[newlib-cygwin] Cygwin: uinfo: prefer token primary group

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=dc7b67316d01c77d81ad6561869b9b89527c2ac8

commit dc7b67316d01c77d81ad6561869b9b89527c2ac8
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Fri Dec 2 16:37:33 2022 +0100
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Fri Dec 2 16:37:33 2022 +0100

    Cygwin: uinfo: prefer token primary group
    
    internal_getlogin overwrites the process token primary group if it
    differs from the primary group as stored in the passwd DB.
    
    However, this also overwrites the primary group of the process if
    it has been deliberately changed by a former process (e. g., newgrp),
    and the current process has a non-Cygwin process as parent.
    
    Our docs claim we restrict overwriting the primary group to local,
    non-domain user accounts anyway, and it was actually meant this way.
    
    So check for exactly that before overwriting the primary group
    in the token:  It's only allowed if the user is a local account
    and the primary group in the token is still the default group
    "None".
    
    Fixes: 6cc7c925ce861 ("(internal_getlogin): Give primary group
    from user token more weight.")
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/uinfo.cc | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/winsup/cygwin/uinfo.cc b/winsup/cygwin/uinfo.cc
index a96c5e7fc6a0..db475d710eb2 100644
--- a/winsup/cygwin/uinfo.cc
+++ b/winsup/cygwin/uinfo.cc
@@ -163,11 +163,20 @@ internal_getlogin (cygheap_user &user)
 
       user.set_name (pwd->pw_name);
       myself->uid = pwd->pw_uid;
-      myself->gid = pwd->pw_gid;
+      myself->gid = pgrp ? pgrp->gr_gid : pwd->pw_gid;
+
       /* If the primary group in the passwd DB is different from the primary
-	 group in the user token, we have to find the SID of that group and
-	 try to override the token primary group. */
-      if (!pgrp || myself->gid != pgrp->gr_gid)
+	 group in the user token, and if the primary group is the default
+	 group of a local user ("None", localized), we have to find the SID
+	 of that group and try to override the token primary group.  Also
+	 makes sure we're not on a domain controller, where account_sid ()
+	 == primary_sid (). */
+      gsid = cygheap->dom.account_sid ();
+      gsid.append (DOMAIN_GROUP_RID_USERS);
+      if (!pgrp
+	  || (myself->gid != pgrp->gr_gid
+	      && cygheap->dom.account_sid () != cygheap->dom.primary_sid ()
+	      && RtlEqualSid (gsid, user.groups.pgsid)))
 	{
 	  if (gsid.getfromgr (grp = internal_getgrgid (pwd->pw_gid, &cldap)))
 	    {