login: use some ideas from util-linux's login.

O_NONBLOCKing output on login timeout.
fchmod instead of chmod (latter is racy).
is_my_tty() is not needed anymore after race is fixed
(is_my_tty() was racy too anyway...).

1 files changed, 15 insertions, 25 deletions

diff --git a/loginutils/login.c b/loginutils/login.c
index 19f865550..c0f4b72bb 100644
--- a/loginutils/login.c
+++ b/loginutils/login.c
@@ -51,14 +51,22 @@ static inline int check_securetty(void) { return 1; }
 
 #endif
 
-static int is_my_tty(void);
 static void get_username_or_die(char *buf, int size_buf);
 static void motd(void);
 
+static void nonblock(int fd)
+{
+        fcntl(fd, F_SETFL, O_NONBLOCK | fcntl(fd, F_GETFL));
+}
 
 static void alarm_handler(int sig ATTRIBUTE_UNUSED)
 {
-        fprintf(stderr, "\r\nLogin timed out after %d seconds\r\n", TIMEOUT);
+        /* This is the escape hatch!  Poor serial line users and the like
+         * arrive here when their connection is broken.
+         * We don't want to block here */
+        nonblock(1);
+        nonblock(2);
+        bb_info_msg("\r\nLogin timed out after %d seconds\r", TIMEOUT);
         exit(EXIT_SUCCESS);
 }
 
@@ -218,13 +226,10 @@ auth_failed:
                 }
         }
 #endif
-        if (!is_my_tty())
-                syslog(LOG_ERR, "unable to determine TTY name, got %s", full_tty);
-
-        /* Try these, but don't complain if they fail
-         * (for example when the root fs is read only) */
-        chown(full_tty, pw->pw_uid, pw->pw_gid);
-        chmod(full_tty, 0600);
+        /* Try these, but don't complain if they fail.
+         * _f_chown is safe wrt race t=ttyname(0);...;chown(t); */
+        fchown(0, pw->pw_uid, pw->pw_gid);
+        fchmod(0, 0600);
 
         if (ENABLE_LOGIN_SCRIPTS) {
                 char *script = getenv("LOGIN_PRE_SUID_SCRIPT");
@@ -257,7 +262,7 @@ auth_failed:
         signal(SIGALRM, SIG_DFL);       /* default alarm signal */
 
         if (pw->pw_uid == 0)
-                syslog(LOG_INFO, "root login %s", fromhost);
+                syslog(LOG_INFO, "root login%s", fromhost);
 #ifdef CONFIG_SELINUX
         /* well, a simple setexeccon() here would do the job as well,
          * but let's play the game for now */
@@ -348,21 +353,6 @@ static int check_securetty(void)
 
 #endif
 
-/* returns 1 if true */
-static int is_my_tty(void)
-{
-        struct stat by_name, by_fd;
-
-        if (stat(full_tty, &by_name) || fstat(0, &by_fd))
-                return 0;
-
-        if (by_name.st_rdev != by_fd.st_rdev)
-                return 0;
-        else
-                return 1;
-}
-
-
 static void motd(void)
 {
         FILE *fp;