Post on 18-Dec-2015
Advanced Char Driver Operations
Sarah Diesburg
CIS 4930
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.html
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
Topics
Managing ioctl command numbers Block/unblocking a process Seeking on a device Access control
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
ioctl
User-level interfaceint 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
ioctl
Driver-level interfaceint (*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/
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/
Choosing the ioctl Commands A command number uses four bitfields
Defined in <linux/ioctl.h> < direction, type, number, size> direction: direction of data transfer
_IOC_NONE _IOC_READ _IOC_WRITE _IOC_READ | WRITE
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)
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)
Choosing the ioctl Commands Useful macros to decode ioctl command
numbers _IOC_DIR(nr) _IOC_TYPE(nr) _IOC_NR(nr) _IOC_SIZE(nr)
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)
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
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
The Return Value
When the command number is not supported Return –EINVAL Or –ENOTTY (according to the POSIX standard)
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
Using the ioctl Argument
If it is an integer, just use it directly If it is a pointer
Need to check for valid user addressint 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 <linux/uaccess.h> Mostly called by memory-access routines
Using the ioctl Argument
The scull exampleint 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) > SCULL_IOC_MAXNR) {
return -ENOTTY;
}
…
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;
…
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 <linux/uaccess.h>
put_user(datum, ptr) Writes to a user-space address Calls access_ok() Returns 0 on success, -EFAULT on error
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
Capabilities and Restricted Operations Limit certain ioctl operations to privileged users See <linux/capability.h> 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
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;
…
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 (> 0) */
return scull_quantum;
…
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;
…
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 */
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);
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
Blocking I/O
Needed when no data is available for reads When the device is not ready to accept data
Output buffer is full
Introduction to Sleeping
Introduction to Sleeping
A process is removed from the scheduler’s 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
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
Introduction to Sleeping
Wait queue: contains a list of processes waiting for a specific event #include <linux/wait.h> To initialize statically, callDECLARE_WAIT_QUEUE_HEAD(my_queue);
To initialize dynamically, callwait_queue_head_t my_queue;
init_waitqueue_head(&my_queue);
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
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)
Simple Sleeping
To wake up, call variants of wake_up functionsvoid 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
Simple Sleeping
Example module: sleepystatic 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->pid, current->comm);
wait_event_interruptible(wq, flag != 0);
flag = 0;
printk(KERN_DEBUG "awoken %i (%s)\n", current->pid,
current->comm);
return 0; /* EOF */
}
Multiple threads can wake up at this point
Simple Sleeping
Example module: sleepyssize_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->pid, current->comm);
flag = 1;
wake_up_interruptible(&wq);
return count; /* succeed, to avoid retrial */
}
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->f_flags Defined in <linux/fcntl.h> Only open, read, and write calls are affected Returns –EAGAIN immediately instead of block Applications need to distinguish non-blocking
returns vs. EOFs
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
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 */
};
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->private_data;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
while (dev->rp == dev->wp) { /* nothing to read */
up(&dev->sem); /* release the lock */
if (filp->f_flags & O_NONBLOCK)
return -EAGAIN;
if (wait_event_interruptible(dev->inq, (dev->rp != dev->wp)))
return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
}
A Blocking I/O Example
if (dev->wp > dev->rp)
count = min(count, (size_t)(dev->wp - dev->rp));
else /* the write pointer has wrapped */
count = min(count, (size_t)(dev->end - dev->rp));
if (copy_to_user(buf, dev->rp, count)) {
up (&dev->sem);
return -EFAULT;
}
dev->rp += count;
if (dev->rp == dev->end) dev->rp = dev->buffer; /* wrapped */
up (&dev->sem);
/* finally, awake any writers and return */
wake_up_interruptible(&dev->outq);
return count;
}
Advanced Sleeping
Advanced Sleeping
Uses low-level functions to affect a sleep How a process sleeps
1. Allocate and initialize a wait_queue_t structureDEFINE_WAIT(my_wait); Or
wait_queue_t my_wait;
init_wait(&my_wait);
Queue element
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);
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 */
}
Advanced Sleeping
4. Return from sleep
Remove the process from the wait queue if schedule() was not calledvoid finish_wait(wait_queue_head_t *queue,
wait_queue_t *wait);
Advanced Sleeping
scullpipe write method
/* How much space is free? */
static int spacefree(struct scull_pipe *dev) {
if (dev->rp == dev->wp)
return dev->buffersize - 1;
return ((dev->rp + dev->buffersize - dev->wp)
% dev->buffersize) - 1;
}
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->private_data;
int result;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
/* Wait for space for writing */
result = scull_getwritespace(dev, filp);
if (result)
return result; /* scull_getwritespace called up(&dev->sem) */
/* ok, space is there, accept something */
count = min(count, (size_t)spacefree(dev));
Advanced Sleeping
if (dev->wp >= dev->rp)
count = min(count, (size_t)(dev->end - dev->wp));
else /* the write pointer has wrapped, fill up to rp - 1 */
count = min(count, (size_t)(dev->rp - dev->wp - 1));
if (copy_from_user(dev->wp, buf, count)) {
up (&dev->sem); return -EFAULT;
}
dev->wp += count;
if (dev->wp == dev->end) dev->wp = dev->buffer; /* wrapped */
up(&dev->sem);
wake_up_interruptible(&dev->inq);
if (dev->async_queue)
kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
return count;
}
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->sem);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);
if (spacefree(dev) == 0) schedule();
finish_wait(&dev->outq, &wait);
if (signal_pending(current)) return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
}
return 0;
}
Task state: RUNNINGQueue: full
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->sem);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);
if (spacefree(dev) == 0) schedule();
finish_wait(&dev->outq, &wait);
if (signal_pending(current)) return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
}
return 0;
}
Task state: RUNNING INTERRUPTIBLEQueue: full
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->sem);
if (filp->f_flags & O_NONBLOCK) return -EAGAIN;
prepare_to_wait(&dev->outq, &wait, TASK_INTERRUPTIBLE);
if (spacefree(dev) == 0) schedule();
finish_wait(&dev->outq, &wait);
if (signal_pending(current)) return -ERESTARTSYS;
if (down_interruptible(&dev->sem)) return -ERESTARTSYS;
}
return 0;
}
Task state: INTERRUPTIBLE /* sleep */Queue: full
Exclusive Waits
Avoid waking up all processes waiting on a queue Wakes up only one process
Callvoid 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
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);
Testing the scullpipe Driver
Window 1% cat /dev/scullpipe
Window2%
Testing the scullpipe Driver
Window 1% cat /dev/scullpipe
Window2% ls –aF > /dev/scullpipe
Testing the scullpipe Driver
Window 1% cat /dev/scullpipe
./
../
file1
file2
Window2% ls –aF > /dev/scullpipe
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
poll and select
All three calls supported through the poll methodunsigned 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
poll and select
poll_table defined in <linux/poll.h> To add a wait queue into the poll_table,
callvoid poll_wait(struct file *,
wait_queue_head_t *,
poll_table *);
Bit mask flags defined in <linux/poll.h> POLLIN
Set if the device can be read without blocking
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
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
poll and select
Examplestatic unsigned int scull_p_poll(struct file *filp,
poll_table *wait) {
struct scull_pipe *dev = filp->private_data;
unsigned int mask = 0;
down(&dev->sem);
poll_wait(filp, &dev->inq, wait);
poll_wait(filp, &dev->outq, wait);
if (dev->rp != dev->wp) /* circular buffer not empty */
mask |= POLLIN | POLLRDNORM; /* readable */
if (spacefree(dev)) /* circular buffer not full */
mask |= POLLOUT | POLLWRNORM; /* writable */
up(&dev->sem);
return mask;
}
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->nwriters in read and poll Problem when a reader opens the scullpipe before
the writer Need blocking within open
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
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
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
Interaction with read and write To flush pending output, call fsyncint (*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
The Underlying Data Structure
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
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
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);
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
The Driver’s Point of View
1. When F_SETOWN is invoked, a value is assigned to filp->f_owner
2. When F_SETFL is executed to change the status of FASYNC The driver’s fasync method is calledstatic int
scull_p_fasync(int fd, struct file *filp, int mode) {
struct scull_pipe *dev = filp->private_data;
return fasync_helper(fd, filp, mode, &dev->async_queue);
}
The Driver’s 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 readersif (dev->async_queue)
kill_fasync(&dev->async_queue, SIGIO, POLL_IN);
Similarly for read, as needed
The Driver’s Point of View
4. When the file is closed, remove the file from the list of asynchronous readers in the release methodscull_p_fasync(-1, filp, 0);
The llseek Implementation
Implements lseek and llseek system calls Modifies filp->f_pos
loff_t scull_llseek(struct file *filp, loff_t off, int whence) {
struct scull_dev *dev = filp->private_data;
loff_t newpos;
switch(whence) {
case 0: /* SEEK_SET */
newpos = off;
break;
case 1: /* SEEK_CUR, relative to the current position */
newpos = filp->f_pos + off;
break;
The llseek Implementation
case 2: /* SEEK_END, relative to the end of the file */
newpos = dev->size + off;
break;
default: /* can't happen */
return -EINVAL;
}
if (newpos < 0) return -EINVAL;
filp->f_pos = newpos;
return newpos;
}
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 <linux/fs.h> in your file_operations structure
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
Single-Open Devices
Example: scullsinglestatic 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->f_flags & O_ACCMODE) == O_WRONLY) scull_trim(dev);
filp->private_data = dev;
return 0; /* success */
}
Returns true, if the tested value is 0
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;
}
Restricting Access to a Single User (with multiple processes) at a Time Example: sculluid Includes the following in the open callspin_lock(&scull_u_lock);
if (scull_u_count && /* someone is using the device */
(scull_u_owner != current->uid) && /* not the same user */
(scull_u_owner != current->euid) && /* 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->uid;
scull_u_count++;
spin_unlock(&scull_u_lock);
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;
}
Blocking open as an Alternative to EBUSY (scullwuid) A user might prefer to wait over getting errors
E.g., data communication channelspin_lock(&scull_w_lock);
while (!scull_w_available()) {
spin_unlock(&scull_w_lock);
if (filp->f_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->uid;
scull_w_count++;
spin_unlock(&scull_w_lock);
Blocking open as an Alternative to EBUSY (scullwuid) The release method wakes pending
processesstatic 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;
}
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
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
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->signal->tty) {
PDEBUG("Process \"%s\" has no ctl tty\n", current->comm);
return -EINVAL;
}
key = tty_devnum(current->signal->tty);
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 */
}
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;
Cloning the Device on open
/* Look for a device or create one if missing */
static struct scull_dev *scull_c_lookfor_device(dev_t key) {
struct scull_listitem *lptr;
list_for_each_entry(lptr, &scull_c_list, list) {
if (lptr->key == key)
return &(lptr->device);
}
/* not found */
lptr = kmalloc(sizeof(struct scull_listitem), GFP_KERNEL);
if (!lptr) return NULL;
Cloning the Device on open
/* initialize the device */
memset(lptr, 0, sizeof(struct scull_listitem));
lptr->key = key;
scull_trim(&(lptr->device)); /* initialize it */
init_MUTEX(&(lptr->device.sem));
/* place it in the list */
list_add(&lptr->list, &scull_c_list);
return &(lptr->device);
}
What’s going on?
scull_c_list
struct list_head { struct list_head *next; struct list_head *prev;};
struct list_head { struct list_head *next; struct list_head *prev;} list;
scull_listitem
struct scull_dev device;dev_t key;