Title: Char Drivers
1Char Drivers
- Ted Baker ? Andy Wang
- COP 5641 / CIS 4930
2Goal
- Write a complete char device driver
- scull
- Simple Character Utility for Loading Localities
- Not hardware dependent
- Just acts on some memory allocated from the
kernel
3The Design of scull
- Implements various devices
- scull0 to scull3
- Four device drivers, each consisting of a memory
area - Global
- Data contained within the device is shared by all
the file descriptors that opened it - Persistent
- If the device is closed and reopened, data isnt
lost
4The Design of scull
- scullpipe0 to scullpipe3
- Four FIFO devices
- Act like pipes
- Show how blocking and nonblocking read and write
can be implemented - Without resorting to interrupts
- scullpriv
- Private to each virtual console
5The Design of scull
- scullsingle
- Similar to scull0
- Allows only one process to use the driver at a
time - sculluid
- Can be opened multiple times by one user at a
time - Returns Device Busy if another user is locking
the device
6The Design of scull
- scullwuid
- Blocks open if another user is locking the device
7Major and Minor Numbers
- Char devices are accessed through names in the
file system - Special files/nodes in /dev
- gtcd /dev
- gtls l
- crw------- 1 root root 5, 1 Apr 12 1650
console - brw-rw---- 1 awang floppy 2, 0 Apr 12 1650
fd0 - brw-rw---- 1 awang floppy 2, 84 Apr 12 1650
fd0u1040
8Major and Minor Numbers
- Char devices are accessed through names in the
file system - Special files/nodes in /dev
- gtcd /dev
- gtls l
- crw------- 1 root root 5, 1 Apr 12 1650
console - brw-rw---- 1 awang floppy 2, 0 Apr 12 1650
fd0 - brw-rw---- 1 awang floppy 2, 84 Apr 12 1650
fd0u1040
Major numbers
Char drivers are identified by a c
Block drivers are identified by a b
Minor numbers
9Major and Minor Numbers
- Major number identifies the driver associated
with the device - /dev/fd0 and /dev/fd0u1040 are managed by driver
2 - gtcd /dev
- gtls l
- crw------- 1 root root 5, 1 Apr 12 1650
console - brw-rw---- 1 awang floppy 2, 0 Apr 12 1650
fd0 - brw-rw---- 1 awang floppy 2, 84 Apr 12 1650
fd0u1040 - Minor number indicates which device is being
referred to
10Representation of Device Numbers
- dev_t type, defined in ltlinux/kdev_t.hgt
- 12 bits for the major number
- Use MAJOR(dev_t dev) to obtain the major number
- 20 bits for the minor number
- Use MINOR(dev_t dev) to obtain the minor number
- Use MKDEV(int major, int minor) to turn them into
a dev_t
11Allocation and Deallocation
- To obtain one or more device numbers, use
- int register_chrdev_region(dev_t first, unsigned
int count, char name) - first
- Beginning device number
- Minor device number is often 0
- count
- Requested number of contiguous device numbers
- name
- Name of the device
12Allocation and Deallocation
- To obtain one or more device numbers, use
- int register_chrdev_region(dev_t first, unsigned
int count, char name) - Returns 0 on success, error code on failure
13Allocation and Deallocation
- Kernel can allocate a major number on the fly
- int alloc_chrdev_region(dev_t dev, unsigned int
firstminor, unsigned int count, char name) - dev
- Output-only parameter that holds the first number
on success - firstminor
- Requested first minor number
- Often 0
14Allocation and Deallocation
- To free your device numbers, use
- int unregister_chrdev_region(dev_t first,
unsigned int count)
15Dynamic Allocation
- Some major device numbers are statically assigned
- See Documentation/devices.txt
- To avoid conflicts, use dynamically allocation
16scull_load Shell Script
- !/bin/sh
- modulescull
- devicescull
- mode664
- invoke insmod with all arguments we got and use
a pathname, - as newer modutils dont look in . by default
- /sbin/insmod ./module.ko exit 1
- remove stale nodes
- rm f /dev/device0-3
- major(awk \2\module\ print \1
/proc/devices)
Arguments to scull_load
Textbook typos
17scull_load Shell Script
- mknod /dev/device0 c major 0
- mknod /dev/device1 c major 1
- mknod /dev/device2 c major 2
- mknod /dev/device3 c major 3
- give appropriate group/permissions, and change
the group. - Not all distributions have staff, some have
wheel instead. - groupstaff
- grep q staff /etc/group groupwheel
- chgrp group /dev/device0-3
- chmod mode /dev/device0-3
18Overview of Data Structures
struct scull_dev
cdev_add()
struct file_operations scull_fops
struct cdev
struct i_node
data
data
19Some Important Data Structures
- file_operations
- file
- inode
- Defined in ltlinux/fs.hgt
20File Operations
- struct file_operations
- struct module owner
- / pointer to the module that owns the
structure prevents - the module from being unloaded while in
use / - loff_t (llseek) (struct file , loff_t, int)
- / change the current position in a file
returns a 64-bit - offset, or a negative value on errors /
- ssize_t (read) (struct file , char __user ,
size_t, - loff_t )
- / returns the number of bytes read, or a
negative value - on errors /
- ssize_t (aio_read) (struct kiocb , const
struct iovec , - unsigned long, loff_t)
- / might return before a read completes /
21File Operations
- ssize_t (write) (struct file , const char
__user , - size_t, loff_t )
- / returns the number bytes written, or a
negative - value on error /
- ssize_t (aio_write) (struct kiocb ,
- const struct __iovec ,
- unsigned long, loff_t)
- int (readdir) (struct file , void ,
filldir_t) - / this function pointer should be NULL for
devices / - unsigned int (poll) (struct file ,
- struct poll_table_struct
) - / query whether a read or write would block
/ - int (ioctl) (struct inode , struct file ,
unsigned int, - unsigned long)
- / provides a way to issue device-specific
commands - (e.g., formatting) /
22File Operations
- int (mmap) (struct file , struct
vm_area_struct ) - / map a device memory to a processs address
/ - int (open) (struct inode , struct file )
- / first operation performed on the device
file if not - defined, opening always succeeds, but
driver is not - notified /
- int (flush) (struct file , fl_owner_t)
- / invoked when a process closes its copy of
a file - descriptor for a device not to be
confused with fsync - /
- int (release) (struct inode , struct file )
- / invoked when the file structure is being
released / - int (fsync) (struct file , struct dentry ,
int) - / flush pending data for a file /
- int (aio_fsync) (struct kiocb , int)
- / asynchronous version of fsync /
- int (fasync) (int, struct file , int)
- / notifies the device of a change in its
FASYNC flag / -
23File Operations
- int (lock) (struct file , int, struct
file_lock ) - / file locking for regular files, almost
never - implemented by device drivers /
- ssize_t (splice_read) (struct file , loff_t
, struct - pipe_inode_info ,
size_t, - unsigned int)
- ssize_t (splice_write) (struct pipe_inode_info
, file , - loff_t , size_t,
unsigned int) - / implement gather/scatter read and write
operations / - ssize_t (sendfile) (struct file , loff_t ,
size_t, - read_actor_t, void )
- / moves the data from one file descriptor to
another - usually not used by device drivers /
- ssize_t (sendpage) (struct file , struct page
, int, - size_t, loff_t , int)
- / called by kernel to send data, one page at
a time - usually not used by device drivers /
24File Operations
- unsigned long (get_unmapped_area) (struct file
, - unsigned
long, - unsigned
long, - unsigned
long, - unsigned
long) - / finds a location in the processs memory
to map in a - memory segment on the underlying device
used to - enforce alignment requirements most
drivers do not use - this function /
- int (check_flags) (int)
- / allows a module to check flags passed to
an fcntl call - /
- int (dir_notify) (struct file , unsigned
long) - / invoked when an application uses fcntl to
request - directory change notifications usually
not used by - device drivers /
25scull device driver
- Implements only the most important methods
- struct file_operations scull_fops
- .owner THIS_MODULE,
- .llseek scull_llseek,
- .read scull_read,
- .write scull_write,
- .ioctl scull_ioctl,
- .open scull_open,
- .release scull_release,
26The File Structure
- struct file
- Nothing to do with the FILE pointers
- Defined in the C Library
- Represents an open file
- A pointer to file is often called filp
27The File Structure
- Some important fields
- mode_t f_mode
- Identifies the file as either readable or
writable - loff_t f_pos
- Current reading/writing position (64-bits)
- unsigned int f_flags
- File flags, such as O_RDONLY, O_NONBLOCK, O_SYNC
28The File Structure
- Some important fields
- struct file_operations f_op
- Operations associated with the file
- Dynamically replaceable pointer
- Equivalent of method overriding in OO programming
- void private data
- Can be used to store additional data structures
- Needs to be freed during the release method
29The File Structure
- Some important fields
- struct dentry f_dentry
- Directory entry associated with the file
- Used to access the inode data structure
- filp-gtf_dentry-gtd_inode
30The i-node Structure
- There can be numerous file structures (multiple
open descriptors) for a single file - Only one inode structure per file
31The i-node Structure
- Some important fields
- dev_t i_rdev
- Contains device number
- For portability, use the following macros
- unsigned int iminor(struct inode inode)
- unsigned int imajor(struct inode inode)
- struct cdev i_cdev
- Contains a pointer to the data structure that
refers to a char device file
32Char Device Registration
- Need to allocate struct cdev to represent char
devices - include ltlinux/cdev.hgt
- / first way /
- struct cdev my_cdev cdev_alloc()
- my_cdev-gtops my_fops
- / second way, for embedded cdev structure, call
this function / - void cdev_init(struct cdev cdev, struct
file_operations fops)
33Char Device Registration
- Either way
- Need to initialize file_operations and set owner
to THIS_MODULE - Inform the kernel by calling
- int cdev_add(struct cdev dev, dev_t num,
unsigned int count) - num first device number
- count number of device numbers
- Remove a char device, call this function
- void cdev_del(struct cdev dev)
34sculls Memory Usage
struct scull_qset void data struct
scull_qset next
SCULL_QUANTUM 1KB
Quantum set, SCULL_QSET 1K quanta
35Device Registration in scull
- scull represents each device with struct
scull_dev - struct scull_dev
- struct scull_qset data / pointer to first
quantum set / - int quantum / the current quantum
size / - int qset / the current array
size / - unsigned long size / amount of data
stored here / - unsigned int access_key / used by sculluid
scullpriv / - struct semaphore sem / mutual exclusion
semaphore / - struct cdev cdev / char device
structure /
36Char Device Initialization Steps
- Register device driver name and numbers
- Allocation of the struct scull_dev objects
- Initialization of scull cdev objects
- Calls cdev_init to initialize the struct cdev
component - Sets cdev.owner to this module
- Sets cdev.ops to scull_fops
- Calls cdev_add to complete registration
37Char Device Cleanup Steps
- Clean up internal data structures
- cdev_del scull devices
- Deallocate scull devices
- Unregister device numbers
38Device Registration in scull
- To add struct scull_dev to the kernel
- static void scull_setup_cdev(struct scull_dev
dev, int index) -
- int err, devno MKDEV(scull_major, scull_minor
index) - cdev_init(dev-gtcdev, scull_fops)
- dev-gtcdev.owner THIS_MODULE
- dev-gtcdev.ops scull_fops / redundant? /
- err cdev_add(dev-gtcdev, devno, 1)
- if (err)
- printk(KERN_NOTICE Error d adding
sculld, err, - index)
-
39The open Method
- In most drivers, open should
- Check for device-specific errors
- Initialize the device (if opened for the first
time) - Update the f_op pointer, as needed
- Allocate and fill data structure in
- filp-gtprivate_data
40The open Method
- int scull_open(struct inode inode, struct file
file) - struct scull_dev dev / device info /
-
- / include ltlinux/kernel.hgt
- container_of(pointer, container_type,
container_field - returns the starting address of struct
scull_dev / - dev container_of(inode-gti_cdev, struct
scull_dev, cdev) - filp-gtprivate_data dev
- / now trim to 0 the length of the device if
open was write-only / - if ((filp-gtf_flags O_ACCMODE) O_WRONLY)
- scull_trim(dev) / ignore errors /
-
- return 0 / success /
41The release Method
- Deallocate filp-gtprivate_data
- Shut down the device on last close
- One release call per open
- Potentially multiple close calls per open due to
fork/dup - scull has no hardware to shut down
- int scull_release(struct inode inode, struct
file filp) - return 0
42sculls Memory Usage
- Dynamically allocated
- include ltlinux/slab.hgt
- void kmalloc(size_t size, int flags)
- Allocate size bytes of memory
- For now, always use GFP_KERNEL
- Return a pointer to the allocated memory, or NULL
if the allocation fails - void kfree(void ptr)
43sculls Memory Usage
struct scull_qset void data struct
scull_qset next
SCULL_QUANTUM 1KB
Quantum set, SCULL_QSET 1K quanta
44sculls Memory Usage
- int scull_trim(struct scull_dev dev)
- struct scull_qset next, dptr
- int qset dev-gtqset / dev is not NULL /
- int i
- for (dptr dev-gtdata dptr dptr next)
- if (dptr-gtdata)
- for (i 0 i lt qset i)
kfree(dptr-gtdatai) - kfree(dptr-gtdata)
- dptr-gtdata NULL
-
- next dptr-gtnext
- kfree(dptr)
-
- dev-gtsize 0 dev-gtdata NULL
- dev-gtquantum scull_quantum dev-gtqset
scull_qset - return 0
45Race Condition Protection
- Different processes may try to execute operations
on the same scull device concurrently - There would be trouble if both were able to
access the data of the same device at once - scull avoids this using per-device semaphore
- All operations that touch the devices data need
to lock the semaphore
46Race Condition Protection
- Some semaphore usage rules
- No double locking
- No double unlocking
- Always lock at start of critical section
- Dont release until end of critical section
- Dont forget to release before exiting
- return, break, or goto
- If you need to hold two locks at once, lock them
in a well-known order, unlock them in the reverse
order (e.g., lock1, lock2, unlock2, unlock1)
47Semaphore Usage Examples
- Initialization
- init_MUTEX(scull_devicesi.sem)
- Critial section
- if (down_interruptible(dev-gtsem))
- return ERESTARTSYS
- scull_trim(dev) / ignore errors /
- up(dev-gtsem)
Can be interrupted by signals (e.g. Ctrl-C)
48Semaphore vs. Spinlock
- Semaphores may block
- Calling process is blocked until the lock is
released - Spinlock may spin (loop)
- Calling processor spins until the lock is
released - Never call down unless it is OK for the current
thread to block - Do not call down while holding a spinlock
- Do not call down within an interrupt handler
49read and write
- ssize_t (read) (struct file filp, char __user
buff, - size_t count, loff_t offp)
- ssize_t (write) (struct file filp, const char
__user buff, - size_t count, loff_t offp)
- filp file pointer
- buff a user-space pointer
- May not be valid in kernel mode
- Might be swapped out
- Could be malicious
- count size of requested transfer
- offp file position pointer
50read and write
- To safely access user-space buffer
- Use kernel-provided functions
- include ltasm/uaccess.hgt
- unsigned long copy_to_user(void __user to,
- const void from,
- unsigned long
count) - unsigned long copy_from_user(void to,
- const void __user
from, - unsigned long
count) - Check whether the user-space pointer is valid
- Return the amount of memory still to be copied
51read and write
52The read Method
- Return values
- Equals to the count argument, we are done
- Positive lt count, retry
- 0, end-of-file
- Negative, check ltlinux/errno.hgt
- Common errors
- -EINTR (interrupted system call)
- -EFAULT (bad address)
- No data, but will arrive later
- read system call should block
53The read Method
- Each scull_read deals only with a single data
quantum - I/O library will reiterate the call to read
additional data - If read position gt device size, return 0
(end-of-file)
54The read Method
- ssize_t scull_read(struct file filp, char __user
buf, - size_t count, loff_t f_pos)
- struct scull_dev dev filp-gtprivate_data
- struct scull_qset dptr / the first listitem
/ - int quantum dev-gtquantum, qset dev-gtqset
- int itemsize quantum qset / bytes in the
listitem / - int item, s_pos, q_pos, rest
- ssize_t retval 0
- if (down_interruptible(dev-gtsem))
- return ERESTARTSYS
- if (fpos gt dev-gtsize)
- goto out
- if (f_pos count gt dev-gtsize)
- count dev-gtsize - fpos
55The read Method
- / find listitem, qset index, and offset in the
quantum / - item (long) f_pos / itemsize
- rest (long) f_pos itemsize
- s_pos rest / quantum
- q_pos rest quantum
- / follow the list up to the right position
(defined elsewhere / - dptr scull_follow(dev, item)
- if (dptr NULL !dptr-gtdata
!dptr-gtdatas_pos) - goto out / dont fill holes /
- / read only up to the end of this quantum /
- if (count gt quantum q_pos)
- count quantum q_pos
56The read Method
- if (copy_to_user(buf, dptr-gtdatas_pos
q_pos, count)) - retval -EFAULT
- goto out
-
- f_pos count
- retval count
- out
- up(dev-gtsem)
- return retval
-
57The write Method
- Return values
- Equals to the count argument, we are done
- Positive lt count, retry
- 0, nothing was written
- Negative, check ltlinux/errno.hgt
58The write Method
- ssize_t scull_write(struct file filp, const char
__user buf, - size_t count, loff_t f_pos)
- struct scull_dev dev filp-gtprivate_data
- struct scull_qset dptr
- int quantum dev-gtquantum, qset dev-gtqset
- int itemsize quantum qset
- int item, s_pos, q_pos, rest
- ssize_t retval -ENOMEM / default error
value / - if (down_interruptible(dev-gtsem))
- return ERESTARTSYS
-
59The write Method
- / find listitem, qset index and offset in the
quantum / - item (long) f_pos / itemsize
- rest (long) f_pos itemsize
- s_pos rest / quantum
- q_pos rest quantum
- / follow the list up the right position /
- dptr scull_follow(dev, item)
-
60The write Method
- if (dptr NULL)
- goto out
- if (!dptr-gtdata)
- dptr-gtdata kmalloc(qsetsizeof(char ),
GFP_KERNEL) - if (!dptr-gtdata)
- goto out
-
- memset(dptr-gtdata, 0, qsetsizeof(char ))
-
- if (!dptr-gtdatas_pos)
- dptr-gtdatas_pos kmalloc(quantum,
GPF_KERNEL) - if (!dptr-gtdatas_pos)
- goto out
-
61The write Method
- / write only up to the end of this quantum /
- if (count gt quantum q_pos)
- count quantum q_pos
- if (copy_from_user(dptr-gtdatas_pos q_pos,
buf, count)) - return EFAULT
- goto out
-
-
62The write Method
- f_pos count
- retval count
- / update the size /
- if (dev-gtsize lt f_pos)
- dev-gtsize f_pos
- out
- up(dev-gtsem)
- return retval
63readv and writev
- Vector versions of read and write
- Take an array of structures
- Each contains a pointer to a buffer and a length
64Playing with the New Devices
- With open, release, read, and write, a driver can
be compiled and tested - Use free command to see the memory usage of scull
- Use strace to monitor various system calls and
return values - strace ls l gt /dev/scull0 to see quantized reads
and writes