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);
}

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);
}