Module to find a task from its pid

Here is a module which find the task_struct of a process from its pid. To find the task_struct of a process we can make use of the function pid_task defined in kernel/pid.c . 

struct task_struct *pid_task(struct pid *pid, enum pid_type type)

Arguments : 
pid : Pointer to the struct pid of the process.
pid_type:  PIDTYPE_PID,
       PIDTYPE_PGID,
          PIDTYPE_SID,
          PIDTYPE_MAX

To find the pid structure if we have the pid of a process we can use the functionfind_get_pid which is also defined in kernel/pid.c 

In the below module we create a read/write proc entry named task_by_pid. Which ever process we want to find the task_struct of using its pid we can write to number into the proc entry. 

When we read the proc entry, it will display the name of the process corresponding to the pid we wrote into it. 

proc_task_pid: 

#include <linux/module.h>
#include <linux/kernel.h>
#include <asm/uaccess.h>
#include <linux/fs.h> 
#include <linux/cdev.h>
#include <linux/proc_fs.h>
#include <linux/pid.h>

#include <linux/pid_namespace.h>
int p_id;
struct pid *pid_struct;
struct task_struct *task;
static struct proc_dir_entry *proc_write_entry;
char *proc_name="task_by_pid";

int read_proc(char *buf,char **start,off_t offset,int count,int *eof,void *data )
{
int len=0;
pid_struct = find_get_pid(p_id);
task = pid_task(pid_struct,PIDTYPE_PID);

 len = sprintf(buf,"\nname %s\n ",task->comm);

return len;
}

int write_proc(struct file *file,const char *buf,int count,void *data )
{
int ret;
char *id;
id = (char *)kmalloc(1000*sizeof(char),GFP_KERNEL);
printk(KERN_INFO "buf passed %s",buf);
if(copy_from_user(id,buf,count))
    return -EFAULT;
printk(KERN_INFO "id passed %s",id);
p_id = simple_strtoul(id,NULL,0);
printk(KERN_INFO "pid %d ret %d",p_id,ret);
return sizeof(buf);
}

void create_new_proc_entry()
{
proc_write_entry = create_proc_entry(proc_name,0666,NULL);
if(!proc_write_entry)
      {
    printk(KERN_INFO "Error creating proc entry");
    return -ENOMEM;
    }
proc_write_entry->read_proc = read_proc ;
proc_write_entry->write_proc = write_proc;
printk(KERN_INFO "proc initialized");

}



int proc_init (void) {
    create_new_proc_entry();
    return 0;
}

void proc_cleanup(void) {
    printk(KERN_INFO " Inside cleanup_module\n");
    remove_proc_entry(proc_name,NULL);
}
MODULE_LICENSE("GPL");   
module_init(proc_init);
module_exit(proc_cleanup);


compilation:
ifneq ($(KERNELRELEASE),)
   obj-m := proc_task_pid.o
else

KERNELDIR ?= /lib/modules/$(shell uname -r)/build

PWD := $(shell pwd)

default:
 $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
endif
clean:
 $(MAKE) -C $(KERNELDIR) M=$(PWD) clean

Compile it using

$make

insert modules
Now let us try to find the name of the process with pid "1", which is always init. 

$ printf "1" > /proc/task_by_pid 
$ cat /proc/task_by_pid

name init