Какво бихте направили, ако някой ви помоли да свършите
нещо, а вие нямате възможност точно в този момент? Ако
сте човешко същество, и ви притеснява някой друг
човек, единственият начин е да кажете: "Не мога сега,
зает съм! Разкарай се!" Но ако сте модул на ядрото и
ви притеснява някой процес, имате още една възможност
- да "приспите" процеса до момента, в който можете да
го обслужите. В края на краищата кернелът постоянно
слага разни процеси да спят и ги буди (по този начин
изглежда, като че ли множество процеси работят по едно
и също време на един и същ процесор).
Този модул на кернела е пример за това. Файлът
(наречен /proc/sleep) може да бъде отворен само от
един процес едновременно. Ако файлът е вече отворен,
модулът на кернела извиква
module_interuptible_sleep_on. Тази функция променя
състоянието на задачата (задачата, или task,
представлява структура от данни на кернела, която
съдържа информация за един процес и за системното
обръщение в което е той, ако въобще има такова) към
TASK_INTERRUPTIBLE, което означава, че задачата няма
да тръгне отново, докато не бъде "събудена" по някакъв
начин, и я допълва към WaitQ, опашката от задачи,
чакащи достъп до файла. Тогава функцията се обръща към
разписанието за контекстно превключване към различен
процес - такъв, от който има някаква полза за CPU-то.
Когато процесът е свършил работата си с файла, той го
затваря, и се обръща към module_close. Тази функция
събужда всички процеси в опашката (понеже няма
механизъм за събуждане на само един от тях). След това
се връща и процесът, който току-що е затворил файла,
може да продължи да работи. По някое време
определящият разписанието решава, че на този процес му
стига толкова, и предава контрола над CPU-то на друг
процес. Впоследствие на един от процесите, който е бил
на опашката, му се дава контрол над процесора от
разписанието. Той се стартира точно от мястото веднага
след обръщението към module_interruptible_sleep_on.
След това може да продължи, за да установи една
глобална променлива, която да съобщи на всички други
процеси, че този файл все още е отворен, и продължава
да "живее". Когато другите процеси получат малко
процесорно време, те поглеждат тази глобална
променлива и се връщат да спят.
За да стане животът още по-интересен, module_close
няма монопол над събуждането на процесите, които чакат
за достъп до файла. Един сигнал, като например Ctrl+C
(SIGINT) може също да събуди процес. В този случай ние
искаме незабавно да се върнем с EINTR. Това е важно,
за да могат потребителите например да убият процеса
преди той да получи файла.
Има още едно нещо, което трябва да се запомни.Понякога
процесите не искат да спят - те или искат да получат
това, което им трябва, и то незабавно, или да им бъде
казано, че това не може да стане. Когато отварят
файла, тези процеси използват флага O_NONBLOCK.
Предполага се в такъв случай кернелът да отговори с
връщане на код за грешка -EAGAIN от операциите, които
иначе би блокирал, като например отварянето на файла в
този случай.
ex sleep.c
/* sleep.c - create a /proc file, and if several
* processes try to open it at the same time, put all
* but one to sleep */
/* Copyright (C) 1998-99 by Ori Pomerantz */
/* The necessary header files */
/* Standard in kernel modules */
#include /* We're doing kernel work*/
#include /* Specifically, a module*/
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include
#endif
/* Necessary because we use proc fs */
#include
/* For putting processes to sleep and waking them up*/
#include
#include
/* In 2.2.3 /usr/include/linux/version.h includes a
/* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include /* for get_user and put_user*/
#endif
/* The module's file functions ***********************/
/* Here we keep the last message received, to prove
* that we can process our input */
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];
/* Since we use the file operations struct, we can't use
* the special proc output provisions - we have to use
* a standard read function, which is this function */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_output(
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the
* user segment) */
size_t len, /* The length of the buffer */
loff_t *offset) /* Offset in the file - ignore */
#else
static int module_output(
struct inode *inode, /* The inode read */
struct file *file, /* The file read */
char *buf, /* The buffer to put data to (in the
* user segment) */
int len) /* The length of the buffer */
#endif
{
static int finished = 0;
int i;
char message[MESSAGE_LENGTH+30];
/* Return 0 to signify end of file - that we have
* nothing more to say at this point. */
if (finished) {
finished = 0;
return 0;
}
/* If you don't understand this by now, you're
* hopeless as a kernel programmer. */
sprintf(message, "Last input:%s\n", Message);
for(i=0; i= KERNEL_VERSION(2,2,0)
static ssize_t module_input(
struct file *file, /* The file itself */
const char *buf, /* The buffer with input */
size_t length, /* The buffer's length */
loff_t *offset) /* offset to file - ignore */
#else
static int module_input(
struct inode *inode, /* The file's inode */
struct file *file, /* The file itself */
const char *buf, /* The buffer with the input
*/
int length) /* The buffer's length */
#endif
{
int i;
/* Put the input into Message, where module_output
* will later be able to use it */
for(i=0; i= KERNEL_VERSION(2,2,0)
get_user(Message[i], buf+i);
#else
Message[i] = get_user(buf+i);
#endif
/* we want a standard, zero terminated string */
Message[i] = '\0';
/* We need to return the number of input
* characters used */
return i;
}
/* 1 if the file is currently open by somebody */
int Already_Open = 0;
/* Queue of processes who want our file */
static struct wait_queue *WaitQ = NULL;
/* Called when the /proc file is opened */
static int module_open(struct inode *inode,
struct file *file)
{
/* If the file's flags include O_NONBLOCK, it means
* the process doesn't want to wait for the file.
* In this case, if the file is already open, we
* should fail with -EAGAIN, meaning "you'll have to
* try again", instead of blocking a process which
* would rather stay awake. */
if ((file->f_flags & O_NONBLOCK) && Already_Open)
return -EAGAIN;
/* This is the correct place for MOD_INC_USE_COUNT
* because if a process is in the loop, which is
* within the kernel module, the kernel module must
* not be removed. */
MOD_INC_USE_COUNT;
/* If the file is already open, wait until it isn't*/
while (Already_Open)
{
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int i, is_sig=0;
#endif
/* This function puts the current process,
* including any system calls, such as us, to sleep.
* Execution will be resumed right after the function
* call, either because somebody called
* wake_up(&WaitQ) (only module_close does that,
* when the file is closed) or when a signal, such
* as Ctrl-C, is sent to the process */
module_interruptible_sleep_on(&WaitQ);
/* If we woke up because we got a signal we're not
* blocking, return -EINTR (fail the system call).
* This allows processes to be killed or stopped.*/
/*
* Emmanuel Papirakis:
*
* This is a little update to work with 2.2.*. Signals
* now are contained in two words (64 bits) and are
* stored in a structure that contains an array of two
* unsigned longs. We now have to make 2 checks in our if.
*
* Ori Pomerantz:
*
* Nobody promised me they'll never use more than 64
* bits, or that this book won't be used for a version
* of Linux with a word size of 16 bits. This code
* would work in any case.
*/
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
for(i=0; i<_NSIG_WORDS && !is_sig; i++)
is_sig = current->signal.sig[i] &
~current->blocked.sig[i];
if (is_sig) {
#else
if (current->signal & ~current->blocked) {
#endif
/* It's important to put MOD_DEC_USE_COUNT here,
* because for processes where the open is
* interrupted there will never be a corresponding
* close. If we don't decrement the usage count
* here, we will be left with a positive usage
* count which we'll have no way to bring down to
* zero, giving us an immortal module, which can
* only be killed by rebooting the machine. */
MOD_DEC_USE_COUNT;
return -EINTR;
}
}
/* If we got here, Already_Open must be zero */
/* Open the file */
Already_Open = 1;
return 0; /* Allow the access */
}
/* Called when the /proc file is closed */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int module_close(struct inode *inode, struct file *file)
#else
void module_close(struct inode *inode, struct file *file)
#endif
{
/* Set Already_Open to zero, so one of the processes
* in the WaitQ will be able to set Already_Open back
* to one and to open the file. All the other processes
* will be called when Already_Open is back to one, so
* they'll go back to sleep. */
Already_Open = 0;
/* Wake up all the processes in WaitQ, so if anybody
* is waiting for the file, they can have it. */
module_wake_up(&WaitQ);
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0; /* success */
#endif
}
/* This function decides whether to allow an operation
* (return zero) or not allow it (return a non-zero
* which indicates why it is not allowed).
*
* The operation can be one of the following values:
* 0 - Execute (run the "file" - meaningless in our case)
* 2 - Write (input to the kernel module)
* 4 - Read (output from the kernel module)
*
* This is the real function that checks file
* permissions. The permissions returned by ls -l are
* for referece only, and can be overridden here.
*/
static int module_permission(struct inode *inode, int op)
{
/* We allow everybody to read from our module, but
* only root (uid 0) may write to it */
if (op == 4 || (op == 2 && current->euid == 0))
return 0;
/* If it's anything else, access is denied */
return -EACCES;
}
/* Structures to register as the /proc file, with
* pointers to all the relevant functions. ************/
/* File operations for our proc file. This is where
* we place pointers to all the functions called when
* somebody tries to do something to our file. NULL
* means we don't want to deal with something. */
static struct file_operations File_Ops_4_Our_Proc_File =
{
NULL, /* lseek */
module_output, /* "read" from the file */
module_input, /* "write" to the file */
NULL, /* readdir */
NULL, /* select */
NULL, /* ioctl */
NULL, /* mmap */
module_open,/* called when the /proc file is opened */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush */
#endif
module_close /* called when it's classed */
};
/* Inode operations for our proc file. We need it so
* we'll have somewhere to specify the file operations
* structure we want to use, and the function we use for
* permissions. It's also possible to specify functions
* to be called for anything else which could be done to an
* inode (although we don't bother, we just put NULL). */
static struct inode_operations
Inode_Ops_4_Our_Proc_File =
{
&File_Ops_4_Our_Proc_File,
NULL, /* create */
NULL, /* lookup */
NULL, /* link */
NULL, /* unlink */
NULL, /* symlink */
NULL, /* mkdir */
NULL, /* rmdir */
NULL, /* mknod */
NULL, /* rename */
NULL, /* readlink */
NULL, /* follow_link */
NULL, /* readpage */
NULL, /* writepage */
NULL, /* bmap */
NULL, /* truncate */
module_permission /* check for permissions */
};
/* Directory entry */
static struct proc_dir_entry Our_Proc_File =
{
0, /* Inode number - ignore, it will be filled by
* proc_register[_dynamic] */
5, /* Length of the file name */
"sleep", /* The file name */
S_IFREG | S_IRUGO | S_IWUSR,
/* File mode - this is a regular file which
* can be read by its owner, its group, and everybody
* else. Also, its owner can write to it.
*
* Actually, this field is just for reference, it's
* module_permission that does the actual check. It
* could use this field, but in our implementation it
* doesn't, for simplicity. */
1, /* Number of links (directories where the
* file is referenced) */
0, 0, /* The uid and gid for the file - we give
* it to root */
80, /* The size of the file reported by ls. */
&Inode_Ops_4_Our_Proc_File,
/* A pointer to the inode structure for
* the file, if we need it. In our case we
* do, because we need a write function. */
NULL /* The read function for the file.
* Irrelevant, because we put it
* in the inode structure above */
};
/* Module initialization and cleanup **************/
/* Initialize the module - register the proc file */
int init_module()
{
/* Success if proc_register_dynamic is a success,
* failure otherwise */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return proc_register(&proc_root, &Our_Proc_File);
#else
return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
/* proc_root is the root directory for the proc
* fs (/proc). This is where we want our file to be
* located.
*/
}
/* Cleanup - unregister our file from /proc. This could
* get dangerous if there are still processes waiting in
* WaitQ, because they are inside our open function,
* which will get unloaded. I'll explain how to avoid
* removal of a kernel module in such a case in
* chapter 10. */
void cleanup_module()
{
proc_unregister(&proc_root, Our_Proc_File.low_ino);
}
