[patch 1/7] tty_port: allow a port to be opened with a tty that has no file handle

[Samuel Thibault]

okash.khawaja at gmail.com, on sam. 11 mars 2017 13:05:25 +0000, wrote:
> Allow access to TTY device from kernel. This is based on Alan Cox's patch
> (http://www.mail-archive.com/linux-kernel at vger.kernel.org/msg1215095.html),
> with description quoted below.
> 
> "tty_port: allow a port to be opened with a tty that has no file handle
> 
> Let us create tty objects entirely in kernel space.
> 
> With this a kernel created non file backed tty object could be used to handle
> data, and set terminal modes. Not all ldiscs can cope with this as N_TTY in
> particular has to work back to the fs/tty layer.
> 
> The tty_port code is however otherwise clean of file handles as far as I can
> tell as is the low level tty port write path used by the ldisc, the
> configuration low level interfaces and most of the ldiscs.
> 
> Currently you don't have any exposure to see tty hangups because those are
> built around the file layer. However a) it's a fixed port so you probably
> don't care about that b) if you do we can add a callback and c) you almost
> certainly don't want the userspace tear down/rebuild behaviour anyway.
> 
> This should however be sufficient if we wanted for example to enumerate all
> the bluetooth bound fixed ports via ACPI and make them directly available.
> 
> It doesn't deal with the case of a user opening a port that's also kernel
> opened and that would need some locking out (so it returned EBUSY if bound
> to a kernel device of some kind). That needs resolving along with how you
> "up" or "down" your new bluetooth device, or enumerate it while providing
> the existing tty API to avoid regressions (and to debug)."
> 
> Signed-off-by: Okash Khawaja <okash.khawaja at gmail.com>

Reviewed-by: Samuel Thibault <samuel.thibault at ens-lyon.org>

> Index: linux-4.10.1/drivers/tty/tty_io.c
> ===================================================================
> --- linux-4.10.1.orig/drivers/tty/tty_io.c
> +++ linux-4.10.1/drivers/tty/tty_io.c
> @@ -855,7 +855,7 @@
>  
>  int tty_hung_up_p(struct file *filp)
>  {
> -	return (filp->f_op == &hung_up_tty_fops);
> +	return (filp && filp->f_op == &hung_up_tty_fops);
>  }
>  
>  EXPORT_SYMBOL(tty_hung_up_p);
> @@ -1368,7 +1368,10 @@
>  	struct tty_struct *tty;
>  
>  	if (driver->ops->lookup)
> -		tty = driver->ops->lookup(driver, file, idx);
> +		if (!file)
> +			tty = ERR_PTR(-EIO);
> +		else
> +			tty = driver->ops->lookup(driver, file, idx);
>  	else
>  		tty = driver->ttys[idx];
>  
> @@ -1986,7 +1989,7 @@
>  		struct tty_driver *console_driver = console_device(index);
>  		if (console_driver) {
>  			driver = tty_driver_kref_get(console_driver);
> -			if (driver) {
> +			if (driver && filp) {
>  				/* Don't let /dev/console block */
>  				filp->f_flags |= O_NONBLOCK;
>  				break;
> @@ -2019,7 +2022,7 @@
>   *	  - concurrent tty driver removal w/ lookup
>   *	  - concurrent tty removal from driver table
>   */
> -static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
> +struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
>  					     struct file *filp)
>  {
>  	struct tty_struct *tty;
> @@ -2064,6 +2067,7 @@
>  	tty_driver_kref_put(driver);
>  	return tty;
>  }
> +EXPORT_SYMBOL(tty_open_by_driver);
>  
>  /**
>   *	tty_open		-	open a tty device
> Index: linux-4.10.1/drivers/tty/tty_port.c
> ===================================================================
> --- linux-4.10.1.orig/drivers/tty/tty_port.c
> +++ linux-4.10.1/drivers/tty/tty_port.c
> @@ -335,7 +335,7 @@
>   *	tty_port_block_til_ready	-	Waiting logic for tty open
>   *	@port: the tty port being opened
>   *	@tty: the tty device being bound
> - *	@filp: the file pointer of the opener
> + *	@filp: the file pointer of the opener or NULL
>   *
>   *	Implement the core POSIX/SuS tty behaviour when opening a tty device.
>   *	Handles:
> @@ -369,7 +369,7 @@
>  		tty_port_set_active(port, 1);
>  		return 0;
>  	}
> -	if (filp->f_flags & O_NONBLOCK) {
> +	if (filp ==  NULL || filp->f_flags & O_NONBLOCK) {
>  		/* Indicate we are open */
>  		if (C_BAUD(tty))
>  			tty_port_raise_dtr_rts(port);
> Index: linux-4.10.1/include/linux/tty.h
> ===================================================================
> --- linux-4.10.1.orig/include/linux/tty.h
> +++ linux-4.10.1/include/linux/tty.h
> @@ -394,6 +394,8 @@
>  /* tty_io.c */
>  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);
>  #else
>  static inline void console_init(void)
>  { }
> 

-- 
Samuel
 > et sinon, quand on s'interesse a un media que l'on ne maitrise pas,
 > on essaye de le comprendre d'abord.
 (Suivi par l'intégralité du message initial de 45 lignes.)
 -+-BM in : GNU - La maîtrise est un long apprentissage petit scarabé -+-