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.
 */

#include <linux/atomic.h>
#include <linux/fs.h>
#include <linux/kernel.h> /* for sprintf() */
#include <linux/module.h> /* Specifically, a module */
#include <linux/printk.h>
#include <linux/proc_fs.h> /* Necessary because we use proc fs */
#include <linux/types.h>
#include <linux/uaccess.h> /* for get_user and put_user */
#include <linux/version.h>
#include <linux/wait.h> /* For putting processes to sleep and
                                   waking them up */

#include <asm/current.h>
#include <asm/errno.h>

#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0)
#define HAVE_PROC_OPS
#endif

/* Here we keep the last message received, to prove that we can process our
 * input.
 */
#define MESSAGE_LENGTH 80
static char message[MESSAGE_LENGTH];

static struct proc_dir_entry *our_proc_file;
#define PROC_ENTRY_FILENAME "sleep"

/* 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.
 */
static ssize_t module_output(struct file *file, /* see include/linux/fs.h   */
                             char __user *buf, /* The buffer to put data to
                                                   (in the user segment)    */
                             size_t len, /* The length of the buffer */
                             loff_t *offset)
{
    static int finished = 0;
    int i;
    char output_msg[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;
    }

    sprintf(output_msg, "Last input:%s\n", message);
    for (i = 0; i < len && output_msg[i]; i++)
        put_user(output_msg[i], buf + i);

    finished = 1;
    return i; /* Return the number of bytes "read" */
}

/* This function receives input from the user when the user writes to the
 * /proc file.
 */
static ssize_t module_input(struct file *file, /* The file itself */
                            const char __user *buf, /* The buffer with input */
                            size_t length, /* The buffer's length */
                            loff_t *offset) /* offset to file - ignore */
{
    int i;

    /* Put the input into message, where module_output will later be able
     * to use it.
     */
    for (i = 0; i < MESSAGE_LENGTH - 1 && i < length; i++)
        get_user(message[i], buf + i);
    /* 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 */
static atomic_t already_open = ATOMIC_INIT(0);

/* Queue of processes who want our file */
static DECLARE_WAIT_QUEUE_HEAD(waitq);

/* Called when the /proc file is opened */
static int module_open(struct inode *inode, struct file *file)
{
    /* Try to get without blocking  */
    if (!atomic_cmpxchg(&already_open, 0, 1)) {
        /* Success without blocking, allow the access */
        try_module_get(THIS_MODULE);
        return 0;
    }
    /* If the file's flags include O_NONBLOCK, it means the process does not
     * want to wait for the file. In this case, because the file is already open,
     * we should fail with -EAGAIN, meaning "you will have to try again",
     * instead of blocking a process which would rather stay awake.
     */
    if (file->f_flags & O_NONBLOCK)
        return -EAGAIN;

    /* This is the correct place for try_module_get(THIS_MODULE) because if
     * a process is in the loop, which is within the kernel module,
     * the kernel module must not be removed.
     */
    try_module_get(THIS_MODULE);

    while (atomic_cmpxchg(&already_open, 0, 1)) {
        int i, is_sig = 0;

        /* 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
         */
        wait_event_interruptible(waitq, !atomic_read(&already_open));

        /* 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.
         */
        for (i = 0; i < _NSIG_WORDS && !is_sig; i++)
            is_sig = current->pending.signal.sig[i] & ~current->blocked.sig[i];

        if (is_sig) {
            /* It is important to put module_put(THIS_MODULE) here, because
             * for processes where the open is interrupted there will never
             * be a corresponding close. If we do not decrement the usage
             * count here, we will be left with a positive usage count
             * which we will have no way to bring down to zero, giving us
             * an immortal module, which can only be killed by rebooting
             * the machine.
             */
            module_put(THIS_MODULE);
            return -EINTR;
        }
    }

    return 0; /* Allow the access */
}

/* Called when the /proc file is closed */
static int module_close(struct inode *inode, struct file *file)
{
    /* 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.
     */
    atomic_set(&already_open, 0);

    /* Wake up all the processes in waitq, so if anybody is waiting for the
     * file, they can have it.
     */
    wake_up(&waitq);

    module_put(THIS_MODULE);

    return 0; /* success */
}

/* 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.
 */
#ifdef HAVE_PROC_OPS
static const struct proc_ops file_ops_4_our_proc_file = {
    .proc_read = module_output, /* "read" from the file */
    .proc_write = module_input, /* "write" to the file */
    .proc_open = module_open, /* called when the /proc file is opened */
    .proc_release = module_close, /* called when it's closed */
    .proc_lseek = noop_llseek, /* return file->f_pos */
};
#else
static const struct file_operations file_ops_4_our_proc_file = {
    .read = module_output,
    .write = module_input,
    .open = module_open,
    .release = module_close,
    .llseek = noop_llseek,
};
#endif

/* Initialize the module - register the proc file */
static int __init sleep_init(void)
{
    our_proc_file =
        proc_create(PROC_ENTRY_FILENAME, 0644, NULL, &file_ops_4_our_proc_file);
    if (our_proc_file == NULL) {
        pr_debug("Error: Could not initialize /proc/%s\n", PROC_ENTRY_FILENAME);
        return -ENOMEM;
    }
    proc_set_size(our_proc_file, 80);
    proc_set_user(our_proc_file, GLOBAL_ROOT_UID, GLOBAL_ROOT_GID);

    pr_info("/proc/%s created\n", PROC_ENTRY_FILENAME);

    return 0;
}

/* 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.
 */
static void __exit sleep_exit(void)
{
    remove_proc_entry(PROC_ENTRY_FILENAME, NULL);
    pr_debug("/proc/%s removed\n", PROC_ENTRY_FILENAME);
}

module_init(sleep_init);
module_exit(sleep_exit);

MODULE_LICENSE("GPL");