how to send the kernel data to the user the space using procfs?

问题

I am calculating a timestamp in the kernel and later I want to tranfer the tmestamp from kernel to the user space. So I am using procfs for communication between kernel and user. I am using the procfile_read function for sending the data from kernel as shown below.

I modified and calculated the timestamp of the kernel code as shown below.

//this code is at network device driver level.

int netif_rx(struct sk_buff *skb) 
{
    __net_timestamp(skb);//I modify the code in kernel to get the timestamp and store in buffer
    // or I can use : skb->tstamp = ktime_get_real(); //to get the timestamp
}

/** * procfs2.c - create a "file" in /proc * */

#include <linux/module.h>   /* Specifically, a module */
#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/proc_fs.h>  /* Necessary because we use the proc fs */
#include <asm/uaccess.h>    /* for copy_from_user */

#define PROCFS_MAX_SIZE     1024
#define PROCFS_NAME         "buffer1k"

/**
 * This structure hold information about the /proc file
 *
 */
static struct proc_dir_entry *Our_Proc_File;

/**
 * The buffer used to store character for this module
 *
 */
static char procfs_buffer[PROCFS_MAX_SIZE];

/**
 * The size of the buffer
 *
 */
static unsigned long procfs_buffer_size = 0;

/** 
 * This function is called then the /proc file is read
 *
 */
int 
procfile_read(char *buffer,
          char **buffer_location,
          off_t offset, int buffer_length, int *eof, void *data)
{
    int ret;

    printk(KERN_INFO "procfile_read (/proc/%s) called\n", PROCFS_NAME);

    if (offset > 0) {
        /* we have finished to read, return 0 */
        ret  = 0;
    } else {
        /* fill the buffer, return the buffer size */
        memcpy(buffer, procfs_buffer, procfs_buffer_size);
        ret = procfs_buffer_size;
    }

    return ret;
}

/**
 *This function is called when the module is loaded
 *
 */
int init_module()
{
    /* create the /proc file */
    Our_Proc_File = create_proc_entry(PROCFS_NAME, 0644, NULL);

    if (Our_Proc_File == NULL) {
        remove_proc_entry(PROCFS_NAME, &proc_root);
        printk(KERN_ALERT "Error: Could not initialize /proc/%s\n",
            PROCFS_NAME);
        return -ENOMEM;
    }

    Our_Proc_File->read_proc  = procfile_read;
    Our_Proc_File->owner      = THIS_MODULE;
    Our_Proc_File->mode       = S_IFREG | S_IRUGO;
    Our_Proc_File->uid    = 0;
    Our_Proc_File->gid    = 0;
    Our_Proc_File->size       = 37;

    printk(KERN_INFO "/proc/%s created\n", PROCFS_NAME);    
    return 0;   /* everything is ok */
}

/**
 *This function is called when the module is unloaded
 *
 */
void cleanup_module()
{
    remove_proc_entry(PROCFS_NAME, &proc_root);
    printk(KERN_INFO "/proc/%s removed\n", PROCFS_NAME);
}

The module initialization establishes a procfs entry with create_proc_entry(). The functions procfile_write and procfile_read are initialized to handle writes and reads on this entry. The module's cleanup_module() function, called when the module is unloaded, removes the procfs entry by calling cleanup_module().

my question is how to add the timestamp calculated into the procfile_read function and send it to the user space ??

回答1:

Crude and simple: set a global variable in your module with the timestamp and use snprintf to copy it into your profs_buffer. In fact, you might do snprintf directly into the provided buffer and loose the procfs_buffer all together.

There are multiple problems with this approach (it's not atomic, for one) but if you just want a crude debug or logging interface it works.

来源：https://stackoverflow.com/questions/23101224/how-to-send-the-kernel-data-to-the-user-the-space-using-procfs

标签

Linux

linux-kernel

proc

procfs