tty contention resulting from tty_open_by_device export

[Okash Khawaja]

Hi,

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.

I suggest we make kernel access to tty exclusive so that if a user
process or kernel opens a kernel opened tty, it gets -EBUSY. Below is
a patch which 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.

I am far from an expert on tty and this patch might contain the wrong
approach. But it should convey what I mean.

Thanks,
Okash

---
drivers/staging/speakup/spk_ttyio.c | 2 -
drivers/tty/tty_io.c | 56 ++++++++++++++++++++++++++++++++++--
include/linux/tty.h | 7 +---
3 files changed, 58 insertions(+), 7 deletions(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -164,7 +164,7 @@ 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);

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1786,6 +1786,53 @@ 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) {
+ tty_kref_put(tty); /* drop kref from tty_driver_lookup_tty() */
+ tty = ERR_PTR(-EBUSY);
+ } else { /* Returns with the tty_lock held for now */
+ 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(tty_kopen);
+
+/**
* tty_open_by_driver - open a tty device
* @device: dev_t of device to open
* @inode: inode of device file
@@ -1801,7 +1848,7 @@ static struct tty_driver *tty_lookup_dri
* - 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;
@@ -1824,6 +1871,12 @@ 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() */
@@ -1846,7 +1899,6 @@ 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
@@ -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
@@ -399,8 +400,7 @@ 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 int tty_dev_name_to_number(const char *name, dev_t *number);
#else
static inline void tty_kref_put(struct tty_struct *tty)
@@ -422,8 +422,7 @@ 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)
+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; }

[Okash Khawaja]

Post by Okash Khawaja
+
+ if (tty) {
+ tty_kref_put(tty); /* drop kref from tty_driver_lookup_tty() */

Put the comment above the line?

Sure

Post by Okash Khawaja
+ tty = ERR_PTR(-EBUSY);
+ } else { /* Returns with the tty_lock held for now */

I don't understand this comment.

This is same comment in tty_open_by_driver. Basically, tty_init_dev
returns tty locked so that it doesn't dissapear from underneath the
caller. I am not sure about the "for now" part of the comment. I'll
remove it.

Post by Okash Khawaja
+ tty = tty_init_dev(driver, index);
+ set_bit(TTY_KOPENED, &tty->flags);
+ }
+ mutex_unlock(&tty_mutex);
+ tty_driver_kref_put(driver);
+ return tty;
+}
+EXPORT_SYMBOL(tty_kopen);

EXPORT_SYMBOL_GPL()? :)

Of course, will update

Post by Okash Khawaja
/**
* tty_open - open a tty device
--- 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 */

Minor nit, please use tabs here.

Will do

Overall, the idea looks sane to me. Keeping userspace from opening a
tty that the kernel has opened internally makes sense, hopefully
userspace doesn't get too confused when that happens. I don't think we
normally return -EBUSY from an open call, have you seen what happens
with apps when you do this (like minicom?)

Yes, returning -EBUSY is a change in the interface. I will test against
applications like minicom before resubmitting this.

Thanks,
Okash

[Okash Khawaja]

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 <***@gmail.com>

---
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; }
static inline int tty_dev_name_to_number(const char *name, dev_t *number)
{ return -ENOTSUPP; }
#endif

[Okash Khawaja]

Hi,

Above
1. take mutex
2. take reference
1. give mutex back
2. give reference back
I think we usually see symmetrical calls, i.e.
1. give reference back
2. give mutex back
So, what did I miss?

After we hold the kref, driver won't disappear from underneath us so we
don't need tty_mutex just for decrementing the refcount.

Thanks,
Okash

[Okash Khawaja]

Post by Okash Khawaja
+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);

Hmm... perhaps
tty = ERR_CAST(driver);
goto out_unlock;
See below for further details.

Sorry missed this one out. Since tty_lookup_driver has failed, we don't
need to down the refcount on driver. So we can return here, without
going to out_unlock.

Thanks,
Okash

[Okash Khawaja]

Post by Okash Khawaja

Post by Okash Khawaja
+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);

Hmm... perhaps
tty = ERR_CAST(driver);
goto out_unlock;
See below for further details.

Sorry missed this one out. Since tty_lookup_driver has failed, we don't
need to down the refcount on driver. So we can return here, without
going to out_unlock.

Yeah, and my point is to use goto with the symmetric giveups of lock
and reference.

Ah okay I see your point. Sure, I don't mind either way. However, this
code closely follows tty_open_by_driver, so I have tried to keep the
same pattern for sake of consistency :)

Thanks,
Okash

[Okash Khawaja]

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 <***@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
@@ -1849,7 +1849,7 @@ 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;
@@ -1900,7 +1900,6 @@ 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 int tty_dev_name_to_number(const char *name, dev_t *number);
#else
@@ -424,9 +422,6 @@ 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 NULL; }
static inline int tty_dev_name_to_number(const char *name, dev_t *number)

[Okash Khawaja]

This patch replaces call to tty_open_by_driver with a tty_kopen.

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

---
drivers/staging/speakup/spk_ttyio.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -164,7 +164,7 @@ 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);

[Okash Khawaja]

Post by Okash Khawaja
This patch replaces call to tty_open_by_driver with a tty_kopen.
---
drivers/staging/speakup/spk_ttyio.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/drivers/staging/speakup/spk_ttyio.c
+++ b/drivers/staging/speakup/spk_ttyio.c
@@ -164,7 +164,7 @@ 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))

Hm, the "no tty layer" return value for this will be NULL. I doubt
that's really a big deal, but perhaps just have that return
PTR_ERR(-ENODEV) or something?

Good point, thanks. Will send a follow up patch

Okash

[Okash Khawaja]

When TTY is not built, tty_kopen should return an error. This way
calling code remains consistent, regardless of whether tty is built or
not. This patch returns -ENODEV when there is no tty.

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

---
include/linux/tty.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -423,7 +423,7 @@ static inline int __init tty_init(void)
static inline const char *tty_name(const struct tty_struct *tty)
{ return "(none)"; }
static inline struct tty_struct *tty_kopen(dev_t device)
-{ return NULL; }
+{ return ERR_PTR(-ENODEV); }
static inline int tty_dev_name_to_number(const char *name, dev_t *number)
{ return -ENOTSUPP; }
#endif

[Okash Khawaja]

On Sun, 09 Jul 2017 12:41:53 +0100

Overall, the idea looks sane to me. Keeping userspace from opening a
tty that the kernel has opened internally makes sense, hopefully
userspace doesn't get too confused when that happens. I don't think we
normally return -EBUSY from an open call, have you seen what happens
with apps when you do this (like minicom?)

I tested this wil minincom, picocom and commands like "echo foo >
/dev/ttyS0". They all correctly report "Device or resource busy".
I have addressed all the comments you made. I have also split the patch
into three. Following is summary of each.

If the tty counts are being misreported then it would be better to fix
the code to actually manage the counts properly. The core tty code is
telling you that the tty is not in a valid state. While this is of
itself a good API to have, the underlying reference miscounting ought
IMHO to be fixed as well.

When opening from kernel, we don't use file pointer. The count mismatch
is between tty->count and #fd's. So opening from kernel leads to #fd's
being less than tty->count. I thought this difference is relevant to
user-space opening of tty, and not to kernel opening of tty. Can you
suggest how to address this mismatch?

Also you don't need a new TTY_KOPENED flag as far as I can see. All tty's
have a usage count and active bit flags already. Use those.

Ah may be I didn't notice the active bit. Is it one of the #defines in
tty.h? Can usage count and active bit be used to differentiate between
whether the tty was opened by kernel or user?

Thanks,
Okash

[Okash Khawaja]

Post by Okash Khawaja

If the tty counts are being misreported then it would be better to fix
the code to actually manage the counts properly. The core tty code is
telling you that the tty is not in a valid state. While this is of
itself a good API to have, the underlying reference miscounting ought
IMHO to be fixed as well.

When opening from kernel, we don't use file pointer. The count mismatch
is between tty->count and #fd's. So opening from kernel leads to #fd's
being less than tty->count. I thought this difference is relevant to
user-space opening of tty, and not to kernel opening of tty. Can you
suggest how to address this mismatch?

Idea is tty_kopen only ever returns a newly initialised tty - returned
by tty_init_dev. Since the access is exclusive, tty->count shouldn't
matter. When the caller is done with it, it calls tty_release_struct
which frees up the tty itself.

But yes, all the while a tty is kopened, its tty->count and #fds will be
unequal.

Thanks,
Okash

[Okash Khawaja]

Post by Okash Khawaja
When opening from kernel, we don't use file pointer. The count mismatch
is between tty->count and #fd's. So opening from kernel leads to #fd's
being less than tty->count. I thought this difference is relevant to
user-space opening of tty, and not to kernel opening of tty. Can you
suggest how to address this mismatch?

Your kernel reference is the same as having a file open reference so I
think this actually needs addressing in the maths. In other words count
the number of kernel references and also add that into the test for
check_tty_count (kernel references + #fds == count).
I'd really like to keep this right because that check has a long history
of catching really nasty race conditions in the tty code. The
open/close/hangup code is really fragile so worth the debugability.

I see. Okay based this, check_tty_count can be easily updated to take
into account kernel references.

Post by Okash Khawaja
Ah may be I didn't notice the active bit. Is it one of the #defines in
tty.h? Can usage count and active bit be used to differentiate between
whether the tty was opened by kernel or user?

It only tells you whether the port is currently active for some purpose,
not which. If you still want to implement exclusivity between kernel and
user then it needs another flag, but I think that flag should be in
port->flags as it is a property of the physical interface.
(Take a look at tty_port_open for example)

Okay I can add TTY_PORT_KOPENED to port->flags and that should work too.

However, can you please help me understand this:
Our use case requires kernel access to tty_struct and accordingly
tty_kopen returns tty_struct. The exclusivity between user and kernel
space is also meant to prevent one side from opening tty_struct while
another has it opened. In all this, it is tty_struct and not tty_port
which is the key resource we are concerned with. So shouldn't the
exclusivity flag belong to tty_struct?

Adding a the flag to port->flags but controlling it from code for
opening and closing tty will also mean we have tty_port_kopened,
tty_port_set_kopen etc inside tty open/close code.

Am I viewing this problem incorrectly?

Thanks,
Okash

[Okash Khawaja]

Post by Okash Khawaja

Post by Okash Khawaja
When opening from kernel, we don't use file pointer. The count mismatch
is between tty->count and #fd's. So opening from kernel leads to #fd's
being less than tty->count. I thought this difference is relevant to
user-space opening of tty, and not to kernel opening of tty. Can you
suggest how to address this mismatch?

Your kernel reference is the same as having a file open reference so I
think this actually needs addressing in the maths. In other words count
the number of kernel references and also add that into the test for
check_tty_count (kernel references + #fds == count).
I'd really like to keep this right because that check has a long history
of catching really nasty race conditions in the tty code. The
open/close/hangup code is really fragile so worth the debugability.

I see. Okay based this, check_tty_count can be easily updated to take
into account kernel references.

Ok, I'll drop this series from my "to-apply" queue and wait for you to
redo it.

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.

Thanks,
Okash

[Okash Khawaja]

Post by Okash Khawaja
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);

[Okash Khawaja]

Post by Okash Khawaja
+ 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

[Okash Khawaja]

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 <***@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
@@ -1851,7 +1851,7 @@ 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;
@@ -1902,7 +1902,6 @@ 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 int tty_dev_name_to_number(const char *name, dev_t *number);
#else
@@ -424,9 +422,6 @@ 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 int tty_dev_name_to_number(const char *name, dev_t *number)

[Okash Khawaja]

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.

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 <***@gmail.com>

---
drivers/tty/tty_io.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++----
include/linux/tty.h | 4 +++
2 files changed, 65 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 (test_bit(TTY_KOPENED, &tty->flags))
+ 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;
@@ -1786,6 +1788,54 @@ 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 +1874,12 @@ 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 ERR_PTR(-ENODEV); }
static inline int tty_dev_name_to_number(const char *name, dev_t *number)
{ return -ENOTSUPP; }
#endif

[Okash Khawaja]

This patch replaces call to tty_open_by_driver with a tty_kopen.

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

---
drivers/staging/speakup/spk_ttyio.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- 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);