above is implemented by adding functions to JAccess and
JRules:

JRules::removeActions($action_regexp)

• Remove all actions from the rule whose action name matches the regular
  expression. This a general function and could be used to remove core
  actions from the root rule, if the calling code requested it (not
  recommended).

JAccess::getGroupId($groupname)

• Returns a group ID for the given group name.
• This could be moved elsewhere, but there did not seem to be a suitable
  place. We may want to add a JUserGroup class (in
  libraries/joomla/user/group.php) and move this function into that class.

JAccess::removeDescendentGroups($groupIds)

• Removes all groups in the provided set of groups that have ancestors in the
  set of groups. This is part of the code that helps select the least
  authoritative group since it eliminates children of groups that are already
  in the set since the child group is likely to have more permissions than
  its parent group.

JAccess::leastAuthoritativeGroup($groupIds, $asset)

• Selects the least authoritative group out of a set of groups.
  Read the code for a clearer idea of how this works.

JAccess::installComponentDefaultRules($component, $file = null)

• Parse the rules for custom rules in component's 'access.xml' file and
  install new permissions in the root asset, as needed.
• Does not allow modifying core rules.
• If a group with the required permission is not found or the group name
  specified in the hint part is unknown by the Joomla installation, the
  default specified will be ignored. Currently a log message is being added.
  This should probably be a JApplication::enqueueMessage() call but I could
  not get that to work in the unit tests.
• This function can be called any time, but it would make sense to call this
  during component installation in the postflight section.
• The optional $file can be used to specify an alternate location for the
  'acess.xml' file (for testing).

JAccess::purgeComponentDefaultRules($component)

• Removes all custom rules for $component defined in the root asset.
• Core rules will not be modified.
• This function can be called at any time, but it would make sense to call
  this during the uninstallation of a component (during preflight or the
  uninstall function).

Unit tests

• Unit tests for Joomla 3 that cover the features above have been added to
  the tests/unit/suites/libraries/access folder.
• The custom unit test configuration file
  tests/system/servers/configdef.php was added to the toplevel .gitignore
  since it could contain private data and should be ignored (like .gitignore
  already ignores the top-level phpunit config file phpunit.xml).

Jonathan Cameron
April 28, 2013