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tty: Re-open /dev/tty without tty_mutex 

Opening /dev/tty (ie., the controlling tty for the current task)
is always a re-open of the underlying tty. Because holding the
tty_lock is sufficient for safely re-opening a tty, and because
having a tty kref is sufficient for safely acquiring the tty_lock [1],
tty_open_current_tty() does not require holding tty_mutex.

Repurpose tty_open_current_tty() to perform the re-open itself and
refactor tty_open().

[1] Analysis of safely re-opening the current tty w/o tty_mutex

get_current_tty() gets a tty kref from the already kref'ed tty value of
current->signal->tty while holding the sighand lock for the current
task. This guarantees that the tty pointer returned from
get_current_tty() points to a tty which remains referenceable
while holding the kref.

Although release_tty() may run concurrently, and thus the driver
reference may be removed, release_one_tty() cannot have run, and
won't while holding the tty kref.

This, in turn, guarantees the tty_lock() can safely be acquired
(since tty->magic and tty->legacy_mutex are still a valid dereferences).
The tty_lock() also gets a tty kref to prevent the tty_unlock() from
dereferencing a released tty. Thus, the kref returned from
get_current_tty() can be released.

Lastly, the first operation of tty_reopen() is to check the tty count.
If non-zero, this ensures release_tty() is not running concurrently,
and the driver references have not been removed.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2014-11-05 12:12:50 -05:00 committed by Greg Kroah-Hartman
parent 55199ea3bd
commit 52494eeb99
1 changed files with 28 additions and 27 deletions
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55
drivers/tty/tty_io.c
View File

@ -1935,20 +1935,20 @@ int tty_release(struct inode *inode, struct file *filp)
}
/**
* tty_open_current_tty - get tty of current task for open
* tty_open_current_tty - get locked tty of current task
* @device: device number
* @filp: file pointer to tty
* @return: tty of the current task iff @device is /dev/tty
* @return: locked tty of the current task iff @device is /dev/tty
*
* Performs a re-open of the current task's controlling tty.
*
* We cannot return driver and index like for the other nodes because
* devpts will not work then. It expects inodes to be from devpts FS.
*
* We need to move to returning a refcounted object from all the lookup
* paths including this one.
*/
static struct tty_struct *tty_open_current_tty(dev_t device, struct file *filp)
{
struct tty_struct *tty;
int retval;
if (device != MKDEV(TTYAUX_MAJOR, 0))
return NULL;
@ -1959,9 +1959,14 @@ static struct tty_struct *tty_open_current_tty(dev_t device, struct file *filp)
filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
/* noctty = 1; */
tty_kref_put(tty);
/* FIXME: we put a reference and return a TTY! */
/* This is only safe because the caller holds tty_mutex */
tty_lock(tty);
tty_kref_put(tty); /* safe to drop the kref now */
retval = tty_reopen(tty);
if (retval < 0) {
tty_unlock(tty);
tty = ERR_PTR(retval);
}
return tty;
}
@ -2059,13 +2064,9 @@ static int tty_open(struct inode *inode, struct file *filp)
index = -1;
retval = 0;
mutex_lock(&tty_mutex);
/* This is protected by the tty_mutex */
tty = tty_open_current_tty(device, filp);
if (IS_ERR(tty)) {
retval = PTR_ERR(tty);
goto err_unlock;
} else if (!tty) {
if (!tty) {
mutex_lock(&tty_mutex);
driver = tty_lookup_driver(device, filp, &noctty, &index);
if (IS_ERR(driver)) {
retval = PTR_ERR(driver);
@ -2078,21 +2079,21 @@ static int tty_open(struct inode *inode, struct file *filp)
retval = PTR_ERR(tty);
goto err_unlock;
}
if (tty) {
tty_lock(tty);
retval = tty_reopen(tty);
if (retval < 0) {
tty_unlock(tty);
tty = ERR_PTR(retval);
}
} else /* Returns with the tty_lock held for now */
tty = tty_init_dev(driver, index);
mutex_unlock(&tty_mutex);
tty_driver_kref_put(driver);
}
if (tty) {
tty_lock(tty);
retval = tty_reopen(tty);
if (retval < 0) {
tty_unlock(tty);
tty = ERR_PTR(retval);
}
} else /* Returns with the tty_lock held for now */
tty = tty_init_dev(driver, index);
mutex_unlock(&tty_mutex);
if (driver)
tty_driver_kref_put(driver);
if (IS_ERR(tty)) {
retval = PTR_ERR(tty);
goto err_file;