From: Jeshwanth Kumar N K Jeshu <jeshkumar555@gmail.com>
To: SaNtosh kuLkarni <santosh.yesoptus@gmail.com>; moon.linux@yahoo.com
Cc: Bernd Petrovitsch <bernd@petrovitsch.priv.at>; kernelnewbies@kernelnewbies.org
Sent: Saturday, November 10, 2012 1:47 PM
Subject: Re: Callback function from kernel module

Hello Santosh and Anand,

Thanks for the reply and very good suggestions.

As Anand Moon suggestedhttp://people.ee.ethz.ch/%7Earkeller/linux/multi/kernel_user_space_howto-3.html

link in http://lists.kernelnewbies.org/pipermail/kernelnewbies/2012-November/006489.html previous thread, 


I read all the kernel to user space communication mechanism ( Except Upcall , ll read it). As of now for my application,


the signals passing from kernel to user space is enough. I was trying the sample code provided in  signals section of above link.

1. In the code he used find_task_by_pid_type, but find_task_by_pid_type not available in sched.h. So I have changed it to 


find_task_by_vpid(pid), is this right way ?

2. I tried to install the module but it is not finding the symbol, but the symbol is present in /proc/kallsyms.

Anything I am missing while compiling ? as I am getting warning.

Compile:

jeshwanth@jeshwanth:~/linux/kernel_user_space/code/signals$ make
make -C /lib/modules/3.0.0-26-generic-pae/build M=/home/jeshwanth/linux/kernel_user_space/code/signals modules
make[1]: Entering directory `/usr/src/linux-headers-3.0.0-26-generic-pae'
  Building modules, stage 2.
  MODPOST 1 modules
WARNING: "find_task_by_vpid" [/home/jeshwanth/linux/kernel_user_space/code/signals/signal_kernel.ko] undefined!
make[1]: Leaving directory `/usr/src/linux-headers-3.0.0-26-generic-pae'
[1]+  Done                    gedit signal_kernel.c

install Command:
root@jeshwanth:/home/jeshwanth/linux/kernel_user_space/code/signals# insmod signal_kernel.ko
insmod: error inserting 'signal_kernel.ko': -1 Unknown symbol in module

/proc/kallsyms:

00000000 T find_task_by_pid_ns
00000000 T find_task_by_vpid

dmesg:
[15012.556740] signal_kernel: Unknown symbol find_task_by_vpid (err 0)

Code:
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <asm/siginfo.h>    //siginfo
#include <linux/rcupdate.h>    //rcu_read_lock
#include <linux/sched.h>    //find_task_by_pid_type
#include <linux/debugfs.h>
#include <linux/uaccess.h>


#define SIG_TEST 44    // we choose 44 as our signal number (real-time signals are in the range of 33 to 64)

struct dentry *file;

static ssize_t write_pid(struct file *file, const char __user *buf,
                                size_t count, loff_t *ppos)
{
    char mybuf[10];
    int pid = 0;
    int ret;
    struct siginfo info;
    struct task_struct *t;
    /* read the value from user space */
    if(count > 10)
        return -EINVAL;
    copy_from_user(mybuf, buf, count);
    sscanf(mybuf, "%d", &pid);
    printk("pid = %d\n", pid);

    /* send the signal */
    memset(&info, 0, sizeof(struct siginfo));
    info.si_signo = SIG_TEST;
    info.si_code = SI_QUEUE;    // this is bit of a trickery: SI_QUEUE is normally used by sigqueue from user space,
                    // and kernel space should use SI_KERNEL. But if SI_KERNEL is used the real_time data
                    // is not delivered to the user space signal handler function.
    info.si_int = 1234;          //real time signals may have 32 bits of data.

    rcu_read_lock();
    //t = find_task_by_pid_type(PIDTYPE_PID, pid);  //find the task_struct associated with this pid
    t = find_task_by_vpid(pid);
    if(t == NULL){
        printk("no such pid\n");
        rcu_read_unlock();
        return -ENODEV;
    }
    rcu_read_unlock();
    ret = send_sig_info(SIG_TEST, &info, t);    //send the signal
    if (ret < 0) {
        printk("error sending signal\n");
        return ret;
    }
    return count;
}

static const struct file_operations my_fops = {
    .write = write_pid,
};

static int __init signalexample_module_init(void)
{
    /* we need to know the pid of the user space process
      * -> we use debugfs for this. As soon as a pid is written to
      * this file, a signal is sent to that pid
      */
    /* only root can write to this file (no read) */
    file = debugfs_create_file("signalconfpid", 0200, NULL, NULL, &my_fops);
    return 0;
}
static void __exit signalexample_module_exit(void)
{
    debugfs_remove(file);

}

module_init(signalexample_module_init);
module_exit(signalexample_module_exit);
MODULE_LICENSE("GPL");





--
Regards
Jeshwanth Kumar N K
+91-7411483498