Print a warning if not setuid root.

Document the need for the setuid bit and how to set it.
Explain why it isn't set by default, and suggest simply adding users
to groups instead.

PR:		docs/167741
MFC after:	3 weeks

usr.bin/newgrp/newgrp.1

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 23, 2002
+.Dd February 8, 2013
 .Dt NEWGRP 1
 .Os
 .Sh NAME
@@ -90,6 +90,15 @@ A
 utility appeared in
 .At v6 .
 .Sh BUGS
+For security reasons, the
+.Nm
+utility is normally installed without the setuid bit.
+To enable it, run the following command:
+.Bd -literal -offset indent
+chmod u+s /usr/bin/newgrp
+.Ed
+.Pp
 Group passwords are inherently insecure as there is no way to stop
-users obtaining the crypted passwords from the group database.
+users obtaining the password hash from the group database.
 Their use is discouraged.
+Instead, users should simply be added to the necessary groups.

usr.bin/newgrp/newgrp.c

@@ -73,7 +73,8 @@ main(int argc, char *argv[])
 {
 	int ch, login;
 
-	euid = geteuid();
+	if ((euid = geteuid()) != 0)
+		warnx("need root permissions to function properly, check setuid bit");
 	if (seteuid(getuid()) < 0)
 		err(1, "seteuid");