[newlib-cygwin/main] Cygwin: uinfo: correctly check and override primary group

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=9edeb1d8b9c672436a42488e13d34f6eb6a4660d

commit 9edeb1d8b9c672436a42488e13d34f6eb6a4660d
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Fri Dec 5 17:07:08 2025 +0100
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Mon Dec 22 12:38:33 2025 +0100

    Cygwin: uinfo: correctly check and override primary group
    
    Commit dc7b67316d01 ("Cygwin: uinfo: prefer token primary group")
    broke the code overriding the primary group in two different ways:
    
    - It changed the way myself->gid was set before checking its value.
    
      Prior to dc7b67316d01, myself->gid was always set to the primary group
      from the passwd entry (pw_gid).  With the patch, it was set to the
      primary group from the Windows user token (token_gid) in the first
      place.
    
      The following condition checking if pw_gid is different
      from token_gid did so, by checking token_gid against myself->gid,
      rather than against pw_gid.  After dc7b67316d01 this was always
      false and the code block overriding the primary group in Cygwin and
      the Windows user token with pw_gid was never called anymore.
    
      The solution is obvious: Do not check token_gid against myself->gid,
      but against the desires primary GID value in pw_gid instead.
    
    - The code block overriding the primary group simply assumed that
      myself->gid was already set to pw_gid, but, as outlined above,
      this was not true anymore after dc7b67316d01.
    
      This is a subtle error, because it leads to having the wrong primary
      GID in `id' output, while the primary group SID in the user token was
      correctly set.  But as soon as you run this under strace or GDB, the
      problem disappears, because the second process tree under GDB or
      strace takes over from the already changed user token.
    
      The solution is to override myself->gid with pw_gid once more, after
      successfully changing the primary GID to pw_gid.
    
    Fixes: dc7b67316d01 ("Cygwin: uinfo: prefer token primary group")
    Addresses: https://cygwin.com/pipermail/cygwin/2025-December/259068.html
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/uinfo.cc | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/uinfo.cc b/winsup/cygwin/uinfo.cc
index ffe71ee0726c..8e9b9e07de9d 100644
--- a/winsup/cygwin/uinfo.cc
+++ b/winsup/cygwin/uinfo.cc
@@ -174,7 +174,7 @@ internal_getlogin (cygheap_user &user)
       gsid = cygheap->dom.account_sid ();
       gsid.append (DOMAIN_GROUP_RID_USERS);
       if (!pgrp
-	  || (myself->gid != pgrp->gr_gid
+	  || (pwd->pw_gid != pgrp->gr_gid
 	      && cygheap->dom.account_sid () != cygheap->dom.primary_sid ()
 	      && RtlEqualSid (gsid, user.groups.pgsid)))
 	{
@@ -209,7 +209,10 @@ internal_getlogin (cygheap_user &user)
 			myself->gid = pwd->pw_gid = pgrp->gr_gid;
 		    }
 		  else
-		    user.groups.pgsid = gsid;
+		    {
+		      user.groups.pgsid = gsid;
+		      myself->gid = pwd->pw_gid;
+		    }
 		  clear_procimptoken ();
 		}
 	    }