id,groups: with no user name, print only real and/or effective IDs,

... i.e., don't use the getpw* functions.

Before this change, running groups or id with no user name argument
would include a group name or ID from /etc/passwd.  Thus, under unusual
circumstances (default group is changed, but has not taken effect for a
given session), those programs could print a name or ID that is neither
real nor effective.

To demonstrate, run this:

    echo 'for i in 1 2; do id -G; sleep 1.5; done' \
      |su -s /bin/sh ftp - &
    sleep 1; perl -pi -e 's/^(ftp:x:\d+):(\d+)/$1:9876/' /etc/passwd

Those id -G commands printed the following:

    50
    50 9876

With this change, they print this:

    50
    50

Similarly, running those programs set-GID could make them
print one ID too many.

* src/group-list.c (print_group_list): When username is NULL, pass
egid, not getpwuid(ruid)->pw_gid), to xgetgroups, per the API
requirements of xgetgroups callee, mgetgroups.
When not using the password database, don't call getpwuid.
* NEWS (Bug fixes): Mention it.
* tests/misc/id-setgid: New file.
* tests/Makefile.am (TESTS): Add it.
(root_tests): It's a root-only test, so add it here, too.
Originally reported by Brynnen Owen as http://bugs.gnu.org/7320.
Raised again by Marc Mengel in http://bugzilla.redhat.com/816708.

1 files changed, 8 insertions, 6 deletions

diff --git a/src/group-list.c b/src/group-list.c
index cf499118f..edbb34213 100644
--- a/src/group-list.c
+++ b/src/group-list.c
@@ -38,11 +38,14 @@ print_group_list (const char *username,
                   bool use_names)
 {
   bool ok = true;
-  struct passwd *pwd;
+  struct passwd *pwd = NULL;
 
-  pwd = getpwuid (ruid);
-  if (pwd == NULL)
-    ok = false;
+  if (username)
+    {
+      pwd = getpwuid (ruid);
+      if (pwd == NULL)
+        ok = false;
+    }
 
   if (!print_group (rgid, use_names))
     ok = false;
@@ -58,8 +61,7 @@ print_group_list (const char *username,
     gid_t *groups;
     int i;
 
-    int n_groups = xgetgroups (username, (pwd ? pwd->pw_gid : (gid_t) -1),
-                               &groups);
+    int n_groups = xgetgroups (username, (pwd ? pwd->pw_gid : egid), &groups);
     if (n_groups < 0)
       {
         if (username)