write: tell when effective gid and tty path group mismatch

Most commonly this error happens when write(1) executable does not have
correct group ownership and setgid bit.  The earlier message was unclear
what exactly was wrong.

$ mesg
is y
$ write testuser
write: you have write permission turned off

Alternatively the 'getegid() == s.st_gid' could be considered unnecessary.
Afterall if to write to destination tty is denied that does not go unnoticed
at thet time when tty is opened.

Reviewed-by: Benno Schulenberg <bensberg@justemail.net>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

diff --git a/term-utils/write.c b/term-utils/write.c
index 8855c1e931c1f641f8e2f7fb61caf558d0615802..1b476473f1589a0cf340a3c4edb2757338117b41 100644 (file)
--- a/term-utils/write.c
+++ b/term-utils/write.c
@@ -113,8 +113,13 @@ static int check_tty(char *tty, int *tty_writeable, time_t *tty_atime, int showe
        }
        if (getuid() == 0)      /* root can always write */
                *tty_writeable = 1;
-       else
-               *tty_writeable = (s.st_mode & S_IWGRP) && (getegid() == s.st_gid);
+       else {
+               if (getegid() != s.st_gid) {
+                       warnx(_("effective gid does not match group of %s"), path);
+                       return 1;
+               }
+               *tty_writeable = s.st_mode & S_IWGRP;
+       }
        if (tty_atime)
                *tty_atime = s.st_atime;
        return 0;