Intercept System call using Kernel module is 2.6 kernel

[Meswani, Mitesh]

Hello 
 
 
I am attempting to run some user code with kernel space permission. I am using the ppc64 kernel version 2.6.16-rc4-3-ppc64 for IBM Power5 processors. 
In this kernel module I am trying to implement a function that can be called from user space. 
 
I have found through various posts that using unused system calls and replacing them temporarily can acheive this objective. 
 
This is what I am doing, but its not working, please bear with the slightly long code that follows: 
 
1) since the 2.6 kernel does not export sys_call_table, I grep it from the boot image
 
2) Next I write the kernel module as : 
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/syscalls.h>
unsigned long **sctable;
void *org_func;  /***** Copy of the original calls address ********/

asmlinkage int mitesh_func(void)   
{ 
        printk(KERN_ALERT "Executing mitesh_func...\n"); 
        return 2;
} 

int init_module(void)
{
 unsigned long ptr;
 unsigned long *p;
 ptr = 0x23203404;  /*** some hard coded addresses from grepping for sys_call_table *****/
  p = (unsigned long *)ptr;
  sctable = (unsigned long **)p;
  printk("The address of the system call table is: 0x%x\n",&sctable[0]);
  printk("The address of syscall #137 is: 0x%x\n",sctable[137]);

org_func = (void *) (sctable[137]);  /**** Store the original sys call ****/
 printk("Original func address 0x%x stored \n",org_func);
 sctable[137] = (void *) mitesh_func;  /**** replace with mitesh_func ****/
printk("The new sys call address is 0x%x and stored as : 0x%x\n",mitesh_func, sctable[137]);

  return 0; 
}
void cleanup_module(void)

{
        sctable[137] = (void *) org_func; 
        printk("Upon module unload the sctable #137 address is :0x%x\n",sctable[137]);
        printk("Module is unloaded!\n");
}

3) My user app looks like this:
#include <stdio.h> 
#include <errno.h> 
#include <asm-ppc64/unistd.h> 
#define __NR_mitesh_func 137 
 
_syscall0(int, mitesh_func); 
void main() 
{
        int x=0; 
        x=mitesh_func(); 
        printf("mitesh_func returned %d\n",x);
}  

 
4) I verify from the system logs that when I insmod the kernel module I get all the print statements. I verified from the logs  that the address of the sys_call_table is correctly passed and from /proc/kallsysms I can see that my function mitesh_func has been defined and has the address as indicated in the logs. 
 
The problem is that when I execute my user app I expect to see two things: 
 a) I should see a message in the log "Executing mitesh_func..." and 
 b) A return value of 2 
 
However I get an error value -1 returned. 
 
Any help and ideas are highly appreciated.  
 
Thank you in advance, 
Mitesh 
 
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