Treat ACLs with extra ACEs for Admins and SYSTEM like a trivial ACL

	POSIX.1e requires that chmod changes the MASK rather than the
	GROUP_OBJ value if the ACL is non-trivial.

	On Windows, especially on home machines, a standard ACL often
	consists of entries for the user, maybe the group, and additional
	entries for SYSTEM and the Administrators group.  A user calling
	chmod on a file with bog standard Windows perms usually expects
	that chmod changes the GROUP_OBJ perms, but given the rules from
	POSIX.1e we can't do that.

	However, since we already treat Admins and SYSTEM special in a
	ACL (they are not used in MASK computations) we go a step in the
	Windows direction to follow user expectations.  If an ACL only
	consists of the three POSIX permissions, plus entries for Admins
	and SYSTEM *only*, then we change the permissions of the GROUP_OBJ
	entry *and* the MASK entry.

	* fhandler_disk_file.cc (fhandler_disk_file::chmod): Drop unused
	code.  Add special handling for a "standard" Windows ACL.  Add
	comment to explain.
	* sec_acl.cc (get_posix_access): Allow to return "standard-ness"
	of an ACL to the caller.  Add preceeding comment to explain a bit.
	* security.h (get_posix_access): Align prototype.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

3 files changed, 30 insertions, 20 deletions

diff --git a/winsup/cygwin/fhandler_disk_file.cc b/winsup/cygwin/fhandler_disk_file.cc
index 4ee67e20d..7d729e3ad 100644
--- a/winsup/cygwin/fhandler_disk_file.cc
+++ b/winsup/cygwin/fhandler_disk_file.cc
@@ -787,35 +787,35 @@ fhandler_disk_file::fchmod (mode_t mode)
       gid_t gid;
       tmp_pathbuf tp;
       aclent_t *aclp;
+      bool standard_acl = false;
       int nentries, idx;
 
       if (!get_file_sd (get_handle (), pc, sd, false))
 	{
 	  aclp = (aclent_t *) tp.c_get ();
 	  if ((nentries = get_posix_access (sd, NULL, &uid, &gid,
-					    aclp, MAX_ACL_ENTRIES)) >= 0)
+					    aclp, MAX_ACL_ENTRIES,
+					    &standard_acl)) >= 0)
 	    {
 	      /* Overwrite ACL permissions as required by POSIX 1003.1e
 		 draft 17. */
 	      aclp[0].a_perm = (mode >> 6) & S_IRWXO;
-#if 0
-	      /* Deliberate deviation from POSIX 1003.1e here.  We're not
-		 writing CLASS_OBJ *or* GROUP_OBJ, but both.  Otherwise we're
-		 going to be in constant trouble with user expectations. */
-	      if ((idx = searchace (aclp, nentries, GROUP_OBJ)) >= 0)
-		aclp[idx].a_perm = (mode >> 3) & S_IRWXO;
-	      if (nentries > MIN_ACL_ENTRIES
-		  && (idx = searchace (aclp, nentries, CLASS_OBJ)) >= 0)
-		aclp[idx].a_perm = (mode >> 3) & S_IRWXO;
-#else
+
 	      /* POSIXly correct: If CLASS_OBJ is present, chmod only modifies
-		 CLASS_OBJ, not GROUP_OBJ. */
+		 CLASS_OBJ, not GROUP_OBJ.
+
+		 Deliberate deviation from POSIX 1003.1e:  If the ACL is a
+		 "standard" ACL, that is, it only contains POSIX permissions
+		 as well as entries for the Administrators group and SYSTEM,
+		 then it's kind of a POSIX-only ACL in a twisted, Windowsy
+		 way.  If so, we change GROUP_OBJ and CLASS_OBJ perms. */
+	      if (standard_acl
+		  && (idx = searchace (aclp, nentries, GROUP_OBJ)) >= 0)
+		aclp[idx].a_perm = (mode >> 3) & S_IRWXO;
 	      if (nentries > MIN_ACL_ENTRIES
 		  && (idx = searchace (aclp, nentries, CLASS_OBJ)) >= 0)
 		aclp[idx].a_perm = (mode >> 3) & S_IRWXO;
-	      else if ((idx = searchace (aclp, nentries, GROUP_OBJ)) >= 0)
-		aclp[idx].a_perm = (mode >> 3) & S_IRWXO;
-#endif
+
 	      if ((idx = searchace (aclp, nentries, OTHER_OBJ)) >= 0)
 		aclp[idx].a_perm = mode & S_IRWXO;
 	      if (pc.isdir ())
diff --git a/winsup/cygwin/sec_acl.cc b/winsup/cygwin/sec_acl.cc
index be5442336..96c6fc314 100644
--- a/winsup/cygwin/sec_acl.cc
+++ b/winsup/cygwin/sec_acl.cc
@@ -561,11 +561,17 @@ getace (aclent_t &acl, int type, int id, DWORD win_ace_mask,
 
 /* From the SECURITY_DESCRIPTOR given in psd, compute user, owner, posix
    attributes, as well as the POSIX acl.  The function returns the number
-   of entries returned in aclbufp, or -1 in case of error. */
+   of entries returned in aclbufp, or -1 in case of error.
+
+   When called from chmod, it also returns the fact if the ACL is a "standard"
+   ACL.  A "standard" ACL is an ACL which only consists of ACEs for owner,
+   group, other, as well as (this is Windows) the Administrators group and
+   SYSTEM.  See fhandler_disk_file::fchmod for how this is used to fake
+   stock POSIX perms even if Administrators and SYSTEM is in the ACE. */
 int
 get_posix_access (PSECURITY_DESCRIPTOR psd,
 		  mode_t *attr_ret, uid_t *uid_ret, gid_t *gid_ret,
-		  aclent_t *aclbufp, int nentries)
+		  aclent_t *aclbufp, int nentries, bool *std_acl)
 {
   tmp_pathbuf tp;
   NTSTATUS status;
@@ -852,7 +858,10 @@ get_posix_access (PSECURITY_DESCRIPTOR psd,
 			 GROUP_OBJ entry. */
 		      if (ace_sid != well_known_system_sid
 			  && ace_sid != well_known_admins_sid)
-			class_perm |= lacl[pos].a_perm;
+			{
+			  class_perm |= lacl[pos].a_perm;
+			  standard_ACEs_only = false;
+			}
 		    }
 		}
 	      /* For a newly created file, we'd like to know if we're running
@@ -861,7 +870,6 @@ get_posix_access (PSECURITY_DESCRIPTOR psd,
 		 a standard ACL, we apply umask.  That's not entirely correct,
 		 but it's probably the best we can do. */
 	      else if (type & (USER | GROUP)
-		       && just_created
 		       && standard_ACEs_only
 		       && ace_sid != well_known_system_sid
 		       && ace_sid != well_known_admins_sid)
@@ -1104,6 +1112,8 @@ out:
 	aclbufp[idx].a_perm &= S_IRWXO;
       aclsort32 (pos, 0, aclbufp);
     }
+  if (std_acl)
+    *std_acl = standard_ACEs_only;
   return pos;
 }
 
diff --git a/winsup/cygwin/security.h b/winsup/cygwin/security.h
index b4c7a023f..0d744dfd4 100644
--- a/winsup/cygwin/security.h
+++ b/winsup/cygwin/security.h
@@ -467,7 +467,7 @@ int searchace (struct acl *, int, int, uid_t id = ILLEGAL_UID);
 PSECURITY_DESCRIPTOR set_posix_access (mode_t, uid_t, gid_t, struct acl *, int,
 				       security_descriptor &, bool);
 int get_posix_access (PSECURITY_DESCRIPTOR, mode_t *, uid_t *, gid_t *,
-		      struct acl *, int);
+		      struct acl *, int, bool * = NULL);
 int getacl (HANDLE, path_conv &, int, struct acl *);
 int setacl (HANDLE, path_conv &, int, struct acl *, bool &);