Writing a new system call

Question

I have been trying to write a new system call(called sys_defclose) in the raspberry's kernel, but upon compiling i get this error:

arch/arm/kernel/built-in.o: In function `__sys_trace_return':

:(.text+0xd50): undefined reference to `sys_defclose'

i have modified the following file:

-include/linux/syscalls.h : where i put the prototype of my syscall

-arch/arm/include/asm/unistd.h : where i put the new raw of the syscall table:

       #define __NR_sys_defclose    (__NR_SYSCALL_BASE+380)

-arch/arm/kernel/calls.S : where i put:

       CALL(sys_defclose)

-i put the source of sys_defclose in arch/arm/kernel and i have modified the makefile in the same directory with the new line

       obj-y    +=sys_defclose.o

the kernel version is 3.6 of raspberrypi.
can somebody explain me how to solve this error?
thanks
this is the implementation of my syscall

static struct task_struct* get_task_by_pid(pid_t pid)

{

return pid_task(find_pid_ns(pid, task_active_pid_ns(current)), PIDTYPE_PID);

}



static void close_files(struct files_struct * files)

 {

     int i, j;

     struct fdtable *fdt;



    j = 0;



    rcu_read_lock();

    fdt = files_fdtable(files);

    rcu_read_unlock();

    for (;;) {

            unsigned long set;

            i = j * BITS_PER_LONG;

            if (i >= fdt->max_fds)

                    break;

           set = fdt->open_fds[j++];

           while (set) {

                    if (set & 1) {

                             struct file * file = xchg(&fdt->fd[i], NULL);

                             if (file) {

                                 filp_close(file, files);

                                    cond_resched();

                           }

                    }

                  i++;

                  set >>= 1;

            }

    }

}

asmlinkage long sys_defclose(pid_t pid)

{

struct task_struct *result = NULL;



rcu_read_lock(); 

result = get_task_by_pid(pid);

rcu_read_unlock(); 

close_files(result->files);

}

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Answer 1

You should use SYSCALL_DEFINE* to define syscall (I think, this step you did wrong), then add your syscall into sys_call_table, which is architecture-dependent (arch/arm/kernel/calls.S for arm).

Change your sys_defclose to look like this:

SYSCALL_DEFINE1(defclose, pid_t, pid)
{
        struct task_struct *result = NULL;

        rcu_read_lock(); 
        result = get_task_by_pid(pid);
        rcu_read_unlock(); 
        close_files(result->files);
}

Writing a new system call

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