ACL sorting, qualifier uniqueness

[Robert Watson]

In my first implementation of ACLs on FreeBSD, I required that userland
processes submit only ACLs with strictly sorted entries.  The intent in
doing so was to allow the kernel to iterate through the ACL once when
doing a syntax check, and determine that there were no duplicated
qualifiers for any of the tags.  That is, there were no duplicate uid's or
gid's in the list; if sorted, it's only necessary to walk down the list
and (a) verify that they are sorted, and (b) that there are no adjacent
entries with the same qualifier/tag.  In later revisions of the code, I
remove this requirement, and disabled the check for duplicated qualifiers,
as the verification code simply does a first match on users, and best
match on groups.  I have two questions:

1) What sorting requirements, if any, are enforced in other
   implementations?  Also, what syntactic requirements, if any, are
   enforced with regards to duplicated qualifiers for a tag?

2) Many calls specifically disclaim ordering as a property that will
   be preserved over the call.  To what extent is this the case on
   other platforms?

If desirable, I can reinstate the ordering verification in kernel along
with the associated syntactic check for qualifiers, and similarly
introduce an acl_sort() invocation in acl_set_{fd,file}().  Just want to
get a feel for what other platforms do.

Robert N M Watson             FreeBSD Core Team, TrustedBSD Project
robert at fledge.watson.org      NAI Labs, Safeport Network Services

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