From: utzoo!decvax!decwrl!sun!megatest!fortune!hpda!hplabs!sri-unix!chris.umcp-cs@UDel-Relay
Newsgroups: net.unix-wizards
Title: Re:  root executing setuid programs
Article-I.D.: sri-arpa.763
Posted: Thu Mar 10 00:28:03 1983
Received: Fri Apr  1 07:20:56 1983

From:  Chris Torek <chris.umcp-cs@UDel-Relay>

From: hplabs!hao!csu-cs!bentson@UCB-C70

	The line printer daemon and other utilities have a need for
	mutual exclusion because they can be simultaneously invoked
	by many users. Since there is no such intrinsic in U*nx, there
	is an idiomatic expression that provides the same effect....
	The U*nx idiom is to create a file with "0" permission so
	that a second attempt to create a file will fail....

	HOWEVER, if either the program is owned by superuser and is setuid
	or the program is run by superuser the example will not work as
	predicted....

There is a better idiomatic expression:

	unlink (tempfile);
	creat (tempfile, mode);
	if (link (tempfile, lockfile) < 0) {
		/* someone else is doing the work */
		exit (0);
	}

The link system call will not let ANYONE link one file to another if
the second file exists.