OpenZFS 6765 - zfs_zaccess_delete() comments do not accurately

reflect delete permissions for ACLs

Authored by: Kevin Crowe <kevin.crowe@nexenta.com>
Reviewed by: Gordon Ross <gwr@nexenta.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Paul B. Henson <henson@acm.org>

Porting Notes:
* Only comments are updated

OpenZFS-issue: https://www.illumos.org/issues/6765
OpenZFS-commit: openzfs/openzfs@da412744bc
Closes openzfs#10266

module/os/linux/zfs/zfs_acl.c

@@ -2687,8 +2687,10 @@ int zfs_write_implies_delete_child = 1;
 /*
  * Determine whether delete access should be granted.
  *
- * The following chart is the recommended NFSv4 enforcement for
- * ability to delete an object.
+ * The following chart outlines how we handle delete permissions which is
+ * how recent versions of windows (Windows 2008) handles it.  The efficiency
+ * comes from not having to check the parent ACL where the object itself grants
+ * delete:
  *
  *      -------------------------------------------------------
  *      |   Parent Dir  |      Target Object Permissions      |
@@ -2697,14 +2699,14 @@ int zfs_write_implies_delete_child = 1;
  *      |               | ACL Allows | ACL Denies| Delete     |
  *      |               |  Delete    |  Delete   | unspecified|
  *      -------------------------------------------------------
- *      |  ACL Allows   | Permit     | Permit *  | Permit     |
- *      |  DELETE_CHILD |            |           |            |
+ *      | ACL Allows    | Permit     | Deny *    | Permit     |
+ *      | DELETE_CHILD  |            |           |            |
  *      -------------------------------------------------------
- *      |  ACL Denies   | Permit *   | Deny      | Deny       |
- *      |  DELETE_CHILD |            |           |            |
+ *      | ACL Denies    | Permit     | Deny      | Deny       |
+ *      | DELETE_CHILD  |            |           |            |
  *      -------------------------------------------------------
  *      | ACL specifies |            |           |            |
- *      | only allow    | Permit     | Permit *  | Permit     |
+ *      | only allow    | Permit     | Deny *    | Permit     |
  *      | write and     |            |           |            |
  *      | execute       |            |           |            |
  *      -------------------------------------------------------
@@ -2717,24 +2719,21 @@ int zfs_write_implies_delete_child = 1;
  *         Re. execute permission on the directory:  if that's missing,
  *	   the vnode lookup of the target will fail before we get here.
  *
- * Re [*] in the table above:  We are intentionally disregarding the
- * NFSv4 committee recommendation for these three cells of the matrix
- * because that recommendation conflicts with the behavior expected
- * by Windows clients for ACL evaluation.  See acl.h for notes on
- * which ACE_... flags should be checked for which operations.
- * Specifically, the NFSv4 committee recommendation is in conflict
- * with the Windows interpretation of DENY ACEs, where DENY ACEs
+ * Re [*] in the table above:  NFSv4 would normally Permit delete for
+ * these two cells of the matrix.
+ * See acl.h for notes on which ACE_... flags should be checked for which
+ * operations.  Specifically, the NFSv4 committee recommendation is in
+ * conflict with the Windows interpretation of DENY ACEs, where DENY ACEs
  * should take precedence ahead of ALLOW ACEs.
  *
- * This implementation takes a conservative approach by checking for
- * DENY ACEs on both the target object and it's container; checking
- * the ACE_DELETE on the target object, and ACE_DELETE_CHILD on the
- * container.  If a DENY ACE is found for either of those, delete
- * access is denied.  (Note that DENY ACEs are very rare.)
+ * This implementation always consults the target object's ACL first.
+ * If a DENY ACE is present on the target object that specifies ACE_DELETE,
+ * delete access is denied.  If an ALLOW ACE with ACE_DELETE is present on
+ * the target object, access is allowed.  If and only if no entries with
+ * ACE_DELETE are present in the object's ACL, check the container's ACL
+ * for entries with ACE_DELETE_CHILD.
  *
- * Note that after these changes, entire the second row and the
- * entire middle column of the table above change to Deny.
- * Accordingly, the logic here is somewhat simplified.
+ * A summary of the logic implemented from the table above is as follows:
  *
  * First check for DENY ACEs that apply.
  * If either target or container has a deny, EACCES.