libauth: re-implement procsetuser() to use /proc instead of #c/user

sys/src/libauth/procsetuser.c

@@ -5,16 +5,30 @@
 int
 procsetuser(char *user)
 {
-	int fd, n;
+	char name[32];
+	Dir dir;
 
-	fd = open("#c/user", OWRITE|OCEXEC);
-	if(fd < 0)
-		return -1;
-	n = strlen(user);
-	if(write(fd, user, n) != n){
+	nulldir(&dir);
+	dir.uid = user;
+	snprint(name, sizeof(name), "/proc/%lud/ctl", (ulong)getpid());
+	if(dirwstat(name, &dir) < 0){
+		/*
+		 * this is backwards compatibility code as
+		 * devproc initially didnt allow changing
+		 * the user to none.
+		 */
+		int fd;
+
+		if(strcmp(user, "none") != 0)
+			return -1;
+		fd = open("#c/user", OWRITE|OCEXEC);
+		if(fd < 0)
+			return -1;
+		if(write(fd, "none", 4) != 4){
+			close(fd);
+			return -1;
+		}
 		close(fd);
-		return -1;
 	}
-	close(fd);
 	return 0;
 }