Advanced Char Driver Operations

1
Advanced Char Driver Operations

• Sarah Diesburg
• CIS 4930

2
Resources

• LDD Chapter 3
• Red font in slides where up-to-date code diverges
  from book
• LDD module source code for 3.2.x
• http//ww2.cs.fsu.edu/diesburg/courses/dd/code.ht
  ml

3
Resources

• LXR Cross-referenced Linux
• Go to http//lxr.linux.no/
• Click on Linux 2.6.11 and later
• Select your kernel version from drop-down menu

4
Topics

• Managing ioctl command numbers
• Block/unblocking a process
• Seeking on a device
• Access control

5
ioctl

• For operations beyond simple data transfers
• Eject the media
• Report error information
• Change hardware settings
• Self destruct
• Alternatives
• Embedded commands in the data stream
• Driver-specific file systems

6
ioctl

• User-level interface
• int ioctl(int fd, int request, ...)
• ...
• Variable number of arguments
• Problematic for the system call interface
• In this context, it is meant to pass a single
  optional argument
• Traditionally a char argp
• Just a way to bypass the type checking
• For more information, look at man page

7
ioctl

• Driver-level interface
• int (unlocked_ioctl) (struct file filp,
• unsigned int cmd,
• unsigned long arg)
• cmd is passed from the user unchanged
• arg can be an integer or a pointer
• Compiler does not type check
• Ioctl has changed from the LDD3 era
• Modified to remove the big kernel lock (BKL)
• http//lwn.net/Articles/119652/

8
Choosing the ioctl Commands

• Need a numbering scheme to avoid mistakes
• E.g., issuing a command to the wrong device
  (changing the baud rate of an audio device)
• Check include/linux/ioctl.h and directory
  Documentation/ioctl/

9
Choosing the ioctl Commands

• A command number uses four bitfields
• Defined in ltlinux/ioctl.hgt
• lt direction, type, number, sizegt
• direction direction of data transfer
• _IOC_NONE
• _IOC_READ
• _IOC_WRITE
• _IOC_READ WRITE

10
Choosing the ioctl Commands

• type (ioctl device type)
• 8-bit (_IOC_TYPEBITS) magic number
• Associated with the device
• number
• 8-bit (_IOC_NRBITS) sequential number
• Unique within device
• size size of user data involved
• The width is either 13 or 14 bits (_IOC_SIZEBITS)

11
Choosing the ioctl Commands

• Useful macros to create ioctl command numbers
• _IO(type, nr)
• _IOR(type, nr, datatype)
• _IOW(type, nr, datatype)
• _IOWR(type, nr, datatype)

size sizeof(datatype)
12
Choosing the ioctl Commands

• Useful macros to decode ioctl command numbers
• _IOC_DIR(nr)
• _IOC_TYPE(nr)
• _IOC_NR(nr)
• _IOC_SIZE(nr)

13
Choosing the ioctl Commands

• The scull example
• / Use 'k' as magic number /
• define SCULL_IOC_MAGIC 'k
• / Please use a different 8-bit number in your
  code /
• define SCULL_IOCRESET _IO(SCULL_IOC_MAGIC, 0)

14
Choosing the ioctl Commands

• The scull example
• /
• S means "Set" through a ptr,
• T means "Tell" directly with the argument value
• G means "Get" reply by setting through a
  pointer
• Q means "Query" response is on the return
  value
• X means "eXchange" switch G and S atomically
• H means "sHift" switch T and Q atomically
• /
• define SCULL_IOCSQUANTUM _IOW(SCULL_IOC_MAGIC,
  1, int)
• define SCULL_IOCSQSET _IOW(SCULL_IOC_MAGIC, 2,
  int)
• define SCULL_IOCTQUANTUM _IO(SCULL_IOC_MAGIC, 3)
• define SCULL_IOCTQSET _IO(SCULL_IOC_MAGIC, 4)
• define SCULL_IOCGQUANTUM _IOR(SCULL_IOC_MAGIC,
  5, int)

Set new value and return the old value
15
Choosing the ioctl Commands

• The scull example
• define SCULL_IOCGQSET _IOR(SCULL_IOC_MAGIC, 6,
  int)
• define SCULL_IOCQQUANTUM _IO(SCULL_IOC_MAGIC, 7)
• define SCULL_IOCQQSET _IO(SCULL_IOC_MAGIC, 8)
• define SCULL_IOCXQUANTUM _IOWR(SCULL_IOC_MAGIC,
  9, int)
• define SCULL_IOCXQSET _IOWR(SCULL_IOC_MAGIC,10,
  int)
• define SCULL_IOCHQUANTUM _IO(SCULL_IOC_MAGIC,
  11)
• define SCULL_IOCHQSET _IO(SCULL_IOC_MAGIC, 12)
• define SCULL_IOC_MAXNR 14

16
The Return Value

• When the command number is not supported
• Return EINVAL
• Or ENOTTY (according to the POSIX standard)

17
The Predefined Commands

• Handled by the kernel first
• Will not be passed down to device drivers
• Three groups
• For any file (regular, device, FIFO, socket)
• Magic number T.
• For regular files only
• Specific to the file system type

18
Using the ioctl Argument

• If it is an integer, just use it directly
• If it is a pointer
• Need to check for valid user address
• int access_ok(int type, const void addr,
• unsigned long size)
• type either VERIFY_READ or VERIFY_WRITE
• Returns 1 for success, 0 for failure
• Driver then results EFAULT to the caller
• Defined in ltlinux/uaccess.hgt
• Mostly called by memory-access routines

19
Using the ioctl Argument

• The scull example
• int scull_ioctl(struct file filp,
• unsigned int cmd, unsigned long
  arg)
• int err 0, tmp
• int retval 0
• / check the magic number and whether the
  command is defined /
• if (_IOC_TYPE(cmd) ! SCULL_IOC_MAGIC)
• return -ENOTTY
• if (_IOC_NR(cmd) gt SCULL_IOC_MAXNR)
• return -ENOTTY

20
Using the ioctl Argument

• The scull example
• / the concept of "read" and "write" is
  reversed here /
• if (_IOC_DIR(cmd) _IOC_READ)
• err !access_ok(VERIFY_WRITE, (void __user
  ) arg,
• _IOC_SIZE(cmd))
• else if (_IOC_DIR(cmd) _IOC_WRITE)
• err !access_ok(VERIFY_READ, (void __user )
  arg,
• _IOC_SIZE(cmd))
• if (err) return -EFAULT

21
Using the ioctl Argument

• Data transfer functions optimized for most used
  data sizes (1, 2, 4, and 8 bytes)
• If the size mismatches
• Cryptic compiler error message
• Conversion to non-scalar type requested
• Use copy_to_user and copy_from_user
• include ltlinux/uaccess.hgt
• put_user(datum, ptr)
• Writes to a user-space address
• Calls access_ok()
• Returns 0 on success, -EFAULT on error

22
Using the ioctl Argument

• __put_user(datum, ptr)
• Does not check access_ok()
• Can still fail if the user-space memory is not
  writable
• get_user(local, ptr)
• Reads from a user-space address
• Calls access_ok()
• Stores the retrieved value in local
• Returns 0 on success, -EFAULT on error
• __get_user(local, ptr)
• Does not check access_ok()
• Can still fail if the user-space memory is not
  readable

23
Capabilities and Restricted Operations

• Limit certain ioctl operations to privileged
  users
• See ltlinux/capability.hgt for the full set of
  capabilities
• To check a certain capability call
• int capable(int capability)
• In the scull example
• if (!capable(CAP_SYS_ADMIN))
• return EPERM

A catch-all capability for many system
administration operations
24
The Implementation of the ioctl Commands

• A giant switch statement
• switch(cmd)
• case SCULL_IOCRESET
• scull_quantum SCULL_QUANTUM
• scull_qset SCULL_QSET
• break
• case SCULL_IOCSQUANTUM / Set arg points to
  the value /
• if (!capable(CAP_SYS_ADMIN))
• return -EPERM
• retval __get_user(scull_quantum, (int
  __user )arg)
• break

25
The Implementation of the ioctl Commands

• case SCULL_IOCTQUANTUM / Tell arg is the
  value /
• if (!capable(CAP_SYS_ADMIN))
• return -EPERM
• scull_quantum arg
• break
• case SCULL_IOCGQUANTUM / Get arg is
  pointer to result /
• retval __put_user(scull_quantum, (int
  __user ) arg)
• break
• case SCULL_IOCQQUANTUM / Query return it
  (gt 0) /
• return scull_quantum

26
The Implementation of the ioctl Commands

• case SCULL_IOCXQUANTUM / eXchange use arg
  as pointer /
• if (!capable(CAP_SYS_ADMIN))
• return -EPERM
• tmp scull_quantum
• retval __get_user(scull_quantum, (int
  __user ) arg)
• if (retval 0)
• retval __put_user(tmp, (int __user )
  arg)
• break

27
The Implementation of the ioctl Commands

• case SCULL_IOCHQUANTUM / sHift like Tell
  Query /
• if (!capable(CAP_SYS_ADMIN))
• return -EPERM
• tmp scull_quantum
• scull_quantum arg
• return tmp
• default / redundant, as cmd was checked
  against MAXNR /
• return -ENOTTY
• / switch /
• return retval
• / scull_ioctl /

28
The Implementation of the ioctl Commands

• Six ways to pass and receive arguments from the
  user space
• Need to know command number
• int quantum
• ioctl(fd,SCULL_IOCSQUANTUM, quantum) / Set by
  pointer /
• ioctl(fd,SCULL_IOCTQUANTUM, quantum) / Set by
  value /
• ioctl(fd,SCULL_IOCGQUANTUM, quantum) / Get by
  pointer /
• quantum ioctl(fd,SCULL_IOCQQUANTUM) / Get by
  return value /
• ioctl(fd,SCULL_IOCXQUANTUM, quantum) /
  Exchange by pointer /
• / Exchange by value /
• quantum ioctl(fd,SCULL_IOCHQUANTUM, quantum)

29
Device Control Without ioctl

• Writing control sequences into the data stream
  itself
• Example console escape sequences
• Advantages
• No need to implement ioctl methods
• Disadvantages
• Need to make sure that escape sequences do not
  appear in the normal data stream (e.g., cat a
  binary file)
• Need to parse the data stream

30
Blocking I/O

• Needed when no data is available for reads
• When the device is not ready to accept data
• Output buffer is full

31
Introduction to Sleeping
32
Introduction to Sleeping

• A process is removed from the schedulers run
  queue
• Certain rules
• Never sleep when running in an atomic context
• Multiple steps must be performed without
  concurrent accesses
• Not while holding a spinlock, seqlock, or RCU
  lock
• Not while disabling interrupts

33
Introduction to Sleeping

• Okay to sleep while holding a semaphore
• Other threads waiting for the semaphore will also
  sleep
• Need to keep it short
• Make sure that it is not blocking the process
  that will wake it up
• After waking up
• Make no assumptions about the state of the system
• The resource one is waiting for might be gone
  again
• Must check the wait condition again

34
Introduction to Sleeping

• Wait queue contains a list of processes waiting
  for a specific event
• include ltlinux/wait.hgt
• To initialize statically, call
• DECLARE_WAIT_QUEUE_HEAD(my_queue)
• To initialize dynamically, call
• wait_queue_head_t my_queue
• init_waitqueue_head(my_queue)

35
Simple Sleeping

• Call variants of wait_event macros
• wait_event(queue, condition)
• queue wait queue head
• Passed by value
• Waits until the boolean condition becomes true
• Puts into an uninterruptible sleep
• Usually is not what you want
• wait_event_interruptible(queue, condition)
• Can be interrupted by signals
• Returns nonzero if sleep was interrupted
• Your driver should return -ERESTARTSYS

36
Simple Sleeping

• wait_event_timeout(queue, condition, timeout)
• Wait for a limited time (in jiffies)
• Returns 0 regardless of condition evaluations
• wait_event_interruptible_timeout(queue,
• condition,
• timeout)

37
Simple Sleeping

• To wake up, call variants of wake_up functions
• void wake_up(wait_queue_head_t queue)
• Wakes up all processes waiting on the queue
• void wake_up_interruptible(wait_queue_head_t
  queue)
• Wakes up processes that perform an interruptible
  sleep

38
Simple Sleeping

• Example module sleepy
• static DECLARE_WAIT_QUEUE_HEAD(wq)
• static int flag 0
• ssize_t sleepy_read(struct file filp, char
  __user buf,
• size_t count, loff_t pos)
• printk(KERN_DEBUG "process i (s) going to
  sleep\n",
• current-gtpid, current-gtcomm)
• wait_event_interruptible(wq, flag ! 0)
• flag 0
• printk(KERN_DEBUG "awoken i (s)\n",
  current-gtpid,
• current-gtcomm)
• return 0 / EOF /

Multiple threads can wake up at this point
39
Simple Sleeping

• Example module sleepy
• ssize_t sleepy_write(struct file filp, const
  char __user buf,
• size_t count, loff_t pos)
• printk(KERN_DEBUG "process i (s) awakening
  the readers...\n",
• current-gtpid, current-gtcomm)
• flag 1
• wake_up_interruptible(wq)
• return count / succeed, to avoid retrial /

40
Blocking and Nonblocking Operations

• By default, operations block
• If no data is available for reads
• If no space is available for writes
• Non-blocking I/O is indicated by the O_NONBLOCK
  flag in filp-gtf_flags
• Defined in ltlinux/fcntl.hgt
• Only open, read, and write calls are affected
• Returns EAGAIN immediately instead of block
• Applications need to distinguish non-blocking
  returns vs. EOFs

41
A Blocking I/O Example

• scullpipe
• A read process
• Blocks when no data is available
• Wakes a blocking write when buffer space becomes
  available
• A write process
• Blocks when no buffer space is available
• Wakes a blocking read process when data arrives

42
A Blocking I/O Example

• scullpipe data structure
• struct scull_pipe
• wait_queue_head_t inq, outq / read and write
  queues /
• char buffer, end / begin of buf, end of buf
  /
• int buffersize / used in pointer arithmetic
  /
• char rp, wp / where to read, where to write
  /
• int nreaders, nwriters / number of openings
  for r/w /
• struct fasync_struct async_queue /
  asynchronous readers /
• struct semaphore sem / mutual exclusion
  semaphore /
• struct cdev cdev / Char device structure /

43
A Blocking I/O Example

• static ssize_t scull_p_read(struct file filp,
  char __user buf,
• size_t count, loff_t
  f_pos)
• struct scull_pipe dev filp-gtprivate_data
• if (down_interruptible(dev-gtsem)) return
  -ERESTARTSYS
• while (dev-gtrp dev-gtwp) / nothing to read
  /
• up(dev-gtsem) / release the lock /
• if (filp-gtf_flags O_NONBLOCK)
• return -EAGAIN
• if (wait_event_interruptible(dev-gtinq,
  (dev-gtrp ! dev-gtwp)))
• return -ERESTARTSYS
• if (down_interruptible(dev-gtsem)) return
  -ERESTARTSYS

44
A Blocking I/O Example

• if (dev-gtwp gt dev-gtrp)
• count min(count, (size_t)(dev-gtwp -
  dev-gtrp))
• else / the write pointer has wrapped /
• count min(count, (size_t)(dev-gtend -
  dev-gtrp))
• if (copy_to_user(buf, dev-gtrp, count))
• up (dev-gtsem)
• return -EFAULT
• dev-gtrp count
• if (dev-gtrp dev-gtend) dev-gtrp dev-gtbuffer
  / wrapped /
• up (dev-gtsem)
• / finally, awake any writers and return /
• wake_up_interruptible(dev-gtoutq)
• return count

45
Advanced Sleeping
46
Advanced Sleeping

• Uses low-level functions to affect a sleep
• How a process sleeps
• 1. Allocate and initialize a wait_queue_t
  structure
• DEFINE_WAIT(my_wait)
• Or
• wait_queue_t my_wait
• init_wait(my_wait)

Queue element
47
Advanced Sleeping

• 2. Add to the proper wait queue and mark a
  process as being asleep
• TASK_RUNNING ?TASK_INTERRUPTIBLE or
  TASK_UNINTERRUPTIBLE
• Call
• void prepare_to_wait(wait_queue_head_t queue,
• wait_queue_t wait,
• int state)

48
Advanced Sleeping

• 3. Give up the processor
• Double check the sleeping condition before going
  to sleep
• The wakeup thread might have changed the
  condition between steps 1 and 2
• if (/ sleeping condition /)
• schedule() / yield the CPU /

49
Advanced Sleeping

• 4. Return from sleep
• Remove the process from the wait queue if
  schedule() was not called
• void finish_wait(wait_queue_head_t queue,
• wait_queue_t wait)

50
Advanced Sleeping

• scullpipe write method
• / How much space is free? /
• static int spacefree(struct scull_pipe dev)
• if (dev-gtrp dev-gtwp)
• return dev-gtbuffersize - 1
• return ((dev-gtrp dev-gtbuffersize - dev-gtwp)
• dev-gtbuffersize) - 1

51
Advanced Sleeping

• static ssize_t
• scull_p_write(struct file filp, const char
  __user buf,
• size_t count, loff_t f_pos)
• struct scull_pipe dev filp-gtprivate_data
• int result
• if (down_interruptible(dev-gtsem)) return
  -ERESTARTSYS
• / Wait for space for writing /
• result scull_getwritespace(dev, filp)
• if (result)
• return result / scull_getwritespace called
  up(dev-gtsem) /
• / ok, space is there, accept something /
• count min(count, (size_t)spacefree(dev))

52
Advanced Sleeping

• if (dev-gtwp gt dev-gtrp)
• count min(count, (size_t)(dev-gtend -
  dev-gtwp))
• else / the write pointer has wrapped, fill up
  to rp - 1 /
• count min(count, (size_t)(dev-gtrp - dev-gtwp
  - 1))
• if (copy_from_user(dev-gtwp, buf, count))
• up (dev-gtsem) return -EFAULT
• dev-gtwp count
• if (dev-gtwp dev-gtend) dev-gtwp dev-gtbuffer
  / wrapped /
• up(dev-gtsem)
• wake_up_interruptible(dev-gtinq)
• if (dev-gtasync_queue)
• kill_fasync(dev-gtasync_queue, SIGIO,
  POLL_IN)
• return count

53
Advanced Sleeping

• / Wait for space for writing caller must hold
  device semaphore.
• On error the semaphore will be released before
  returning. /
• static int scull_getwritespace(struct scull_pipe
  dev,
• struct file filp)
  
• while (spacefree(dev) 0) / full /
• DEFINE_WAIT(wait)
• up(dev-gtsem)
• if (filp-gtf_flags O_NONBLOCK) return
  -EAGAIN
• prepare_to_wait(dev-gtoutq, wait,
  TASK_INTERRUPTIBLE)
• if (spacefree(dev) 0) schedule()
• finish_wait(dev-gtoutq, wait)
• if (signal_pending(current)) return
  -ERESTARTSYS
• if (down_interruptible(dev-gtsem)) return
  -ERESTARTSYS
• return 0

Task state RUNNING
Queue full
54
Advanced Sleeping

• / Wait for space for writing caller must hold
  device semaphore.
• On error the semaphore will be released before
  returning. /
• static int scull_getwritespace(struct scull_pipe
  dev,
• struct file filp)
  
• while (spacefree(dev) 0) / full /
• DEFINE_WAIT(wait)
• up(dev-gtsem)
• if (filp-gtf_flags O_NONBLOCK) return
  -EAGAIN
• prepare_to_wait(dev-gtoutq, wait,
  TASK_INTERRUPTIBLE)
• if (spacefree(dev) 0) schedule()
• finish_wait(dev-gtoutq, wait)
• if (signal_pending(current)) return
  -ERESTARTSYS
• if (down_interruptible(dev-gtsem)) return
  -ERESTARTSYS
• return 0

Task state RUNNING ? INTERRUPTIBLE
Queue full
55
Advanced Sleeping

• / Wait for space for writing caller must hold
  device semaphore.
• On error the semaphore will be released before
  returning. /
• static int scull_getwritespace(struct scull_pipe
  dev,
• struct file filp)
  
• while (spacefree(dev) 0) / full /
• DEFINE_WAIT(wait)
• up(dev-gtsem)
• if (filp-gtf_flags O_NONBLOCK) return
  -EAGAIN
• prepare_to_wait(dev-gtoutq, wait,
  TASK_INTERRUPTIBLE)
• if (spacefree(dev) 0) schedule()
• finish_wait(dev-gtoutq, wait)
• if (signal_pending(current)) return
  -ERESTARTSYS
• if (down_interruptible(dev-gtsem)) return
  -ERESTARTSYS
• return 0

Task state INTERRUPTIBLE / sleep /
Queue full
56
Exclusive Waits

• Avoid waking up all processes waiting on a queue
• Wakes up only one process
• Call
• void prepare_to_wait_exclusive(wait_queue_heat_t
  queue,
• wait_queue_t
  wait, int state)
• Set the WQ_FLAG_EXCLUSIVE flag
• Add the queue entry to the end of the wait queue
• wake_up stops after waking the first process with
  the flag set

57
The Details of Waking Up

• / wakes up all processes waiting on the queue /
• void wake_up(wait_queue_head_t queue)
• / wakes up processes that perform an
  interruptible sleep /
• void wake_up_interruptible(wait_queue_head_t
  queue)
• / wake up to nr exclusive waiters /
• void wake_up_nr(wait_queue_head_t queue, int
  nr)
• void wake_up_interruptible_nr(wait_queue_head_t
  queue, int nr)
• / wake up all exclusive waiters /
• void wake_up_all(wait_queue_head_t queue)
• void wake_up_interruptible_all(wait_queue_head_t
  queue)
• / do not lose the CPU during this call /
• void wake_up_interruptible_sync(wait_queue_head_t
  queue)

58
Testing the scullpipe Driver

• Window 1
• cat /dev/scullpipe

• Window2

59
Testing the scullpipe Driver

• Window 1
• cat /dev/scullpipe

• Window2
• ls aF gt /dev/scullpipe

60
Testing the scullpipe Driver

• Window 1
• cat /dev/scullpipe
• ./
• ../
• file1
• file2

• Window2
• ls aF gt /dev/scullpipe

61
poll and select

• Nonblocking I/Os often involve the use of poll,
  select, and epoll system calls
• Allow a process to determine whether it can read
  or write one or more open files without blocking
• Can block a process until any of a set of file
  descriptors becomes available for reading and
  writing
• select introduced in BSD Linux
• poll introduced in System V
• epoll added in 2.5.45 for better scaling

62
poll and select

• All three calls supported through the poll method
• unsigned int (poll) (struct file filp,
• poll_table wait)
• 1. Call poll_wait on one or more wait queues that
  could indicate a change in the poll status
• If no file descriptors are available, wait
• 2. Return a bit mask describing the operations
  that could be immediately performed without
  blocking

63
poll and select

• poll_table defined in ltlinux/poll.hgt
• To add a wait queue into the poll_table, call
• void poll_wait(struct file ,
• wait_queue_head_t ,
• poll_table )
• Bit mask flags defined in ltlinux/poll.hgt
• POLLIN
• Set if the device can be read without blocking

64
poll and select

• POLLOUT
• Set if the device can be written without blocking
• POLLRDNORM
• Set if normal data is available for reading
• A readable device returns (POLLIN POLLRDNORM)
• POLLWRNORM
• Same meaning as POLLOUT
• A writable device returns (POLLOUT POLLWRNORM)
• POLLPRI
• High-priority data can be read without blocking

65
poll and select

• POLLHUP
• Returns when a process reads the end-of-file
• POLLERR
• An error condition has occurred
• POLLRDBAND
• Out-of-band data is available for reading
• Associated with sockets
• POLLWRBAND
• Data with nonzero priority can be written to the
  device

66
poll and select

• Example
• static unsigned int scull_p_poll(struct file
  filp,
• poll_table
  wait)
• struct scull_pipe dev filp-gtprivate_data
• unsigned int mask 0
• down(dev-gtsem)
• poll_wait(filp, dev-gtinq, wait)
• poll_wait(filp, dev-gtoutq, wait)
• if (dev-gtrp ! dev-gtwp) / circular buffer not
  empty /
• mask POLLIN POLLRDNORM / readable /
• if (spacefree(dev)) / circular buffer not full
  /
• mask POLLOUT POLLWRNORM / writable /
• up(dev-gtsem)
• return mask

67
poll and select

• No end-of-file support
• Scull pipe does not implement this
• If it did
• The reader could see an end-of-file when all
  writers close the file
• Check dev-gtnwriters in read and poll
• Problem when a reader opens the scullpipe before
  the writer
• Need blocking within open

68
Interaction with read and write

• Reading from the device
• If there is data in the input buffer, return at
  least one byte
• poll returns POLLIN POLLRDNORM
• If no data is available
• If O_NONBLOCK is set, return EAGAIN
• poll must report the device unreadable until one
  byte arrives
• At the end-of-file, read returns 0, poll returns
  POLLHUP

69
Interaction with read and write

• Writing to the device
• If there is space in the output buffer, accept at
  least one byte
• poll reports that the devices is writable by
  returning POLLOUT POLLWRNORM
• If the output buffer is full, write blocks
• If O_NONBLOCK is set, write returns EAGAIN
• poll reports that the file is not writable
• If the device is full, write returns -ENOSPC

70
Interaction with read and write

• In write, never wait for data transmission before
  returning
• Or, select may block
• To make sure the output buffer is actually
  transmitted, use fsync call

71
Interaction with read and write

• To flush pending output, call fsync
• int (fsync) (struct file file, loff_t, loff_t,
  int datasync)
• Should return only when the device has been
  completely flushed
• datasync
• Used by file systems, ignored by drivers

72
The Underlying Data Structure
73
The Underlying Data Structure

• When the poll call completes, poll_table is
  deallocated with all wait queue entries removed
• epoll reduces this overhead of setting up and
  tearing down the data structure between every I/O

74
Asynchronous Notification

• Polling
• Inefficient for rare events
• A solution asynchronous notification
• Application receives a signal whenever data
  becomes available
• Two steps
• Specify a process as the owner of the file (so
  that the kernel knows whom to notify)
• Set the FASYNC flag in the device via fcntl
  command

75
Asynchronous Notification

• Example (user space)
• / create a signal handler /
• signal(SIGIO, input_handler)
• / set current pid the owner of the stdin /
• fcntl(STDIN_FILENO, F_SETOWN, getpid())
• / obtain the current file control flags /
• oflags fcntl(STDIN_FILENO, F_GETFL)
• / set the asynchronous flag /
• fcntl(STDIN_FILENO, F_SETFL, oflags FASYNC)

76
Asynchronous Notification

• Some catches
• Not all devices support asynchronous notification
• Usually available for sockets and ttys
• Need to know which input file to process
• Still need to use poll or select

77
The Drivers Point of View

• 1. When F_SETOWN is invoked, a value is assigned
  to filp-gtf_owner
• 2. When F_SETFL is executed to change the status
  of FASYNC
• The drivers fasync method is called
• static int
• scull_p_fasync(int fd, struct file filp, int
  mode)
• struct scull_pipe dev filp-gtprivate_data
• return fasync_helper(fd, filp, mode,
  dev-gtasync_queue)

78
The Drivers Point of View

• fasync_helper adds or removes processes from the
  asynchronous list
• void fasync_helper(int fd, struct file filp, int
  mode,
• struct fasync_struct fa)
• 3. When data arrives, send a SIGNO signal to all
  processes registered for asynchronous
  notification
• Near the end of write, notify blocked readers
• if (dev-gtasync_queue)
• kill_fasync(dev-gtasync_queue, SIGIO, POLL_IN)
• Similarly for read, as needed

79
The Drivers Point of View

• 4. When the file is closed, remove the file from
  the list of asynchronous readers in the release
  method
• scull_p_fasync(-1, filp, 0)

80
The llseek Implementation

• Implements lseek and llseek system calls
• Modifies filp-gtf_pos
• loff_t scull_llseek(struct file filp, loff_t
  off, int whence)
• struct scull_dev dev filp-gtprivate_data
• loff_t newpos
• switch(whence)
• case 0 / SEEK_SET /
• newpos off
• break
• case 1 / SEEK_CUR, relative to the current
  position /
• newpos filp-gtf_pos off
• break

81
The llseek Implementation

• case 2 / SEEK_END, relative to the end of
  the file /
• newpos dev-gtsize off
• break
• default / can't happen /
• return -EINVAL
• if (newpos lt 0) return -EINVAL
• filp-gtf_pos newpos
• return newpos

82
The llseek Implementation

• Does not make sense for serial ports and keyboard
  inputs
• Need to inform the kernel via calling
  nonseekable_open in the open method
• int nonseekable_open(struct inode inode,
  struct file filp)
• Replace llseek method with no_llseek (defined in
  ltlinux/fs.hgt in your file_operations structure

83
Access Control on a Device File

• Prevents unauthorized users from using the device
• Sometimes permits only one authorized user to
  open the device at a time

84
Single-Open Devices

• Example scullsingle
• static atomic_t scull_s_available
  ATOMIC_INIT(1)
• static int scull_s_open(struct inode inode,
  struct file filp)
• struct scull_dev dev scull_s_device
• if (!atomic_dec_and_test(scull_s_available))
• atomic_inc(scull_s_available)
• return -EBUSY / already open /
• / then, everything else is the same as before
  /
• if ((filp-gtf_flags O_ACCMODE) O_WRONLY)
  scull_trim(dev)
• filp-gtprivate_data dev
• return 0 / success /

Returns true, if the tested value is 0
85
Single-Open Devices

• In the release call, marks the device idle
• static int
• scull_s_release(struct inode inode, struct file
  filp)
• atomic_inc(scull_s_available) / release the
  device /
• return 0

86
Restricting Access to a Single User (with
multiple processes) at a Time

• Example sculluid
• Includes the following in the open call
• spin_lock(scull_u_lock)
• if (scull_u_count / someone is using the
  device /
• (scull_u_owner ! current-gtuid) / not the
  same user /
• (scull_u_owner ! current-gteuid) / not the
  same effective uid (for su) /
• !capable(CAP_DAC_OVERRIDE)) / not root
  override /
• spin_unlock(scull_u_lock)
• return -EBUSY / -EPERM would confuse the user
  /
• if (scull_u_count 0) scull_u_owner
  current-gtuid
• scull_u_count
• spin_unlock(scull_u_lock)

87
Restricting Access to a Single User (with
Multiple Processes) at a Time

• Includes the following in the release call
• static int scull_u_release(struct inode inode,
• struct file filp)
• spin_lock(scull_u_lock)
• scull_u_count-- / nothing else /
• spin_unlock(scull_u_lock)
• return 0

88
Blocking open as an Alternative to EBUSY
(scullwuid)

• A user might prefer to wait over getting errors
• E.g., data communication channel
• spin_lock(scull_w_lock)
• while (!scull_w_available())
• spin_unlock(scull_w_lock)
• if (filp-gtf_flags O_NONBLOCK) return -EAGAIN
• if (wait_event_interruptible(scull_w_wait,
• scull_w_available()
  ))
• return -ERESTARTSYS / tell the fs layer to
  handle it /
• spin_lock(scull_w_lock)
• if (scull_w_count 0) scull_w_owner
  current-gtuid
• scull_w_count
• spin_unlock(scull_w_lock)

89
Blocking open as an Alternative to EBUSY
(scullwuid)

• The release method wakes pending processes
• static int scull_w_release(struct inode inode,
• struct file filp)
• int temp
• spin_lock(scull_w_lock)
• scull_w_count--
• temp scull_w_count
• spin_unlock(scull_w_lock)
• if (temp 0)
• wake_up_interruptible_sync(scull_w_wait)
• return 0

90
Blocking open as an Alternative to EBUSY

• Might not be the right semantics for interactive
  users
• Blocking on cp vs. getting a return value EBUSY
  or -EPERM
• Incompatible policies for the same device
• One solution one device node per policy

91
Cloning the Device on open

• Allows the creation of private, virtual devices
• E.g., One virtual scull device for each process
  with different tty device number
• Example scullpriv

92
Cloning the Device on open

• static int scull_c_open(struct inode inode,
  struct file filp)
• struct scull_dev dev
• dev_t key
• if (!current-gtsignal-gttty)
• PDEBUG("Process \"s\" has no ctl tty\n",
  current-gtcomm)
• return -EINVAL
• key tty_devnum(current-gtsignal-gttty)
• spin_lock(scull_c_lock)
• dev scull_c_lookfor_device(key)
• spin_unlock(scull_c_lock)
• if (!dev) return -ENOMEM
• .../ then, everything else is the same as
  before /

93
Cloning the Device on open

• / The clone-specific data structure includes a
  key field /
• struct scull_listitem
• struct scull_dev device
• dev_t key
• struct list_head list
• / The list of devices, and a lock to protect it
  /
• static LIST_HEAD(scull_c_list)
• static spinlock_t scull_c_lock
  SPIN_LOCK_UNLOCKED

94
Cloning the Device on open

• / Look for a device or create one if missing /
• static struct scull_dev scull_c_lookfor_device(de
  v_t key)
• struct scull_listitem lptr
• list_for_each_entry(lptr, scull_c_list, list)
  
• if (lptr-gtkey key)
• return (lptr-gtdevice)
• / not found /
• lptr kmalloc(sizeof(struct scull_listitem),
  GFP_KERNEL)
• if (!lptr) return NULL

95
Cloning the Device on open

• / initialize the device /
• memset(lptr, 0, sizeof(struct scull_listitem))
• lptr-gtkey key
• scull_trim((lptr-gtdevice)) / initialize it
  /
• init_MUTEX((lptr-gtdevice.sem))
• / place it in the list /
• list_add(lptr-gtlist, scull_c_list)
• return (lptr-gtdevice)

96
Whats going on?
scull_listitem
struct scull_dev device dev_t key
scull_c_list