Adding a system call to kernel

[jensen galan <jrgalan@yahoo.com>]

Greetings,

I am trying to add a system call to a 2.4 kernel. 
It's a simple system call which merely prints the
value of xtime.  The kernel recompiles OK, and my
user-space program (p5_a.c) actually works using the
added system call when I use syscall() and do not
generate a stub. (The 2 versions of my user-space
programs are included below).  However, when I try to
generate a stub in my user-space program using
_syscall2(), I receive the following compilation
error:

# gcc -Wall -D__KERNEL__ -I
/lib/modules/2.4.28-gentoo-r5/build/include -o p5_b
p5_b.c
/tmp/cc5nBrjZ.o(.text+0x23): In function
'pedagogictime':
: undefined reference to 'errno'
collect2: ld returned 1 exit status

Here is what I did:

I first modified
/usr/src/linux-2.4.28-gentoo-r5/arch/i386/kernel/time.c
to add my system call, which is the following:

asmlinkage int sys_pedagogictime(int flag, struct
timeval *thetime)
{
  int write_failed;
  struct timeval kernel_tv;

  cli(); // blocks interrupts
  kernel_tv.tv_sec = xtime.tv_sec;  //read xtime
  kernel_tv.tv_usec = xtime.tv_usec;
  sti(); // unblock interrupts

  write_failed = copy_to_user(thetime, &kernel_tv,
sizeof(struct timeval));
  if (write_failed) {
    printk("WRITE FAILED: could not write the time
into user space\n");
    return -1;
  }

  /* Print the time if flag is true */
  if (flag) {
    printk("pedagogictime results: %ld.%.6ld\n",
kernel_tv.tv_sec, 
                                                
kernel_tv.tv_usec);
  }

  return 0;

}

I then modified
/usr/src/linux-2.4.28-gentoo-r5/arch/i386/kernel/entry.S
to add my system call table entry:

.long SYMBOL_NAME(sys_pedagogictime)    /* 259 */

I also added the following line in
/usr/src/linux-2.4.28-gentoo-r5/include/asm/unistd.h
to generate a system call stub:

#define __NR_pedagogictime      259

Finally, I cd /usr/src/linux-2.4.28-gentoo-r5 and edit
the Makefile so that "EXTRAVERSION = -gentoo-r5-new",
and recompiled my custom kernel with the following
commands:

make mrproper
make menuconfig
make dep
make bzImage
make modules
make modules-install
make install

Here are the 2 versions of my user-space programs:

/* p5_a.c */
#include <stdio.h>
#include <sys/time.h>

int main(int argc, char **argv)
{
  struct timeval tv;

  /* use syscall to call pedagogictime - entry 259 */
  syscall(259, 1, &tv);

  /* print the results to stdout */
  printf("Pedagocictime: %ld.%.6ld\n", tv.tv_sec,
tv.tv_usec);

  return 0; 
}

/* p5_b.c */
#include <stdio.h>
#include <linux/unistd.h>
#include <sys/time.h>

_syscall2(int, pedagogictime, int, flag, struct
*timeval, thetime);

int main(int argc, char **argv)
{
  struct timeval tv;

  pedagogictime(1, &tv);
  printf("Pedagogictime: %ld.%.6ld\n", tv.tc_sec,
tv.tv_usec);

  return 0;
}

I apologize for being so verbose, but I think it's all
relevant.

Also, now I have 2 kernels, and when I boot from the
original, I get the following error at boot:

Bringing eth0 up via DHCP... [!!]
ERROR: Problem starting needed services.
"netmount" was not started.

The original kernel I compiled with genkernel.  The
new kernel used the method described above.  Here is
my grub.conf:

default 0
timeout 30
splashimage=(hd0,0)/grub/splash.xpm.gz

title=Gentoo Linux 2.4.28-r5
root (hd0,0)
kernel /kernel-2.4.28-gentoo-r5 root=/dev/ram0
init=/linuxrc 
ramdisk=8192 real_root=/dev/hda3
initrd /initrd-2.4.28-gentoo-r5

title=Gentoo Linux 2.4.28-r5-new
root (hd0,0)
kernel /vmlinuz-2.4.28-gentoo-r5-new root=/dev/hda3

Thank you for any help you can provide.  I am new at
kernel programming, but am having a great time!

Jensen



		
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