cttyhack: check sysfs for the name of the active console

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 48 insertions, 31 deletions

diff --git a/shell/cttyhack.c b/shell/cttyhack.c
index d1ac2cd23..522a1ba73 100644
--- a/shell/cttyhack.c
+++ b/shell/cttyhack.c
@@ -14,18 +14,22 @@
 //config:       bool "cttyhack"
 //config:       default y
 //config:       help
-//config:         One common problem reported on the mailing list is "can't access tty;
-//config:         job control turned off" error message which typically appears when
-//config:         one tries to use shell with stdin/stdout opened to /dev/console.
+//config:         One common problem reported on the mailing list is the "can't
+//config:         access tty; job control turned off" error message, which typically
+//config:         appears when one tries to use a shell with stdin/stdout on
+//config:         /dev/console.
 //config:         This device is special - it cannot be a controlling tty.
 //config:
-//config:         Proper solution is to use correct device instead of /dev/console.
+//config:         The proper solution is to use the correct device instead of
+//config:         /dev/console.
 //config:
-//config:         cttyhack provides "quick and dirty" solution to this problem.
+//config:         cttyhack provides a "quick and dirty" solution to this problem.
 //config:         It analyzes stdin with various ioctls, trying to determine whether
 //config:         it is a /dev/ttyN or /dev/ttySN (virtual terminal or serial line).
-//config:         If it detects one, it closes stdin/out/err and reopens that device.
-//config:         Then it executes given program. Opening the device will make
+//config:         On Linux it also checks sysfs for a pointer to the active console.
+//config:         If cttyhack is able to find the real console device, it closes
+//config:         stdin/out/err and reopens that device.
+//config:         Then it executes the given program. Opening the device will make
 //config:         that device a controlling tty. This may require cttyhack
 //config:         to be a session leader.
 //config:
@@ -115,33 +119,46 @@ int cttyhack_main(int argc UNUSED_PARAM, char **argv)
                 close(fd);
         } else {
                 /* We don't have ctty (or don't have "/dev/tty" node...) */
-                if (0) {}
-#ifdef TIOCGSERIAL
-                else if (ioctl(0, TIOCGSERIAL, &u.sr) == 0) {
-                        /* this is a serial console */
-                        sprintf(console + 8, "S%d", u.sr.line);
-                }
-#endif
+                do {
 #ifdef __linux__
-                else if (ioctl(0, VT_GETSTATE, &u.vt) == 0) {
-                        /* this is linux virtual tty */
-                        sprintf(console + 8, "S%d" + 1, u.vt.v_active);
-                }
+                        int s = open_read_close("/sys/class/tty/console/active",
+                                console + 5, sizeof(console) - 5 - 1);
+                        if (s > 0) {
+                                /* found active console via sysfs (Linux 2.6.38+) */
+                                console[5 + s] = '\0';
+                                break;
+                        }
+
+                        if (ioctl(0, VT_GETSTATE, &u.vt) == 0) {
+                                /* this is linux virtual tty */
+                                sprintf(console + 8, "S%d" + 1, u.vt.v_active);
+                                break;
+                        }
 #endif
-                if (console[8]) {
-                        fd = xopen(console, O_RDWR);
-                        //bb_error_msg("switching to '%s'", console);
-                        dup2(fd, 0);
-                        dup2(fd, 1);
-                        dup2(fd, 2);
-                        while (fd > 2)
-                                close(fd--);
-                        /* Some other session may have it as ctty,
-                         * steal it from them:
-                         */
-                        ioctl(0, TIOCSCTTY, 1);
-                }
+#ifdef TIOCGSERIAL
+                        if (ioctl(0, TIOCGSERIAL, &u.sr) == 0) {
+                                /* this is a serial console, asuming it is named /dev/ttySn */
+                                sprintf(console + 8, "S%d", u.sr.line);
+                                break;
+                        }
+#endif
+                        /* nope, could not find it */
+                        goto ret;
+                } while (0);
+
+                fd = xopen(console, O_RDWR);
+                //bb_error_msg("switching to '%s'", console);
+                dup2(fd, 0);
+                dup2(fd, 1);
+                dup2(fd, 2);
+                while (fd > 2)
+                        close(fd--);
+                /* Some other session may have it as ctty,
+                 * steal it from them:
+                 */
+                ioctl(0, TIOCSCTTY, 1);
         }
 
+ret:
         BB_EXECVP_or_die(argv);
 }