Toby Ovod-Everett > Win32-Security > Win32::Security::ACL

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NAME ^

Win32::Security::ACL - Win32 ACL manipulation

SYNOPSIS ^

        use Win32::Security::ACL;

        my $acl =  Win32::Security::ACL->new('FILE', $acl_string);
        my $acl2 = Win32::Security::ACL->new('FILE', @Aces);

DESCRIPTION ^

Win32::Security::ACL and its subclasses provide an interface for interacting with Win32 ACLs (Access Control Lists). The subclasses allow for variation in mask behavior (different privileges apply to files than apply to registry keys and so forth).

Win32::Security::ACL uses the flyweight design pattern in conjunction with an in-memory cache of demand-computed properties. The result is that parsing of ACLs is only done once for each unique ACL, and that the ACL objects themselves are very lightweight. Double-indirection is used in the ACL objects to provide for mutability without invalidating the cache.

Installation instructions

This installs as part of Win32-Security. See Win32::Security::NamedObject for more information.

It depends upon Class::Prototyped which should be installable via PPM or available on CPAN. It also depends upon Win32::Security::ACE , which is installed as part of Win32-Security.

ARCHITECTURE ^

Win32::Security::ACL uses some OO tricks to boost performance and clean up the design. Here's a quick overview of the internal architecture, should you care! It is possible to use Win32::Security::ACL objects without understanding or reading any of this, because the public interface is designed to hide as much of the details as possible. After all, that's the point of OO design. If, however, you want to boost performance or to muck about in the internals, it's worth understanding how things were done.

Class Structure

Win32::Security::ACL uses single inheritance similar to the _ObjectType side of the multiple inheritance in Win32::Security::ACE. While not technically necessary, it was done in order to parallel the ACE design, and so that the data caches could be maintained independently for each Object Type.

With that in mind, the class hierarchy looks like this:

Flyweight Objects w/ Cached Demand-Computed Properties

On the typical computer systems, there are very few unique ACLs. There may be hundred or thousands, but usually there are orders of magnitude fewer ACLs than there are objects to which they are applied. In order to reduce the computation involved in analyzing them, the Win32::Security::ACL caches all the information computed about each ACL in a central store (actually, multiple central stores - one for each Named Object type) based on the binary form (rawAcl). The object returned by a call to new is a a reference to a reference to the hash for that rawAcl in the central store. Because it isn't a direct reference to the hash, it is possible to switch which hash the object points to on the fly. This allows the Win32::Security::ACL objects to be mutable while maintaining the immutability of the central store. It also makes each individual Win32::Security::ACL object incredibly lightweight, since it is only composed of a single blessed scalar. To be safe, you may wish to clone ACLs before modifying them, just to make sure that you aren't modifying someone else's ACL object. The properties are computed as needed, but the results are cached in the central store.

For instance, once aces has been computed for a given rawAcl, it can be found from the object as $$self->{aces}. This should be used with care, although in some instances it is possible to reduce the number of method calls (should this be necessary for performance reasons) by making calls like so:

    $$acl->{aces} || [$acl->aces()];

That provides a fail-safe should the aces value have not yet been computed while eliminating the method call if it has been. Note that $acl->aces() also derefences the array stored in the cache.

In order to defend against accidental manipulation, return values from the calls (although not from the direct access, obviously) are deep-copied one layer deep. That means that the results of $acl->aces() can be safely manipulated without harming the ACL, but that the results of $$acl->{aces} should be treated as read-only.

Win32::Security::ACL and Win32::Security::ACE objects returned are cloned (using inlined code to reduce the performance hit). The values returned from the /^dbm.*/ calls are not cloned, however, so be careful there.

Method Reference ^

new This creates a new Win32::Security::ACL object.

The various calling forms are:

Note that when using $objectType in the package name, the value needs to be canonicalized (i.e. SE_FILE_OBJECT, not the alias FILE). If the $objectType has already been canonicalized, improved performance can be realized by making the call on the fully-qualified package name and thus avoiding the call to redo the canonicalization. Aliases are permitted when passed as a parameter to the call.

To create a NULL ACL, pass an empty string (which will be interpreted as an empty rawAcl). Passing an empty list of ACEs creates an empty ACL, which is totally different from a NULL ACL.

If called on an Win32::Security::ACL object, it creates a new ACL object of the same subclass comprised of the passed list of ACEs.

ACEs can be passed either as Win32::Security::ACE objects or as anonymous arrays of parameters to be passed to Win32::Security::ACE::$objectType->New().

clone

This creates a new Win32::Security::ACL object that is identical in all forms, except for identity, to the original object. Because of the flyweight design pattern, this is a very inexpensive operation. However, should you wish to avoid the overhead of a method call, you can inline the code like so:

    bless(\(my $o = ${$obj}), ref($obj));

Basically, it derefences the scalar reference, assigns it to a temporary lexical, creates a reference to that, and then blesses it into the original package. Nifty, eh? Syntax stolen (with a few modifications) from Data::Dumper output.

dump

This returns a dump of the Win32::Security::ACL object in a format useful for debugging.

dbmObjectType

Returns the Data::BitMask object for interacting with Named Object Types. See Win32::Security::ACE->dbmObjectType() for more explanation.

rawAcl

Returns the binary string form of the ACL

objectType

Returns the type of object to which the ACE is or should be attached.

isNullAcl

Tests for a NULL ACL.

aces

Returns a list of Win32::Security::ACE objects. The ACEs are in the same order as they are in the ACL.

It accepts an optional filter. The filter should be an anonymous subroutine that looks for the ACE in $_ and that returns true or false like the block passed to grep does (note that unlike grep {} @list, it is neccessary to specify sub to ensure that the block is interpreted as an anonymous subroutine and not an anonymous hash). The returned ACEs are cloned to ensure that modifications to them do not modify the cached ACE values for that ACL (this is done before passing them to the optional anonymous subroutine, so it is safe for that subroutine to modify the ACEs).

aclRevision

Returns the ACL Revision for the ACL. In general, this should be 2 (ACL_REVISION) for normal ACLs and 4 (ACL_REVISION_DS) for ACLs that contain object-specific ACEs.

has_creatorowner

Returns 1 if the ACL in question contains a dreaded and evil CREATOR OWNER ACE, 0 if it doesn't.

sids

Returns a list of all unique SIDs present in the ACL, except for CREATOR OWNER and the null SID.

inheritable

Accepts a type (either 'OBJECT' or 'CONTAINER'). Returns the list of ACEs that would be inherited by a newly created child OBJECT or CONTAINER if the parent has this ACL. It handles occluded permissions properly (I hope). For instance, if an container has an inherited permission granting READ access to Domain Users and someone adds explicit fully-inheritable FULL access to Domain Users to that container, child objects will not receive the inherited READ access because it is fully occluded by the also inherited FULL access. The exact algorithms for this had to be developed through trial and error as I could find no documentation on the exact behavior. As in aces, the returned ACEs are cloned for safety.

If the ACL in question contains a dreaded and evil CREATOR OWNER ACE and the ACE applies to the object in question, then a placeholder ACE is returned with a null SID - the null SID should be replaced with whatever the appropriate trustee might be. This may be in addition to the inheritable CREATOR OWNER ACE itself.

compare_inherited

Accepts $inheritable, a Win32::Security::ACL object, which should ideally be generated by a call to inheritable on the parent object. It should be comprised solely of ACEs with the INHERITED_ACE flag.

The method compares the ACEs on the receiver marked as inherited with the ACEs for the passed object using a very simple algorithm. First, it filters out ACEs not marked as INHERITED_ACE from the list of those on the receiver (these will be addressed later). Then it starts at the beginning of the two lists of ACEs and removes ACEs that match. If there are remaining ACEs, it removes matching ACEs from the end.

It deals with null SIDs in the $inheritable object (implying an ACE resulting from a CREATOR OWNER ACE) by testing all of the SIDs in $self as possible standins for the null SID. If any of these result in a perfect match, then life is good. Otherwise, the results are returned after testing with the null SID unchanged. The algorithm does not currently deal with situations where there are multiple CREATOR OWNER permissions that were set at different times and thus the bound owner for the permissions is different, and it does not deal with CREATOR GROUP.

It returns a list of anonymous arrays, the first consisting of an ACL and the second consisting of an $IMWX value that can be interpreted as so:

I

ACE is properly inherited from $inheritable.

M

ACE should have been inherited from $inheritable, but is missing!

W

ACE marked as INHERITED_ACE, but there is no corresponding ACE to inherit in $inheritable.

X

ACE explicitly assigned to object (i.e. INHERITED_ACE is not set).

Note that the I, W, and X ACEs indicate those actually present on the receiver, in the same order they are present on the receiver. The I, M, and X ACEs indicate those that should be present, in the same order they should be present.

If you pass a true value for the optional second parameter $flat, the returned data will be flattened into a single list. This is more difficult to interact with, but because the anonymous arrays don't have to be built, it is faster. In both cases, the returned values are cloned to ensure the safety of the cached data.

addAces

Adds ACEs to the Win32::Security::ACL object. ACEs may be passed as Win32::Security::ACE objects, rawAce strings, or anonymous arrays of parameters to be passed to "Win32::Security::ACE::$objectType"->new(). The $objectType value will be generated from the existing ACL. If the existing ACEs in the ACL are not in the proper order, they will end up reordered as specified in http://support.microsoft.com/default.aspx?scid=kb;en-us;269159 .

deleteAces

Deletes all ACEs matched by the passed filter from the ACL. The filter should be an anonymous subroutine that looks for the ACEs one-by-one in $_ and returns 1 if they should be deleted.

AUTHOR ^

Toby Ovod-Everett, toby@ovod-everett.org

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