static_key.c

/*
 * static_key.c
 */

#include <linux/atomic.h>
#include <linux/device.h>
#include <linux/fs.h>
#include <linux/kernel.h> /* for sprintf() */
#include <linux/module.h>
#include <linux/printk.h>
#include <linux/types.h>
#include <linux/uaccess.h> /* for get_user and put_user */
#include <linux/jump_label.h> /* for static key macros */
#include <linux/version.h>

#include <asm/errno.h>

static int device_open(struct inode *inode, struct file *file);
static int device_release(struct inode *inode, struct file *file);
static ssize_t device_read(struct file *file, char __user *buf, size_t count,
                           loff_t *ppos);
static ssize_t device_write(struct file *file, const char __user *buf,
                            size_t count, loff_t *ppos);

#define DEVICE_NAME "key_state"
#define BUF_LEN 10

static int major;

enum {
    CDEV_NOT_USED,
    CDEV_EXCLUSIVE_OPEN,
};

static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED);

static char msg[BUF_LEN + 1];

static struct class *cls;

static DEFINE_STATIC_KEY_FALSE(fkey);

static struct file_operations chardev_fops = {
#if LINUX_VERSION_CODE < KERNEL_VERSION(6, 4, 0)
    .owner = THIS_MODULE,
#endif
    .open = device_open,
    .release = device_release,
    .read = device_read,
    .write = device_write,
};

static int __init chardev_init(void)
{
    major = register_chrdev(0, DEVICE_NAME, &chardev_fops);
    if (major < 0) {
        pr_alert("Registering char device failed with %d\n", major);
        return major;
    }

    pr_info("I was assigned major number %d\n", major);

#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 4, 0)
    cls = class_create(DEVICE_NAME);
#else
    cls = class_create(THIS_MODULE, DEVICE_NAME);
#endif

    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME);

    pr_info("Device created on /dev/%s\n", DEVICE_NAME);

    return 0;
}

static void __exit chardev_exit(void)
{
    device_destroy(cls, MKDEV(major, 0));
    class_destroy(cls);

    /* Unregister the device */
    unregister_chrdev(major, DEVICE_NAME);
}

/* Methods */

/**
 * Called when a process tried to open the device file, like
 * cat /dev/key_state
 */
static int device_open(struct inode *inode, struct file *file)
{
    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN))
        return -EBUSY;

    sprintf(msg, static_key_enabled(&fkey) ? "enabled\n" : "disabled\n");

    pr_info("fastpath 1\n");
    if (static_branch_unlikely(&fkey))
        pr_alert("do unlikely thing\n");
    pr_info("fastpath 2\n");

    try_module_get(THIS_MODULE);

    return 0;
}

/**
 * Called when a process closes the device file
 */
static int device_release(struct inode *inode, struct file *file)
{
    /* We are now ready for our next caller. */
    atomic_set(&already_open, CDEV_NOT_USED);

    /**
     * Decrement the usage count, or else once you opened the file, you will
     * never get rid of the module.
     */
    module_put(THIS_MODULE);

    return 0;
}

/**
 * Called when a process, which already opened the dev file, attempts to
 * read from it.
 */
static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */
                           char __user *buffer, /* buffer to fill with data */
                           size_t length, /* length of the buffer */
                           loff_t *offset)
{
    /* Number of the bytes actually written to the buffer */
    int bytes_read = 0;
    const char *msg_ptr = msg;

    if (!*(msg_ptr + *offset)) { /* We are at the end of the message */
        *offset = 0; /* reset the offset */
        return 0; /* signify end of file */
    }

    msg_ptr += *offset;

    /* Actually put the data into the buffer */
    while (length && *msg_ptr) {
        /**
         * The buffer is in the user data segment, not the kernel
         * segment so "*" assignment won't work. We have to use
         * put_user which copies data from the kernel data segment to
         * the user data segment.
         */
        put_user(*(msg_ptr++), buffer++);
        length--;
        bytes_read++;
    }

    *offset += bytes_read;

    /* Most read functions return the number of bytes put into the buffer. */
    return bytes_read;
}

/* Called when a process writes to dev file; echo "enable" > /dev/key_state */
static ssize_t device_write(struct file *filp, const char __user *buffer,
                            size_t length, loff_t *offset)
{
    char command[10];

    if (length > 10) {
        pr_err("command exceeded 10 char\n");
        return -EINVAL;
    }

    if (copy_from_user(command, buffer, length))
        return -EFAULT;

    if (strncmp(command, "enable", strlen("enable")) == 0)
        static_branch_enable(&fkey);
    else if (strncmp(command, "disable", strlen("disable")) == 0)
        static_branch_disable(&fkey);
    else {
        pr_err("Invalid command: %s\n", command);
        return -EINVAL;
    }

    /* Again, return the number of input characters used. */
    return length;
}

module_init(chardev_init);
module_exit(chardev_exit);

MODULE_LICENSE("GPL");