如何添加poll函数到内核模块代码中？

问题描述：

据我所知，要从内核空间通知用户空间，一种方法是使用 poll。这意味着内核驱动程序应该首先提供 poll 方法。下面的代码是从互联网上找到的，它确实有效！

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/string.h>
#include <linux/vmalloc.h>
#include <asm/uaccess.h>
 
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Fortune Cookie Kernel Module");
MODULE_AUTHOR("M. Tim Jones");
 
#define MAX_COOKIE_LENGTH       PAGE_SIZE
 
static struct proc_dir_entry *proc_entry;
static char *cookie_buf;  // Space for fortune strings
static int write_index;   // Index to write next fortune
static int read_index;    // Index to read next fortune
 
ssize_t fortune_write( struct file *filp, const char __user *buff,
                        unsigned long len, void *data )
// Refer to: ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
{
  int space_available = (MAX_COOKIE_LENGTH-write_index);
 
  if (len > space_available) {
    printk(KERN_INFO "fortune: cookie buffer is full!
");
    return -ENOSPC;
  }
 
  if (copy_from_user( &cookie_buf[write_index], buff, len )) {
    return -EFAULT;
  }
 
  write_index += len;
  cookie_buf[write_index-1] = 0;
 
  return len;
}
 
ssize_t fortune_read(struct file *file, char *buf, size_t count, loff_t *f_pos){
// Refer to: ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
    int len;
 
    //there's no fortune or a fortune has already been read
    //the *f_pos > 0 hack is needed because `cat /proc/fortune` would otherwise
    //display every thing in the cookie_buf
    if(write_index == 0 || *f_pos > 0){
        return 0;
    }
 
    // cicle through fortunes
    if(read_index >= write_index){
        read_index = 0;
    }
 
    len = sprintf(buf, "%s
", &cookie_buf[read_index]);
 
    read_index += len;
    *f_pos += len;
 
    return len;
}
 
static const struct file_operations proc_test_fops = {
   .owner        = THIS_MODULE,
//    .open        = led_proc_open,
   .read        = fortune_read,
//    .llseek        = seq_lseek,
//    .release    = single_release,
   .write        = fortune_write,
//    unsigned int (*poll) (struct file *, struct poll_table_struct *);
//    int (*fasync) (int, struct file *, int);
};
 
int __init init_fortune_module( void )
{
    int ret = 0;
    cookie_buf = (char *)vmalloc( MAX_COOKIE_LENGTH );
    if (!cookie_buf) {
        ret = -ENOMEM;
    } else {
        memset( cookie_buf, 0, MAX_COOKIE_LENGTH );
//        proc_entry = create_proc_entry( "fortune", 0644, NULL );
        proc_entry = proc_create( "fortune", 0644, NULL, &proc_test_fops );
 
        if (proc_entry == NULL) {
            ret = -ENOMEM;
            vfree(cookie_buf);
            printk(KERN_INFO "fortune: Couldn't create proc entry
");
        } else {
            write_index = 0;
            read_index = 0;
            printk(KERN_INFO "fortune: Module loaded.
");
        }
    }
 
    return ret;
}
 
void __exit exit_fortune_module( void )
{
//    remove_proc_entry("fortune", &proc_entry);
    proc_remove(proc_entry);
    vfree(cookie_buf);
    printk(KERN_INFO "fortune: Module unloaded.
");
}
 
module_init( init_fortune_module );
module_exit( exit_fortune_module );

我可以这样做来让它工作：

echo "hello" > /proc/fortune

进而

cat /proc/fortune

查看结果。

但是如何添加 poll 方法呢？我试了几次，还是失败。

解决方案 1：

你可以在内核中找到一些很好的例子。看看下一个文件：

• 驱动程序/rtc/dev.c，驱动程序/rtc/interface.c

• 内核/printk/printk.c

• 驱动程序/char/random.c

要向您的代码添加poll()功能，请按照以下步骤操作。

1. 包括所需的标题：

 #include <linux/wait.h>
 #include <linux/poll.h>

1. 声明 waitqueue 变量：

 static DECLARE_WAIT_QUEUE_HEAD(fortune_wait);

1. 添加fortune_poll()函数并将其（作为.poll回调）添加到文件操作结构中：

 static unsigned int fortune_poll(struct file *file, poll_table *wait)
 {
     poll_wait(file, &fortune_wait, wait);
     if (new-data-is-ready)
         return POLLIN | POLLRDNORM;
     return 0;
 }

 static const struct file_operations proc_test_fops = {
     ....
     .poll = fortune_poll,
 };

POLLIN | POLLRDNORM请注意，如果有新数据需要读取，则应返回，0如果没有新数据需要读取（poll()调用超时），则应返回。有关详细信息，请参阅man 2 poll 。

1. 一旦有新数据，就通知你的等待队列：

 wake_up_interruptible(&fortune_wait);

这就是实现操作的基本内容poll()。根据您的任务，您可能需要在函数中使用一些waitqueue API.read（例如wait_event_interruptible()）。

另请参阅相关问题：在 Linux 内核模块中实现轮询。

解决方案 2：

最小可运行示例

带有 QEMU + Buildroot 样板的 GitHub 上游：

• poll.ko内核模块

• poll.out用户空间测试

在这个简化的例子中，我们从单独的线程生成轮询事件。在现实生活中，当硬件完成某项工作并且新数据可供用户空间读取时，轮询事件很可能由中断触发。

要记住的要点是，如果poll返回零，内核会再次调用它：为什么我们需要在 poll 中调用 poll_wait？

民意调查

#include <linux/debugfs.h>
#include <linux/delay.h> /* usleep_range */
#include <linux/errno.h> /* EFAULT */
#include <linux/fs.h>
#include <linux/jiffies.h>
#include <linux/kernel.h> /* min */
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/poll.h>
#include <linux/printk.h> /* printk */
#include <linux/uaccess.h> /* copy_from_user, copy_to_user */
#include <linux/wait.h> /* wait_queue_head_t, wait_event_interruptible, wake_up_interruptible  */
#include <uapi/linux/stat.h> /* S_IRUSR */

static int ret0 = 0;
module_param(ret0, int, S_IRUSR | S_IWUSR);
MODULE_PARM_DESC(i, "if 1, always return 0 from poll");

static char readbuf[1024];
static size_t readbuflen;
static struct dentry *debugfs_file;
static struct task_struct *kthread;
static wait_queue_head_t waitqueue;

static ssize_t read(struct file *filp, char __user *buf, size_t len, loff_t *off)
{
    ssize_t ret;
    if (copy_to_user(buf, readbuf, readbuflen)) {
        ret = -EFAULT;
    } else {
        ret = readbuflen;
    }
    /* This is normal pipe behaviour: data gets drained once a reader reads from it. */
    /* https://stackoverflow.com/questions/1634580/named-pipes-fifos-on-unix-with-multiple-readers */
    readbuflen = 0;
    return ret;
}

/* If you return 0 here, then the kernel will sleep until an event
 * happens in the queue. and then call this again, because of the call to poll_wait. */
unsigned int poll(struct file *filp, struct poll_table_struct *wait)
{
    pr_info("poll
");
    /* This doesn't sleep. It just makes the kernel call poll again if we return 0. */
    poll_wait(filp, &waitqueue, wait);
    if (readbuflen && !ret0) {
        pr_info("return POLLIN
");
        return POLLIN;
    } else {
        pr_info("return 0
");
        return 0;
    }
}

static int kthread_func(void *data)
{
    while (!kthread_should_stop()) {
        readbuflen = snprintf(
            readbuf,
            sizeof(readbuf),
            "%llu",
            (unsigned long long)jiffies
        );
        usleep_range(1000000, 1000001);
        pr_info("wake_up
");
        wake_up(&waitqueue);
    }
    return 0;
}

static const struct file_operations fops = {
    .owner = THIS_MODULE,
    .read = read,
    .poll = poll
};

static int myinit(void)
{
    debugfs_file = debugfs_create_file(
        "lkmc_poll", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
    init_waitqueue_head(&waitqueue);
    kthread = kthread_create(kthread_func, NULL, "mykthread");
    wake_up_process(kthread);
    return 0;
}

static void myexit(void)
{
    kthread_stop(kthread);
    debugfs_remove(debugfs_file);
}

module_init(myinit)
module_exit(myexit)
MODULE_LICENSE("GPL");

poll.out 用户空间：

#define _XOPEN_SOURCE 700
#include <assert.h>
#include <fcntl.h> /* creat, O_CREAT */
#include <poll.h> /* poll */
#include <stdio.h> /* printf, puts, snprintf */
#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS */
#include <unistd.h> /* read */

int main(int argc, char **argv) {
    char buf[1024];
    int fd, i, n;
    short revents;
    struct pollfd pfd;

    if (argc < 2) {
        fprintf(stderr, "usage: %s <poll-device>
", argv[0]);
        exit(EXIT_FAILURE);
    }
    fd = open(argv[1], O_RDONLY | O_NONBLOCK);
    if (fd == -1) {
        perror("open");
        exit(EXIT_FAILURE);
    }
    pfd.fd = fd;
    pfd.events = POLLIN;
    while (1) {
        puts("poll");
        i = poll(&pfd, 1, -1);
        if (i == -1) {
            perror("poll");
            assert(0);
        }
        revents = pfd.revents;
        printf("revents = %d
", revents);
        if (revents & POLLIN) {
            n = read(pfd.fd, buf, sizeof(buf));
            printf("POLLIN n=%d buf=%.*s
", n, n, buf);
        }
    }
}

用法：

insmod poll.ko
mount -t debugfs none /sys/kernel/debug
./kernel_modules/poll.out /sys/kernel/debug/lkmc_poll

结果：jiffies每秒从用户空间打印到标准输出，例如：

poll
<6>[    4.275305] poll
<6>[    4.275580] return POLLIN
revents = 1
POLLIN n=10 buf=4294893337
poll
<6>[    4.276627] poll
<6>[    4.276911] return 0
<6>[    5.271193] wake_up
<6>[    5.272326] poll
<6>[    5.273207] return POLLIN
revents = 1
POLLIN n=10 buf=4294893588
poll
<6>[    5.276367] poll
<6>[    5.276618] return 0
<6>[    6.275178] wake_up
<6>[    6.276370] poll
<6>[    6.277269] return POLLIN
revents = 1
POLLIN n=10 buf=4294893839

强制轮询file_operation返回 0，以更清楚地查看发生了什么：

insmod poll.ko ret0=1

示例输出：

poll
<6>[   85.674801] poll
<6>[   85.675788] return 0
<6>[   86.675182] wake_up
<6>[   86.676431] poll
<6>[   86.677373] return 0
<6>[   87.679198] wake_up
<6>[   87.680515] poll
<6>[   87.681564] return 0
<6>[   88.683198] wake_up

由此我们可以看出，控制权并没有返回到用户空间：内核只是不断地调用轮询file_operation。

在 Linux 5.4.3 上测试。