/*
 * 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/kernel.h>  /* We're doing kernel work */
#include <linux/module.h>  /* Specifically, a module */
#include <linux/proc_fs.h> /* Necessary because we use proc fs */
#include <linux/sched.h>   /* For putting processes to sleep and
                                   waking them up */
#include <linux/uaccess.h> /* for get_user and put_user */
#include <linux/version.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 *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 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;
    }

    sprintf(message, "Last input:%s\n", Message);
    for (i = 0; i < len && message[i]; i++)
        put_user(message[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 *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 */
int Already_Open = 0;

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

/* 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 does not
     * want to wait for the file. In this case, if 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) && Already_Open)
        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);

    /* If the file is already open, wait until it is not. */
    while (Already_Open) {
        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, !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;
        }
    }

    /* 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 */
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.
     */
    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 */
};
#else
static const struct file_operations File_Ops_4_Our_Proc_File = {
    .read = module_output,
    .write = module_input,
    .open = module_open,
    .release = module_close,
};
#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) {
        remove_proc_entry(PROC_ENTRY_FILENAME, 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");