Set User (aka su)

[Vadim Antonov avg@postman.ncube.com]

Hi -- there's an equivalent of Unix "su" for Plan9, to
allow to become a different user temporarily.

It uses the file /lib/namespace.su to remount the root file
system under new user name.  On a CPU server or terminal
running kfs that should be

naiad% cat /lib/namespace.su
mount -bc /srv/kfs /
naiad% 

Does anybody know a better way to tell the file system that
user name has changed?  Also, note the magic incantation "cd `{pwd}".
God, if feels like the quote from ken/slp.c.

--vadim

naiad% pwd
/sys/src/cmd/auth
naiad% diff mkfile.old mkfile
16a17
> 	su\
37a39,41
> 
> $BIN/su:V:	$O.su
> 	cp $O.su /$objtype/bin/su
naiad% cat su.c
#include <u.h>
#include <libc.h>
#include <auth.h>
#include "authsrv.h"

void
main(int argc, char *argv[])
{
        char buf[32], pass[32], key[DESKEYLEN];
	int fd;
        Chalstate ch;
	char *s;

	if( argc != 2 ) {
		fprint(2, "usage: su user\n");
		exits("usage");
	}

        s = getenv("service");
        if(s && strcmp(s, "cpu") == 0){
                fprint(2, "su must not be run on the cpu server\n");
                exits("boofhead");
        }

        if(getchal(&ch, argv[1]) < 0) {
		fprint(2, "Authentication failure\n");
		exits("can't get challenge");
	}
        readln("Password: ", pass, sizeof pass, 1);
        passtokey(key, pass);
	strcpy(buf, ch.chal);
	netcrypt(key, buf);
	if( chalreply(&ch, buf) < 0 ) {
		fprint(2, "Authentication failed\n");
		exits("bad response");
	}
	rfork(RFNAMEG|RFENVG);
	if( (fd = create("#e/prompt", OWRITE, 0644)) >= 0 ) {
		snprint(buf, sizeof buf, "[%s]%% ", argv[1]);
		write(fd, buf, strlen(buf));
		close(fd);
	}
	if( (fd = create("#e/user", OWRITE, 0644)) >= 0 ) {
		write(fd, argv[1], strlen(argv[1]));
		close(fd);
	}
	/* Do some magic and start interactive shell */
	execl("/bin/rc", "rc", "-c", 
	      ". /lib/namespace.su; cd `{pwd}; exec /bin/rc -i", 0);
	fprint(2, "Exec failed\n");
	exits("exec");
}
naiad%