How to write system calls on debian/ubuntu

Question

I am trying to write a system call of my own. It would just return the current time. I know the concept of what should I do and I did go through a couple of links like these:

• Implementing a System Call on Linux 2.6 for i386

• Another of the System call implementation

But I am still confused and have not got the desired result. The kernel is not compiling and its crashing due to problems. I have tried it on debian latest stable release of 3.X.X

Could someone point me out to a clean hello world kind of program to develop system calls?

EDIT

To the below answer, here are my problems:

1. File 3: linux-x.x.x/arch/x86/kernel/syscall_table_32.S is not found in my linux folder. I had to improvise and so modified the following file: linux-x.x.x/arch/x86/syscalls/syscall_64.tbl

2. The above (1) new file mentioned had different pattern of <number> <64/x32/common> <name> <entry point> and my entry was `313 common

3. The kernel image did compile successfully, but I couldnt call the function. It gives an undefined reference" error when i compile it with gcc. Why?

Answer 1

This is just example how to write a simple kernel system call. Consider the following C function system_strcpy() that simply copies one string into another: similar to what strcpy() does.

#include<stdio.h>

long system_strcpy(char* dest, const char* src)
{
   int i=0;
   while(src[i]!=0)
      dest[i]=src[i++];

   dest[i]=0;
   return i;
}

Before writing, get a kernel source tar and untar it to get a linux-x.x.x directory.

File 1: linux-x.x.x/test/system_strcpy.c Create a directory within the linux-x.x.x, named test and save this code as file system_strcpy.c in it.

#include<linux/linkage.h>
#include<linux/kernel.h>
asmlinkage long system_strcpy(char*dest, const char* src)
{
   int i=0;
   while(src[i]!=0)
      dest[i]=src[i++];

   dest[i]=0;
   return i;
}

File 2: linux-x.x.x/test/Makefile Create a Makefile within the same test directory you created above and put this line in it:

obj-y := system_strcpy.o

File 3: linux-x.x.x/arch/x86/kernel/syscall_table_32.S Now, you have to add your system call to the system call table. Append to the file the following line:

.long system_strcpy

NOTE: For Kernel 3.3 and higher versions.

*Refer:linux-3.3.xx/arch/x86/syscalls/syscall_64.tbl*

And in there, now add at the end of the following series of lines:

310 64 process_vm_readv sys_process_vm_readv

311 64 process_vm_writev sys_process_vm_writev

312 64 kcmp sys_kcmp

313 64 system_strcpy system_strcpy

The format for the 3.3 version is in: number abi name entry point

File 4: linux-x.x.x/arch/x86/include/asm/unistd_32.h

NOTE: This section is redundant for 3.3 and higher kernel versions

In this file, the names of all the system calls will be associated with a unique number. After the last system call-number pair, add a line

#define __NR_system_strcpy 338

(if 337 was the number associated with the last system call in the system call-number pair).

Then replace NR_syscalls value, stating total number of system calls with (the existing number incremented by 1) i.e. in this case the NR_syscalls should've been 338 and the new value is 339.

#define NR_syscalls 339

File 5: linux-x.x.x/include/linux/syscalls.h

Append to the file the prototype of our function.

asmlinkage long system_strcpy(char *dest,char *src);

just before the #endif line in the file.

File 6: Makefile at the root of source directory.

Open Makefile and find the line where core-y is defined and add the directory test to the end of that line.

core-y += kernel/ mm/ fs/ test/

Now compile the kernel. Issue: make bzImage -j4

Install the kernel by executing the following command as root(or with root permissions): make install

Reboot the system.

To use the recently created system call use:

syscall(338,dest,src); (or syscall(313,dest,src); for kernel 3.3+) instead of the regular strcpy library function.

#include "unistd.h"
#include "sys/syscall.h"
int main()
{
 char *dest=NULL,*src="Hello";
 dest=(char*)malloc(strlen(src)+1);
 syscall(338,dest,src);//syscall(313,dest,src); for kernel 3.3+
 printf("%s \n %s\n",src,dest);
 return 0;
}

Instead of numbers like 313,etc in syscall, you can also directly use __NR_system_strcpy

This is a generic example. You will need to do a little experimentation to see what works for your specific kernel version.