On Sun, Jul 09, 2017 at 12:41:54PM +0100, Okash Khawaja wrote:
> The commit 12e84c71b7d4ee (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 overlaoding
> 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.
>
> 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@xxxxxxxxx>
>
> ---
> drivers/tty/tty_io.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++++
> include/linux/tty.h | 4 +++
> 2 files changed, 58 insertions(+)
>
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -1786,6 +1786,54 @@ static struct tty_driver *tty_lookup_dri
> }
>
> /**
> + * 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);
> + set_bit(TTY_KOPENED, &tty->flags);
> + }
> +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
> * @inode: inode of device file
> @@ -1824,6 +1872,12 @@ struct tty_struct *tty_open_by_driver(de
> }
>
> if (tty) {
> + if (test_bit(TTY_KOPENED, &tty->flags)) {
> + 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() */
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -362,6 +362,7 @@ struct tty_file_private {
> #define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */
> #define TTY_HUPPED 18 /* Post driver->hangup() */
> #define TTY_LDISC_HALTED 22 /* Line discipline is halted */
> +#define TTY_KOPENED 23 /* TTY exclusively opened by kernel */
>
> /* Values for tty->flow_change */
> #define TTY_THROTTLE_SAFE 1
> @@ -401,6 +402,7 @@ 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 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 +427,8 @@ static inline const char *tty_name(const
> 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 NULL; }
Like I said in response to patch 2, maybe this should return an error?
thanks,
greg k-h
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