TTY Internals

Kopen

These functions serve for opening a TTY from the kernelspace:

void tty_kclose(struct tty_struct *tty)

closes tty opened by tty_kopen

Parameters

struct tty_struct *tty
tty device

Description

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.

struct tty_struct * tty_kopen_exclusive(dev_t device)

open a tty device for kernel

Parameters

dev_t device
dev_t of device to open

Description

Opens tty exclusively for kernel. Performs the driver lookup, makes sure it’s not already opened and performs the first-time tty initialization.

Claims the global tty_mutex to serialize:
  • concurrent first-time tty initialization
  • concurrent tty driver removal w/ lookup
  • concurrent tty removal from driver table

Return

the locked initialized tty_struct

struct tty_struct * tty_kopen_shared(dev_t device)

open a tty device for shared in-kernel use

Parameters

dev_t device
dev_t of device to open

Description

Opens an already existing tty for in-kernel use. Compared to tty_kopen_exclusive() above it doesn’t ensure to be the only user.

Locking: identical to tty_kopen() above.

Exported Internal Functions

int tty_dev_name_to_number(const char *name, dev_t *number)

return dev_t for device name

Parameters

const char *name
user space name of device under /dev
dev_t *number
pointer to dev_t that this function will populate

Description

This function converts device names like ttyS0 or ttyUSB1 into dev_t like (4, 64) or (188, 1). If no corresponding driver is registered then the function returns -ENODEV.

Locking: this acquires tty_mutex to protect the tty_drivers list from
being modified while we are traversing it, and makes sure to release it before exiting.
void tty_release_struct(struct tty_struct *tty, int idx)

release a tty struct

Parameters

struct tty_struct *tty
tty device
int idx
index of the tty

Description

Performs the final steps to release and free a tty device. It is roughly the reverse of tty_init_dev().

int tty_get_icount(struct tty_struct *tty, struct serial_icounter_struct *icount)

get tty statistics

Parameters

struct tty_struct *tty
tty device
struct serial_icounter_struct *icount
output parameter

Description

Gets a copy of the tty’s icount statistics.

Locking: none (up to the driver)

Internal Functions

void free_tty_struct(struct tty_struct *tty)

free a disused tty

Parameters

struct tty_struct *tty
tty struct to free

Description

Free the write buffers, tty queue and tty memory itself.

Locking: none. Must be called after tty is definitely unused

void tty_free_file(struct file *file)

free file->private_data

Parameters

struct file *file
to free private_data of

Description

This shall be used only for fail path handling when tty_add_file was not called yet.

struct tty_driver * get_tty_driver(dev_t device, int *index)

find device of a tty

Parameters

dev_t device
device identifier
int *index
returns the index of the tty

Description

This routine returns a tty driver structure, given a device number and also passes back the index number.

Locking: caller must hold tty_mutex

struct file * tty_release_redirect(struct tty_struct *tty)

Release a redirect on a pty if present

Parameters

struct tty_struct *tty
tty device

Description

This is available to the pty code so if the master closes, if the slave is a redirect it can release the redirect.

void __tty_hangup(struct tty_struct *tty, int exit_session)

actual handler for hangup events

Parameters

struct tty_struct *tty
tty device
int exit_session
if non-zero, signal all foreground group processes

Description

This can be called by a “kworker” kernel thread. That is process synchronous but doesn’t hold any locks, so we need to make sure we have the appropriate locks for what we’re doing.

The hangup event clears any pending redirections onto the hung up device. It ensures future writes will error and it does the needed line discipline hangup and signal delivery. The tty object itself remains intact.

Locking:
  • BTM
  • redirect lock for undoing redirection
  • file list lock for manipulating list of ttys
  • tty_ldiscs_lock from called functions
  • termios_rwsem resetting termios data
  • tasklist_lock to walk task list for hangup event
  • ->siglock to protect ->signal/->sighand
void tty_vhangup_self(void)

process vhangup for own ctty

Parameters

void
no arguments

Description

Perform a vhangup on the current controlling tty

void tty_vhangup_session(struct tty_struct *tty)

hangup session leader exit

Parameters

struct tty_struct *tty
tty to hangup

Description

The session leader is exiting and hanging up its controlling terminal. Every process in the foreground process group is signalled SIGHUP.

We do this synchronously so that when the syscall returns the process is complete. That guarantee is necessary for security reasons.

ssize_t tty_read(struct kiocb *iocb, struct iov_iter *to)

read method for tty device files

Parameters

struct kiocb *iocb
kernel I/O control block
struct iov_iter *to
destination for the data read

Description

Perform the read system call function on this terminal device. Checks for hung up devices before calling the line discipline method.

Locking:
Locks the line discipline internally while needed. Multiple read calls may be outstanding in parallel.
void tty_write_message(struct tty_struct *tty, char *msg)

write a message to a certain tty, not just the console.

Parameters

struct tty_struct *tty
the destination tty_struct
char *msg
the message to write

Description

This is used for messages that need to be redirected to a specific tty. We don’t put it into the syslog queue right now maybe in the future if really needed.

We must still hold the BTM and test the CLOSING flag for the moment.

ssize_t tty_write(struct kiocb *iocb, struct iov_iter *from)

write method for tty device file

Parameters

struct kiocb *iocb
kernel I/O control block
struct iov_iter *from
iov_iter with data to write

Description

Write data to a tty device via the line discipline.

Locking:
Locks the line discipline as required Writes to the tty driver are serialized by the atomic_write_lock and are then processed in chunks to the device. The line discipline write method will not be invoked in parallel for each device.
int tty_send_xchar(struct tty_struct *tty, char ch)

send priority character

Parameters

struct tty_struct *tty
the tty to send to
char ch
xchar to send

Description

Send a high priority character to the tty even if stopped.

Locking: none for xchar method, write ordering for write method.

void pty_line_name(struct tty_driver *driver, int index, char *p)

generate name for a pty

Parameters

struct tty_driver *driver
the tty driver in use
int index
the minor number
char *p
output buffer of at least 6 bytes

Description

Generate a name from a driver reference and write it to the output buffer p.

Locking: None

ssize_t tty_line_name(struct tty_driver *driver, int index, char *p)

generate name for a tty

Parameters

struct tty_driver *driver
the tty driver in use
int index
the minor number
char *p
output buffer of at least 7 bytes

Description

Generate a name from a driver reference and write it to the output buffer p.

Locking: None

struct tty_struct * tty_driver_lookup_tty(struct tty_driver *driver, struct file *file, int idx)

find an existing tty, if any

Parameters

struct tty_driver *driver
the driver for the tty
struct file *file
file object
int idx
the minor number

Return

the tty, if found. If not found, return NULL or ERR_PTR() if the driver lookup() method returns an error.

Description

Locking: tty_mutex must be held. If the tty is found, bump the tty kref.

int tty_driver_install_tty(struct tty_driver *driver, struct tty_struct *tty)

install a tty entry in the driver

Parameters

struct tty_driver *driver
the driver for the tty
struct tty_struct *tty
the tty

Description

Install a tty object into the driver tables. The tty->index field will be set by the time this is called. This method is responsible for ensuring any need additional structures are allocated and configured.

Locking: tty_mutex for now

void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct *tty)

remove a tty from the driver tables

Parameters

struct tty_driver *driver
the driver for the tty
struct tty_struct *tty
tty to remove

Description

Remove a tty object from the driver tables. The tty->index field will be set by the time this is called.

Locking: tty_mutex for now

int tty_reopen(struct tty_struct *tty)

fast re-open of an open tty

Parameters

struct tty_struct *tty
the tty to open

Description

Re-opens on master ptys are not allowed and return -EIO.

Locking: Caller must hold tty_lock

Return

0 on success, -errno on error.

struct tty_struct * tty_init_dev(struct tty_driver *driver, int idx)

initialise a tty device

Parameters

struct tty_driver *driver
tty driver we are opening a device on
int idx
device index

Description

Prepare a tty device. This may not be a “new” clean device but could also be an active device. The pty drivers require special handling because of this.

Locking:
The function is called under the tty_mutex, which protects us from the tty struct or driver itself going away.

On exit the tty device has the line discipline attached and a reference count of 1. If a pair was created for pty/tty use and the other was a pty master then it too has a reference count of 1.

WSH 06/09/97: Rewritten to remove races and properly clean up after a failed open. The new code protects the open with a mutex, so it’s really quite straightforward. The mutex locking can probably be relaxed for the (most common) case of reopening a tty.

Return

new tty structure

void tty_flush_works(struct tty_struct *tty)

flush all works of a tty/pty pair

Parameters

struct tty_struct *tty
tty device to flush works for (or either end of a pty pair)

Description

Sync flush all works belonging to tty (and the ‘other’ tty).

void release_one_tty(struct work_struct *work)

release tty structure memory

Parameters

struct work_struct *work
work of tty we are obliterating

Description

Releases memory associated with a tty structure, and clears out the driver table slots. This function is called when a device is no longer in use. It also gets called when setup of a device fails.

Locking:
takes the file list lock internally when working on the list of ttys that the driver keeps.

This method gets called from a work queue so that the driver private cleanup ops can sleep (needed for USB at least)

void release_tty(struct tty_struct *tty, int idx)

release tty structure memory

Parameters

struct tty_struct *tty
tty device release
int idx
index of the tty device release

Description

Release both tty and a possible linked partner (think pty pair), and decrement the refcount of the backing module.

Locking:
tty_mutex takes the file list lock internally when working on the list of ttys that the driver keeps.
int tty_release_checks(struct tty_struct *tty, int idx)

check a tty before real release

Parameters

struct tty_struct *tty
tty to check
int idx
index of the tty

Description

Performs some paranoid checking before true release of the tty. This is a no-op unless TTY_PARANOIA_CHECK is defined.

int tty_release(struct inode *inode, struct file *filp)

vfs callback for close

Parameters

struct inode *inode
inode of tty
struct file *filp
file pointer for handle to tty

Description

Called the last time each file handle is closed that references this tty. There may however be several such references.

Locking:
Takes BKL. See tty_release_dev().

Even releasing the tty structures is a tricky business. We have to be very careful that the structures are all released at the same time, as interrupts might otherwise get the wrong pointers.

WSH 09/09/97: rewritten to avoid some nasty race conditions that could lead to double frees or releasing memory still in use.

struct tty_struct * tty_open_current_tty(dev_t device, struct file *filp)

get locked tty of current task

Parameters

dev_t device
device number
struct file *filp
file pointer to tty

Return

locked tty of the current task iff device is /dev/tty

Description

Performs a re-open of the current task’s controlling tty.

We cannot return driver and index like for the other nodes because devpts will not work then. It expects inodes to be from devpts FS.

struct tty_driver * tty_lookup_driver(dev_t device, struct file *filp, int *index)

lookup a tty driver for a given device file

Parameters

dev_t device
device number
struct file *filp
file pointer to tty
int *index
index for the device in the return driver

Description

If returned value is not erroneous, the caller is responsible to decrement the refcount by tty_driver_kref_put().

Locking: tty_mutex protects get_tty_driver()

Return

driver for this inode (with increased refcount)

struct tty_struct * tty_open_by_driver(dev_t device, struct file *filp)

open a tty device

Parameters

dev_t device
dev_t of device to open
struct file *filp
file pointer to tty

Description

Performs the driver lookup, checks for a reopen, or otherwise performs the first-time tty initialization.

Claims the global tty_mutex to serialize:
  • concurrent first-time tty initialization
  • concurrent tty driver removal w/ lookup
  • concurrent tty removal from driver table

Return

the locked initialized or re-opened tty_struct

int tty_open(struct inode *inode, struct file *filp)

open a tty device

Parameters

struct inode *inode
inode of device file
struct file *filp
file pointer to tty

Description

tty_open() and tty_release() keep up the tty count that contains the number of opens done on a tty. We cannot use the inode-count, as different inodes might point to the same tty.

Open-counting is needed for pty masters, as well as for keeping track of serial lines: DTR is dropped when the last close happens. (This is not done solely through tty->count, now. - Ted 1/27/92)

The termios state of a pty is reset on the first open so that settings don’t persist across reuse.

Locking:

Note

the tty_unlock/lock cases without a ref are only safe due to tty_mutex

__poll_t tty_poll(struct file *filp, poll_table *wait)

check tty status

Parameters

struct file *filp
file being polled
poll_table *wait
poll wait structures to update

Description

Call the line discipline polling method to obtain the poll status of the device.

Locking: locks called line discipline but ldisc poll method may be re-entered freely by other callers.

int tiocsti(struct tty_struct *tty, char __user *p)

fake input character

Parameters

struct tty_struct *tty
tty to fake input into
char __user *p
pointer to character

Description

Fake input to a tty device. Does the necessary locking and input management.

FIXME: does not honour flow control ??

Locking:
  • Called functions take tty_ldiscs_lock
  • current->signal->tty check is safe without locks
int tiocgwinsz(struct tty_struct *tty, struct winsize __user *arg)

implement window query ioctl

Parameters

struct tty_struct *tty
tty
struct winsize __user *arg
user buffer for result

Description

Copies the kernel idea of the window size into the user buffer.

Locking: tty->winsize_mutex is taken to ensure the winsize data is consistent.

int tiocswinsz(struct tty_struct *tty, struct winsize __user *arg)

implement window size set ioctl

Parameters

struct tty_struct *tty
tty side of tty
struct winsize __user *arg
user buffer for result

Description

Copies the user idea of the window size to the kernel. Traditionally this is just advisory information but for the Linux console it actually has driver level meaning and triggers a VC resize.

Locking:
Driver dependent. The default do_resize method takes the tty termios mutex and ctrl.lock. The console takes its own lock then calls into the default method.
int tioccons(struct file *file)

allow admin to move logical console

Parameters

struct file *file
the file to become console

Description

Allow the administrator to move the redirected console device.

Locking: uses redirect_lock to guard the redirect information

int tiocsetd(struct tty_struct *tty, int __user *p)

set line discipline

Parameters

struct tty_struct *tty
tty device
int __user *p
pointer to user data

Description

Set the line discipline according to user request.

Locking: see tty_set_ldisc(), this function is just a helper

int tiocgetd(struct tty_struct *tty, int __user *p)

get line discipline

Parameters

struct tty_struct *tty
tty device
int __user *p
pointer to user data

Description

Retrieves the line discipline id directly from the ldisc.

Locking: waits for ldisc reference (in case the line discipline is changing or the tty is being hungup)

int send_break(struct tty_struct *tty, unsigned int duration)

performed time break

Parameters

struct tty_struct *tty
device to break on
unsigned int duration
timeout in mS

Description

Perform a timed break on hardware that lacks its own driver level timed break functionality.

Locking:
tty->atomic_write_lock serializes
int tty_tiocmget(struct tty_struct *tty, int __user *p)

get modem status

Parameters

struct tty_struct *tty
tty device
int __user *p
pointer to result

Description

Obtain the modem status bits from the tty driver if the feature is supported. Return -ENOTTY if it is not available.

Locking: none (up to the driver)

int tty_tiocmset(struct tty_struct *tty, unsigned int cmd, unsigned __user *p)

set modem status

Parameters

struct tty_struct *tty
tty device
unsigned int cmd
command - clear bits, set bits or set all
unsigned __user *p
pointer to desired bits

Description

Set the modem status bits from the tty driver if the feature is supported. Return -ENOTTY if it is not available.

Locking: none (up to the driver)

struct tty_struct * alloc_tty_struct(struct tty_driver *driver, int idx)

allocate a new tty

Parameters

struct tty_driver *driver
driver which will handle the returned tty
int idx
minor of the tty

Description

This subroutine allocates and initializes a tty structure.

Locking: none - tty in question is not exposed at this point