tty: resolve tty contention between kernel and user space
The commit 12e84c71b7
("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: Okash Khawaja <okash.khawaja@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
parent
8439a69e72
commit
a09ac3974d
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@ -280,7 +280,7 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
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{
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#ifdef CHECK_TTY_COUNT
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struct list_head *p;
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int count = 0;
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int count = 0, kopen_count = 0;
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spin_lock(&tty->files_lock);
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list_for_each(p, &tty->tty_files) {
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@ -291,10 +291,12 @@ static int check_tty_count(struct tty_struct *tty, const char *routine)
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tty->driver->subtype == PTY_TYPE_SLAVE &&
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tty->link && tty->link->count)
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count++;
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if (tty->count != count) {
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tty_warn(tty, "%s: tty->count(%d) != #fd's(%d)\n",
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routine, tty->count, count);
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return count;
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if (tty_port_kopened(tty->port))
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kopen_count++;
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if (tty->count != (count + kopen_count)) {
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tty_warn(tty, "%s: tty->count(%d) != (#fd's(%d) + #kopen's(%d))\n",
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routine, tty->count, count, kopen_count);
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return (count + kopen_count);
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}
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#endif
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return 0;
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@ -1512,6 +1514,38 @@ static int tty_release_checks(struct tty_struct *tty, int idx)
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return 0;
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}
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/**
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* tty_kclose - closes tty opened by tty_kopen
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* @tty: tty device
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*
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* Performs the final steps to release and free a tty device. It is the
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* same as tty_release_struct except that it also resets TTY_PORT_KOPENED
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* flag on tty->port.
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*/
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void tty_kclose(struct tty_struct *tty)
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{
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/*
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* Ask the line discipline code to release its structures
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*/
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tty_ldisc_release(tty);
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/* Wait for pending work before tty destruction commmences */
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tty_flush_works(tty);
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tty_debug_hangup(tty, "freeing structure\n");
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/*
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* The release_tty function takes care of the details of clearing
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* the slots and preserving the termios structure. The tty_unlock_pair
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* should be safe as we keep a kref while the tty is locked (so the
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* unlock never unlocks a freed tty).
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*/
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mutex_lock(&tty_mutex);
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tty_port_set_kopened(tty->port, 0);
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release_tty(tty, tty->index);
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mutex_unlock(&tty_mutex);
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}
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EXPORT_SYMBOL_GPL(tty_kclose);
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/**
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* tty_release_struct - release a tty struct
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* @tty: tty device
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@ -1785,6 +1819,56 @@ static struct tty_driver *tty_lookup_driver(dev_t device, struct file *filp,
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return driver;
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}
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/**
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* tty_kopen - open a tty device for kernel
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* @device: dev_t of device to open
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*
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* Opens tty exclusively for kernel. Performs the driver lookup,
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* makes sure it's not already opened and performs the first-time
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* tty initialization.
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*
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* Returns the locked initialized &tty_struct
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*
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* Claims the global tty_mutex to serialize:
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* - concurrent first-time tty initialization
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* - concurrent tty driver removal w/ lookup
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* - concurrent tty removal from driver table
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*/
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struct tty_struct *tty_kopen(dev_t device)
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{
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struct tty_struct *tty;
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struct tty_driver *driver = NULL;
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int index = -1;
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mutex_lock(&tty_mutex);
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driver = tty_lookup_driver(device, NULL, &index);
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if (IS_ERR(driver)) {
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mutex_unlock(&tty_mutex);
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return ERR_CAST(driver);
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}
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/* check whether we're reopening an existing tty */
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tty = tty_driver_lookup_tty(driver, NULL, index);
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if (IS_ERR(tty))
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goto out;
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if (tty) {
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/* drop kref from tty_driver_lookup_tty() */
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tty_kref_put(tty);
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tty = ERR_PTR(-EBUSY);
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} else { /* tty_init_dev returns tty with the tty_lock held */
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tty = tty_init_dev(driver, index);
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if (IS_ERR(tty))
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goto out;
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tty_port_set_kopened(tty->port, 1);
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}
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out:
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mutex_unlock(&tty_mutex);
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tty_driver_kref_put(driver);
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return tty;
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}
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EXPORT_SYMBOL_GPL(tty_kopen);
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/**
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* tty_open_by_driver - open a tty device
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* @device: dev_t of device to open
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@ -1824,6 +1908,12 @@ struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
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}
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if (tty) {
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if (tty_port_kopened(tty->port)) {
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tty_kref_put(tty);
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mutex_unlock(&tty_mutex);
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tty = ERR_PTR(-EBUSY);
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goto out;
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}
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mutex_unlock(&tty_mutex);
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retval = tty_lock_interruptible(tty);
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tty_kref_put(tty); /* drop kref from tty_driver_lookup_tty() */
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@ -261,6 +261,8 @@ struct tty_port {
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*/
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#define TTY_PORT_CTS_FLOW 3 /* h/w flow control enabled */
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#define TTY_PORT_CHECK_CD 4 /* carrier detect enabled */
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#define TTY_PORT_KOPENED 5 /* device exclusively opened by
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kernel */
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/*
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* Where all of the state associated with a tty is kept while the tty
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@ -401,6 +403,8 @@ extern int __init tty_init(void);
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extern const char *tty_name(const struct tty_struct *tty);
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extern struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
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struct file *filp);
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extern struct tty_struct *tty_kopen(dev_t device);
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extern void tty_kclose(struct tty_struct *tty);
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extern int tty_dev_name_to_number(const char *name, dev_t *number);
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#else
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static inline void tty_kref_put(struct tty_struct *tty)
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static inline struct tty_struct *tty_open_by_driver(dev_t device,
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struct inode *inode, struct file *filp)
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{ return NULL; }
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static inline struct tty_struct *tty_kopen(dev_t device)
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{ return ERR_PTR(-ENODEV); }
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static inline void tty_kclose(struct tty_struct *tty)
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{ }
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static inline int tty_dev_name_to_number(const char *name, dev_t *number)
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{ return -ENOTSUPP; }
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#endif
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@ -652,6 +660,19 @@ static inline void tty_port_set_initialized(struct tty_port *port, bool val)
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clear_bit(TTY_PORT_INITIALIZED, &port->iflags);
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}
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static inline bool tty_port_kopened(struct tty_port *port)
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{
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return test_bit(TTY_PORT_KOPENED, &port->iflags);
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}
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static inline void tty_port_set_kopened(struct tty_port *port, bool val)
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{
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if (val)
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set_bit(TTY_PORT_KOPENED, &port->iflags);
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else
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clear_bit(TTY_PORT_KOPENED, &port->iflags);
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}
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extern struct tty_struct *tty_port_tty_get(struct tty_port *port);
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extern void tty_port_tty_set(struct tty_port *port, struct tty_struct *tty);
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extern int tty_port_carrier_raised(struct tty_port *port);
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