who --mesg (-T) can use a more accurate test for TTY writability

Enabled when coreutils is configured with --with-tty-group.
Based on a patch written by Piotr Gackiewicz.  Details at
http://bugzilla.redhat.com/454261

* src/who.c (is_tty_writable): A new function returning true if a TTY
device is writable by the group.  Additionally it checks the group to be
the same as TTY_GROUP_NAME when compiled with --with-tty-group.
* m4/jm-macros.m4: Introduce a new configure option --with-tty-group.
* NEWS: Mention the change.

NEWS

@@ -2,6 +2,18 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** New features
+
+  who: the "+/-" --mesg (-T) indicator of whether a user/tty is accepting
+  messages could be incorrectly listed as "+", when in fact, the user was
+  not accepting messages (mesg no).  Before, who would examine only the
+  permission bits, and not consider the group of the TTY device file.
+  Thus, if a login tty's group would change somehow e.g., to "root",
+  that would make it unwritable (via write(1)) by normal users, in spite
+  of whatever the permission bits might imply.  Now, when configured
+  using the --with-tty-group[=NAME] option, who also compares the group
+  of the TTY device with NAME (or "tty" if no group name is specified).
+
 
 * Noteworthy changes in release 8.4 (2010-01-13) [stable]
 

THANKS

@@ -495,6 +495,7 @@ Philippe Schnoebelen                Philippe.Schnoebelen@imag.fr
 Phillip Jones                       mouse@datastacks.com
 Piergiorgio Sartor                  sartor@sony.de
 Pieter Bowman                       bowman@math.utah.edu
+Piotr Gackiewicz                    gacek@intertele.pl
 Piotr Kwapulinski                   kwap@univ.gda.pl
 Prashant TR                         tr@eth.net
 Priit Jõerüüt                       jemm4jemm@yahoo.com

m4/jm-macros.m4

@@ -144,6 +144,25 @@ AC_DEFUN([coreutils_MACROS],
     ])
 
   AC_REQUIRE([AM_LANGINFO_CODESET])
+
+  # Accept configure options: --with-tty-group[=GROUP], --without-tty-group
+  # You can determine the group of a TTY via 'stat --format %G /dev/tty'
+  # Omitting this option is equivalent to using --without-tty-group.
+  AC_ARG_WITH([tty-group],
+    AS_HELP_STRING([--with-tty-group[[[=NAME]]]],
+      [group used by system for TTYs, "tty" when not specified]
+      [ (default: do not rely on any group used for TTYs)]),
+    [tty_group_name=$withval],
+    [tty_group_name=no])
+
+  if test "x$tty_group_name" != xno; then
+    if test "x$tty_group_name" = xyes; then
+      tty_group_name=tty
+    fi
+    AC_MSG_NOTICE([TTY group used by system set to "$tty_group_name"])
+    AC_DEFINE_UNQUOTED([TTY_GROUP_NAME], ["$tty_group_name"],
+      [group used by system for TTYs])
+  fi
 ])
 
 AC_DEFUN([gl_CHECK_ALL_HEADERS],

src/who.c

@@ -37,6 +37,10 @@
 #include "hard-locale.h"
 #include "quote.h"
 
+#ifdef TTY_GROUP_NAME
+# include <grp.h>
+#endif
+
 /* The official name of this program (e.g., no `g' prefix).  */
 #define PROGRAM_NAME "who"
 
@@ -308,6 +312,22 @@ print_line (int userlen, const char *user, const char state,
   free (x_exitstr);
 }
 
+/* Return true if a terminal device given as PSTAT allows other users
+   to send messages to; false otherwise */
+static bool
+is_tty_writable (struct stat const *pstat)
+{
+#ifdef TTY_GROUP_NAME
+  /* Ensure the group of the TTY device matches TTY_GROUP_NAME, more info at
+     https://bugzilla.redhat.com/454261 */
+  struct group *ttygr = getgrnam (TTY_GROUP_NAME);
+  if (!ttygr || (pstat->st_gid != ttygr->gr_gid))
+    return false;
+#endif
+
+  return pstat->st_mode & S_IWGRP;
+}
+
 /* Send properly parsed USER_PROCESS info to print_line.  The most
    recent boot time is BOOTTIME. */
 static void
@@ -346,7 +366,7 @@ print_user (const STRUCT_UTMP *utmp_ent, time_t boottime)
 
   if (stat (line, &stats) == 0)
     {
-      mesg = (stats.st_mode & S_IWGRP) ? '+' : '-';
+      mesg = is_tty_writable (&stats) ? '+' : '-';
       last_change = stats.st_atime;
     }
   else