Allow update-game-score to run sgid instead of suid.

* configure.ac (gamegroup): New AC_SUBST.
(--with-gameuser): Allow to specify a group instead of a user.
In the default case, check at configure time if a 'games' user
exists.
* lib-src/update-game-score.c: Allow the program to run sgid
instead of suid, in order to match common practice for most games.
(main): Check if we are running sgid.  Pass appropriate file
permission bits to 'write_scores'.
(write_scores): New 'mode' argument, instead of hardcoding 0644.
(get_prefix): Update error message.
* lib-src/Makefile.in (gamegroup): New variable, set by configure.
($(DESTDIR)${archlibdir}): Handle both suid or sgid when
installing the 'update-game-score' program.
* lisp/play/gamegrid.el (gamegrid-add-score-with-update-game-score):
Allow the 'update-game-score' helper program to run suid or sgid.

1 files changed, 19 insertions, 14 deletions

diff --git a/lib-src/update-game-score.c b/lib-src/update-game-score.c
index d3354af2783..4f154832c81 100644
--- a/lib-src/update-game-score.c
+++ b/lib-src/update-game-score.c
@@ -21,8 +21,8 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 
 
 /* This program allows a game to securely and atomically update a
-   score file.  It should be installed setuid, owned by an appropriate
-   user like `games'.
+   score file.  It should be installed either setuid or setgid, owned
+   by an appropriate user or group like `games'.
 
    Alternatively, it can be compiled without HAVE_SHARED_GAME_DIR
    defined, and in that case it will store scores in the user's home
@@ -88,7 +88,7 @@ static int push_score (struct score_entry **scores, ptrdiff_t *count,
                        ptrdiff_t *size, struct score_entry const *newscore);
 static void sort_scores (struct score_entry *scores, ptrdiff_t count,
                          bool reverse);
-static int write_scores (const char *filename,
+static int write_scores (const char *filename, mode_t mode,
                          const struct score_entry *scores, ptrdiff_t count);
 
 static _Noreturn void
@@ -122,18 +122,19 @@ get_user_id (void)
 }
 
 static const char *
-get_prefix (bool running_suid, const char *user_prefix)
+get_prefix (bool privileged, const char *user_prefix)
 {
-  if (!running_suid && user_prefix == NULL)
-    lose ("Not using a shared game directory, and no prefix given.");
-  if (running_suid)
+  if (privileged)
     {
 #ifdef HAVE_SHARED_GAME_DIR
       return HAVE_SHARED_GAME_DIR;
 #else
-      lose ("This program was compiled without HAVE_SHARED_GAME_DIR,\n and should not be suid.");
+      lose ("This program was compiled without HAVE_SHARED_GAME_DIR,\n"
+            "and should not run with elevated privileges.");
 #endif
     }
+  if (user_prefix == NULL)
+    lose ("Not using a shared game directory, and no prefix given.");
   return user_prefix;
 }
 
@@ -173,7 +174,7 @@ int
 main (int argc, char **argv)
 {
   int c;
-  bool running_suid;
+  bool running_suid, running_sgid;
   void *lockstate;
   char *scorefile;
   char *end, *nl, *user, *data;
@@ -214,8 +215,11 @@ main (int argc, char **argv)
     usage (EXIT_FAILURE);
 
   running_suid = (getuid () != geteuid ());
+  running_sgid = (getgid () != getegid ());
+  if (running_suid && running_sgid)
+    lose ("This program can run either suid or sgid, but not both.");
 
-  prefix = get_prefix (running_suid, user_prefix);
+  prefix = get_prefix (running_suid || running_sgid, user_prefix);
 
   scorefile = malloc (strlen (prefix) + strlen (argv[optind]) + 2);
   if (!scorefile)
@@ -270,7 +274,8 @@ main (int argc, char **argv)
         scores += scorecount - max_scores;
       scorecount = max_scores;
     }
-  if (write_scores (scorefile, scores, scorecount) < 0)
+  if (write_scores (scorefile, running_sgid ? 0664 : 0644,
+                    scores, scorecount) < 0)
     {
       unlock_file (scorefile, lockstate);
       lose_syserr ("Failed to write scores file");
@@ -421,8 +426,8 @@ sort_scores (struct score_entry *scores, ptrdiff_t count, bool reverse)
 }
 
 static int
-write_scores (const char *filename, const struct score_entry *scores,
-              ptrdiff_t count)
+write_scores (const char *filename, mode_t mode,
+              const struct score_entry *scores, ptrdiff_t count)
 {
   int fd;
   FILE *f;
@@ -435,7 +440,7 @@ write_scores (const char *filename, const struct score_entry *scores,
   if (fd < 0)
     return -1;
 #ifndef DOS_NT
-  if (fchmod (fd, 0644) != 0)
+  if (fchmod (fd, mode) != 0)
     return -1;
 #endif
   f = fdopen (fd, "w");