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Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

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  Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device  export Okash Khawaja <okash.khawaja@gmail.com> - 2017-07-18 13:40 +0200
    Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device  export Dan Carpenter <dan.carpenter@oracle.com> - 2017-07-18 14:30 +0200
      Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device  export Okash Khawaja <okash.khawaja@gmail.com> - 2017-07-18 21:30 +0200
    Re: [patch 0/3] Re: tty contention resulting from  tty_open_by_device export Alan Cox <gnomes@lxorguk.ukuu.org.uk> - 2017-07-18 16:00 +0200
      [patch v3 3/3] tty: undo export of tty_open_by_driver Okash Khawaja <okash.khawaja@gmail.com> - 2017-07-20 09:30 +0200
      [patch v3 1/3] tty: resolve tty contention between kernel and user space Okash Khawaja <okash.khawaja@gmail.com> - 2017-07-20 09:30 +0200
      [patch v3 0/3] tty contention resulting from tty_open_by_driver export Okash Khawaja <okash.khawaja@gmail.com> - 2017-07-20 09:30 +0200
        [patch v3 2/3] staging: speakup: use tty_kopen and tty_kclose Okash Khawaja <okash.khawaja@gmail.com> - 2017-07-20 09:40 +0200

#1690187 — Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

On Mon, Jul 17, 2017 at 11:04:38PM +0100, Alan Cox wrote:
> 
> > Sure. I can fix the tty->count mismatch based on Alan's suggestion. However I don't understand why the exclusivity flag should belong to tty_port and not tty_struct. It will be good to know why. 
> 
> We are trying to move all the flags that we can and structs into the
> tty_port, except any that are used internally within the struct tty
> level code. The main reason for this is to make the object lifetimes and
> locking simpler - because the tty_port lasts for the time the hardware is
> present.

I see. That does make sense. I have appended a version of the patch which
adds the flag to tty_port and uses that inside tty_kopen.

From readability point of view however, I think adding the flag to
tty_struct looks more intuitive. At least for now - until we move
other things from tty_struct to tty_port.

The patch is untested but I can work on this if that fits in with the
plans for tty.

Thanks,
Okash


---
 drivers/tty/tty_io.c |   67 +++++++++++++++++++++++++++++++++++++++++++++++----
 include/linux/tty.h  |   17 ++++++++++++
 2 files changed, 79 insertions(+), 5 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -280,7 +280,7 @@ static int check_tty_count(struct tty_st
 {
 #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_st
            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;
@@ -1522,6 +1524,7 @@ static int tty_release_checks(struct tty
  */
 void tty_release_struct(struct tty_struct *tty, int idx)
 {
+       // TODO: reset TTY_PORT_KOPENED on tty->port
        /*
         * Ask the line discipline code to release its structures
         */
@@ -1786,6 +1789,54 @@ 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);
+               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
  *     @inode: inode of device file
@@ -1824,6 +1875,12 @@ struct tty_struct *tty_open_by_driver(de
        }

        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() */
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -261,6 +261,7 @@ 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 +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; }
 static inline int tty_dev_name_to_number(const char *name, dev_t *number)
 { return -ENOTSUPP; }
 #endif
@@ -652,6 +656,19 @@ static inline void tty_port_set_initiali
                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);

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#1690227

On Tue, Jul 18, 2017 at 12:29:52PM +0100, Okash Khawaja wrote:
> +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);
> +               tty_port_set_kopened(tty->port, 1);
                                       ^^^^^^^^^

tty_init_dev() can fail leading to an error pointer dereference here.

> +       }
> +out:
> +       mutex_unlock(&tty_mutex);
> +       tty_driver_kref_put(driver);
> +       return tty;
> +}
> +EXPORT_SYMBOL_GPL(tty_kopen);

regards,
dan carpenter

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#1690586

On Tue, Jul 18, 2017 at 03:26:37PM +0300, Dan Carpenter wrote:
> > +       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);
> > +               tty_port_set_kopened(tty->port, 1);
>                                        ^^^^^^^^^
> 
> tty_init_dev() can fail leading to an error pointer dereference here.

Thanks very much. I will check for error pointer here.

Okash

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#1690282 — Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export

> The patch is untested but I can work on this if that fits in with the
> plans for tty.

I think this is the right direction.

Alan

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#1692489 — [patch v3 3/3] tty: undo export of tty_open_by_driver

Since we have tty_kopen, we no longer need to export tty_open_by_driver.
This patch makes this function static.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/tty/tty_io.c |    3 +--
 include/linux/tty.h  |    5 -----
 2 files changed, 1 insertion(+), 7 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1885,7 +1885,7 @@ EXPORT_SYMBOL_GPL(tty_kopen);
  *	  - concurrent tty driver removal w/ lookup
  *	  - concurrent tty removal from driver table
  */
-struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
+static struct tty_struct *tty_open_by_driver(dev_t device, struct inode *inode,
 					     struct file *filp)
 {
 	struct tty_struct *tty;
@@ -1936,7 +1936,6 @@ out:
 	tty_driver_kref_put(driver);
 	return tty;
 }
-EXPORT_SYMBOL_GPL(tty_open_by_driver);
 
 /**
  *	tty_open		-	open a tty device
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -400,8 +400,6 @@ extern struct tty_struct *get_current_tt
 /* 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);
 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);
@@ -425,9 +423,6 @@ static inline int __init tty_init(void)
 { return 0; }
 static inline const char *tty_name(const struct tty_struct *tty)
 { return "(none)"; }
-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)

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#1692490 — [patch v3 1/3] tty: resolve tty contention between kernel and user space

The commit 12e84c71b7d4 ("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>

---
 drivers/tty/tty_io.c |  100 ++++++++++++++++++++++++++++++++++++++++++++++++---
 include/linux/tty.h  |   21 ++++++++++
 2 files changed, 116 insertions(+), 5 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -280,7 +280,7 @@ static int check_tty_count(struct tty_st
 {
 #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_st
 	    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;
@@ -1513,6 +1515,38 @@ static int tty_release_checks(struct tty
 }
 
 /**
+ *      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
  *	@idx: index of the tty
@@ -1786,6 +1820,56 @@ 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);
+		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
  *	@inode: inode of device file
@@ -1824,6 +1908,12 @@ struct tty_struct *tty_open_by_driver(de
 	}
 
 	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() */
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -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
 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_initiali
 		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);

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#1692497 — [patch v3 0/3] tty contention resulting from tty_open_by_driver export

Hi,

I have updated the patches so that the exclusivity flag is in tty_port.
When closing the struct - by calling tty_release_struct - we also need
to reset the flag. One way to do that is to reset the flag inside
tty_release_struct function, regardless of whether the tty was opened
through tty_kopen or not. In order to keep the code clean, I have
instead created a separate function called tty_kclose which is the same
as tty_release_struct except that it also resets the exclusivity flag.
As a result, any changes to tty_release_struct won't be in tty_kclose
untless manually added. Please advise on this.

Here is a summary of changes when compared to v2:

- Patch 1 uses TTY_PORT_KOPENED flag instead of TTY_KOPENED and as a
        result adds helper functions to read and change the flag
- Patch 1 adds tty_kclose function to close tty opened by tty_kopen
- Patch 2 calls tty_kclose instead of tty_release_struct

Thanks,
Okash

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#1692512 — [patch v3 2/3] staging: speakup: use tty_kopen and tty_kclose

This patch replaces call to tty_open_by_driver with a tty_kopen and
uses tty_kclose instead of tty_release_struct to close it.

Signed-off-by: Okash Khawaja <okash.khawaja@gmail.com>

---
 drivers/staging/speakup/spk_ttyio.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -158,7 +158,7 @@ static int spk_ttyio_initialise_ldisc(st
 	if (ret)
 		return ret;
 
-	tty = tty_open_by_driver(dev, NULL, NULL);
+	tty = tty_kopen(dev);
 	if (IS_ERR(tty))
 		return PTR_ERR(tty);
 
@@ -308,7 +308,7 @@ void spk_ttyio_release(void)
 
 	tty_ldisc_flush(speakup_tty);
 	tty_unlock(speakup_tty);
-	tty_release_struct(speakup_tty, speakup_tty->index);
+	tty_kclose(speakup_tty);
 }
 EXPORT_SYMBOL_GPL(spk_ttyio_release);
 

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