tty: Destroy ldisc instance on hangup

Currently, when the tty is hungup, the ldisc is re-instanced; ie., the
current instance is destroyed and a new instance is created. The purpose
of this design was to guarantee a valid, open ldisc for the lifetime of
the tty.

However, now that tty buffers are owned by and have lifetime equivalent
to the tty_port (since v3.10), any data received immediately after the
ldisc is re-instanced may cause continued driver i/o operations
concurrently with the driver's hangup() operation. For drivers that
shutdown h/w on hangup, this is unexpected and usually bad. For example,
the serial core may free the xmit buffer page concurrently with an
in-progress write() operation (triggered by echo).

With the existing stable and robust ldisc reference handling, the
cleaned-up tty_reopen(), the straggling unsafe ldisc use cleaned up, and
the preparation to properly handle a NULL tty->ldisc, the ldisc instance
can be destroyed and only re-instanced when the tty is re-opened.

If the tty was opened as /dev/console or /dev/tty0, the original behavior
of re-instancing the ldisc is retained (the 'reinit' parameter to
tty_ldisc_hangup() is true). This is required since those file descriptors
are never hungup.

This patch has neglible impact on userspace; the tty file_operations ptr
is changed to point to the hungup file operations _before_ the ldisc
instance is destroyed, so only racing file operations might now retrieve
a NULL ldisc reference (which is simply handled as if the hungup file
operation had been called instead -- see "tty: Prepare for destroying
line discipline on hangup").

This resolves a long-standing FIXME and several crash reports.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 7f556e3c151594f9d447dcaf043b8dc5e03b4bd4..d3ecbb513fa14fd7a9f8b7df8c2c9c8093949812 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -727,7 +727,7 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
        while (refs--)
                tty_kref_put(tty);
 
-       tty_ldisc_hangup(tty);
+       tty_ldisc_hangup(tty, cons_filp != NULL);
 
        spin_lock_irq(&tty->ctrl_lock);
        clear_bit(TTY_THROTTLED, &tty->flags);
@@ -752,10 +752,9 @@ static void __tty_hangup(struct tty_struct *tty, int exit_session)
        } else if (tty->ops->hangup)
                tty->ops->hangup(tty);
        /*
-        * We don't want to have driver/ldisc interactions beyond
-        * the ones we did here. The driver layer expects no
-        * calls after ->hangup() from the ldisc side. However we
-        * can't yet guarantee all that.
+        * We don't want to have driver/ldisc interactions beyond the ones
+        * we did here. The driver layer expects no calls after ->hangup()
+        * from the ldisc side, which is now guaranteed.
         */
        set_bit(TTY_HUPPED, &tty->flags);
        tty_unlock(tty);
@@ -1475,7 +1474,8 @@ static int tty_reopen(struct tty_struct *tty)
 
        tty->count++;
 
-       WARN_ON(!tty->ldisc);
+       if (!tty->ldisc)
+               return tty_ldisc_reinit(tty, tty->termios.c_line);
 
        return 0;
 }

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index e18c8e86411040e5fe452abe9790b03aba5d3b18..c6f970d63060253fdc236b182e62865c3f3439e8 100644 (file)
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -257,6 +257,9 @@ const struct file_operations tty_ldiscs_proc_fops = {
  *     reference to it. If the line discipline is in flux then
  *     wait patiently until it changes.
  *
+ *     Returns: NULL if the tty has been hungup and not re-opened with
+ *              a new file descriptor, otherwise valid ldisc reference
+ *
  *     Note: Must not be called from an IRQ/timer context. The caller
  *     must also be careful not to hold other locks that will deadlock
  *     against a discipline change, such as an existing ldisc reference
@@ -642,14 +645,15 @@ static void tty_reset_termios(struct tty_struct *tty)
  *     @disc: line discipline to reinitialize
  *
  *     Completely reinitialize the line discipline state, by closing the
- *     current instance and opening a new instance. If an error occurs opening
- *     the new non-N_TTY instance, the instance is dropped and tty->ldisc reset
- *     to NULL. The caller can then retry with N_TTY instead.
+ *     current instance, if there is one, and opening a new instance. If
+ *     an error occurs opening the new non-N_TTY instance, the instance
+ *     is dropped and tty->ldisc reset to NULL. The caller can then retry
+ *     with N_TTY instead.
  *
  *     Returns 0 if successful, otherwise error code < 0
  */
 
-static int tty_ldisc_reinit(struct tty_struct *tty, int disc)
+int tty_ldisc_reinit(struct tty_struct *tty, int disc)
 {
        struct tty_ldisc *ld;
        int retval;
@@ -693,11 +697,9 @@ static int tty_ldisc_reinit(struct tty_struct *tty, int disc)
  *     tty itself so we must be careful about locking rules.
  */
 
-void tty_ldisc_hangup(struct tty_struct *tty)
+void tty_ldisc_hangup(struct tty_struct *tty, bool reinit)
 {
        struct tty_ldisc *ld;
-       int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS;
-       int err = 0;
 
        tty_ldisc_debug(tty, "%p: hangup\n", tty->ldisc);
 
@@ -725,25 +727,17 @@ void tty_ldisc_hangup(struct tty_struct *tty)
         */
        tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
 
-       if (tty->ldisc) {
+       if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
+               tty_reset_termios(tty);
 
-               /* At this point we have a halted ldisc; we want to close it and
-                  reopen a new ldisc. We could defer the reopen to the next
-                  open but it means auditing a lot of other paths so this is
-                  a FIXME */
-               if (reset == 0)
-                       err = tty_ldisc_reinit(tty, tty->termios.c_line);
-
-               /* If the re-open fails or we reset then go to N_TTY. The
-                  N_TTY open cannot fail */
-               if (reset || err < 0)
-                       tty_ldisc_reinit(tty, N_TTY);
+       if (tty->ldisc) {
+               if (reinit) {
+                       if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0)
+                               tty_ldisc_reinit(tty, N_TTY);
+               } else
+                       tty_ldisc_kill(tty);
        }
        tty_ldisc_unlock(tty);
-       if (reset)
-               tty_reset_termios(tty);
-
-       tty_ldisc_debug(tty, "%p: re-opened\n", tty->ldisc);
 }
 
 /**

diff --git a/include/linux/tty.h b/include/linux/tty.h
index e3b0afbde7b98a870fbd1f8a3430c5a41706f47f..95e52c6b9696ff30001dac2fee88ff27c2da0c66 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -490,7 +490,8 @@ extern int tty_set_termios(struct tty_struct *tty, struct ktermios *kt);
 extern struct tty_ldisc *tty_ldisc_ref(struct tty_struct *);
 extern void tty_ldisc_deref(struct tty_ldisc *);
 extern struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *);
-extern void tty_ldisc_hangup(struct tty_struct *tty);
+extern void tty_ldisc_hangup(struct tty_struct *tty, bool reset);
+extern int tty_ldisc_reinit(struct tty_struct *tty, int disc);
 extern const struct file_operations tty_ldiscs_proc_fops;
 
 extern void tty_wakeup(struct tty_struct *tty);