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Commit a09ac397 authored by Okash Khawaja's avatar Okash Khawaja Committed by Greg Kroah-Hartman
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tty: resolve tty contention between kernel and user space 



The commit 12e84c71 ("tty: export tty_open_by_driver") exports
tty_open_by_device to allow tty to be opened from inside kernel which
works fine except that it doesn't handle contention with user space or
another kernel-space open of the same tty. For example, opening a tty
from user space while it is kernel opened results in failure and a
kernel log message about mismatch between tty->count and tty's file
open count.

This patch makes kernel access to tty exclusive, so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. It does
this by adding TTY_KOPENED flag to tty->flags. When this flag is set,
tty_open_by_driver returns -EBUSY. Instead of overloading
tty_open_by_driver for both kernel and user space, this
patch creates a separate function tty_kopen which closely follows
tty_open_by_driver. tty_kclose closes the tty opened by tty_kopen.

To address the mismatch between tty->count and #fd's, this patch adds
#kopen's to the count before comparing it with tty->count. That way
check_tty_count reflects correct usage count.

Returning -EBUSY on tty open is a change in the interface. I have
tested this with minicom, picocom and commands like "echo foo >
/dev/ttyS0". They all correctly report "Device or resource busy" when
the tty is already kernel opened.

Signed-off-by: default avatarOkash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 8439a69e

drivers/tty/tty_io.c

+95 −5
Original line number Diff line number Diff line
@@ -280,7 +280,7 @@ static int check_tty_count(struct tty_struct *tty, const char *routine) 
{

#ifdef CHECK_TTY_COUNT

	struct list_head *p;

	int count = 0;

	int count = 0, kopen_count = 0;



	spin_lock(&tty->files_lock);

	list_for_each(p, &tty->tty_files) {
@@ -291,10 +291,12 @@ static int check_tty_count(struct tty_struct *tty, const char *routine) 
	    tty->driver->subtype == PTY_TYPE_SLAVE &&

	    tty->link && tty->link->count)

		count++;

	if (tty->count != count) {

		tty_warn(tty, "%s: tty->count(%d) != #fd's(%d)\n",

			 routine, tty->count, count);

		return count;

	if (tty_port_kopened(tty->port))

		kopen_count++;

	if (tty->count != (count + kopen_count)) {

		tty_warn(tty, "%s: tty->count(%d) != (#fd's(%d) + #kopen's(%d))\n",

			 routine, tty->count, count, kopen_count);

		return (count + kopen_count);

	}

#endif

	return 0;
@@ -1512,6 +1514,38 @@ static int tty_release_checks(struct tty_struct *tty, int idx) 
	return 0;

}



/**

 *      tty_kclose      -       closes tty opened by tty_kopen

 *      @tty: tty device

 *

 *      Performs the final steps to release and free a tty device. It is the

 *      same as tty_release_struct except that it also resets TTY_PORT_KOPENED

 *      flag on tty->port.

 */

void tty_kclose(struct tty_struct *tty)

{

	/*

	 * Ask the line discipline code to release its structures

	 */

	tty_ldisc_release(tty);



	/* Wait for pending work before tty destruction commmences */

	tty_flush_works(tty);



	tty_debug_hangup(tty, "freeing structure\n");

	/*

	 * The release_tty function takes care of the details of clearing

	 * the slots and preserving the termios structure. The tty_unlock_pair

	 * should be safe as we keep a kref while the tty is locked (so the

	 * unlock never unlocks a freed tty).

	 */

	mutex_lock(&tty_mutex);

	tty_port_set_kopened(tty->port, 0);

	release_tty(tty, tty->index);

	mutex_unlock(&tty_mutex);

}

EXPORT_SYMBOL_GPL(tty_kclose);



/**

 *	tty_release_struct	-	release a tty struct

 *	@tty: tty device
@@ -1785,6 +1819,56 @@ static struct tty_driver *tty_lookup_driver(dev_t device, struct file *filp, 
	return driver;

}



/**

 *	tty_kopen	-	open a tty device for kernel

 *	@device: dev_t of device to open

 *

 *	Opens tty exclusively for kernel. Performs the driver lookup,

 *	makes sure it's not already opened and performs the first-time

 *	tty initialization.

 *

 *	Returns the locked initialized &tty_struct

 *

 *	Claims the global tty_mutex to serialize:

 *	  - concurrent first-time tty initialization

 *	  - concurrent tty driver removal w/ lookup

 *	  - concurrent tty removal from driver table

 */

struct tty_struct *tty_kopen(dev_t device)

{

	struct tty_struct *tty;

	struct tty_driver *driver = NULL;

	int index = -1;



	mutex_lock(&tty_mutex);

	driver = tty_lookup_driver(device, NULL, &index);

	if (IS_ERR(driver)) {

		mutex_unlock(&tty_mutex);

		return ERR_CAST(driver);

	}



	/* check whether we're reopening an existing tty */

	tty = tty_driver_lookup_tty(driver, NULL, index);

	if (IS_ERR(tty))

		goto out;



	if (tty) {

		/* drop kref from tty_driver_lookup_tty() */

		tty_kref_put(tty);

		tty = ERR_PTR(-EBUSY);

	} else { /* tty_init_dev returns tty with the tty_lock held */

		tty = tty_init_dev(driver, index);

		if (IS_ERR(tty))

			goto out;

		tty_port_set_kopened(tty->port, 1);

	}

out:

	mutex_unlock(&tty_mutex);

	tty_driver_kref_put(driver);

	return tty;

}

EXPORT_SYMBOL_GPL(tty_kopen);



/**

 *	tty_open_by_driver	-	open a tty device

 *	@device: dev_t of device to open
@@ -1824,6 +1908,12 @@ struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode, 
	}



	if (tty) {

		if (tty_port_kopened(tty->port)) {

			tty_kref_put(tty);

			mutex_unlock(&tty_mutex);

			tty = ERR_PTR(-EBUSY);

			goto out;

		}

		mutex_unlock(&tty_mutex);

		retval = tty_lock_interruptible(tty);

		tty_kref_put(tty);  /* drop kref from tty_driver_lookup_tty() */

include/linux/tty.h

+21 −0
Original line number Diff line number Diff line
@@ -261,6 +261,8 @@ struct tty_port { 
 */

#define TTY_PORT_CTS_FLOW	3	/* h/w flow control enabled */

#define TTY_PORT_CHECK_CD	4	/* carrier detect enabled */

#define TTY_PORT_KOPENED	5	/* device exclusively opened by

					   kernel */



/*

 * Where all of the state associated with a tty is kept while the tty
@@ -401,6 +403,8 @@ extern int __init tty_init(void); 
extern const char *tty_name(const struct tty_struct *tty);

extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,

		struct file *filp);

extern struct tty_struct *tty_kopen(dev_t device);

extern void tty_kclose(struct tty_struct *tty);

extern int tty_dev_name_to_number(const char *name, dev_t *number);

#else

static inline void tty_kref_put(struct tty_struct *tty)
@@ -425,6 +429,10 @@ static inline const char *tty_name(const struct tty_struct *tty) 
static inline struct tty_struct *tty_open_by_driver(dev_t device,

		struct inode *inode, struct file *filp)

{ return NULL; }

static inline struct tty_struct *tty_kopen(dev_t device)

{ return ERR_PTR(-ENODEV); }

static inline void tty_kclose(struct tty_struct *tty)

{ }

static inline int tty_dev_name_to_number(const char *name, dev_t *number)

{ return -ENOTSUPP; }

#endif
@@ -652,6 +660,19 @@ static inline void tty_port_set_initialized(struct tty_port *port, bool val) 
		clear_bit(TTY_PORT_INITIALIZED, &port->iflags);

}



static inline bool tty_port_kopened(struct tty_port *port)

{

	return test_bit(TTY_PORT_KOPENED, &port->iflags);

}



static inline void tty_port_set_kopened(struct tty_port *port, bool val)

{

	if (val)

		set_bit(TTY_PORT_KOPENED, &port->iflags);

	else

		clear_bit(TTY_PORT_KOPENED, &port->iflags);

}



extern struct tty_struct *tty_port_tty_get(struct tty_port *port);

extern void tty_port_tty_set(struct tty_port *port, struct tty_struct *tty);

extern int tty_port_carrier_raised(struct tty_port *port);