> False alarm. What I didn't mention, because I thought it was
> unimportant, was that I was running passwd after having su'd to one
> of the user login-names.  Apparently passwd doesn't like you to do
> this. If I rlogin to the user account, passwd works fine.
>
> Can someone explain to this novice why this is true?

from passwd.c:

	username = getlogin();
	.
	.
	.
	exit(local_passwd(username));

from local_passwd.c:
	local_passwd(uname)
	char *uname;
	{
	.
	.
	pw = getpwnam(uname)
	.
	.
	uid = getuid();
+>	if (uid && uid != pw->pw_uid) {
|       	(void)fprintf(stderr, "passwd: %s\n", strerror(EACCES));
|       	return(1);
|       }
|
|  pw->pw_uid ultimately comes from looking up your account in the
|  utmp file via getlogin().  So if you log in as root, su to fred
|  with a uid of 100, then the if statement evalutates into:
|
+>      if (100 && 100 != 0) {
		(void)fprintf(stderr, "passwd: you're shitting me\n");
		return(1);
	}