Own syscall implementation

[Oleg Kutkov]

I understood my errors. Everything is good now.
Anyway, thank you!

2010/10/11 Oleg Kutkov <elenbert at gmail.com>:
> Thank you for answer.
> Sorry, now i made some changes due to actual kernel version
>
> 2010/10/11 Russell King - ARM Linux <linux at arm.linux.org.uk>:
>> That's fine, except that there's no bounds checking for the value of
>> 'call', which if it were negative or a large positive number would
>> overflow the nargs array.  You're also not propagating the error back.
>>
>
> asmlinkage long sys_mycall(int call, unsigned long __user *args);
>
> SYSCALL_DEFINE2(mycall, int, call, unsigned long __user *, args)
> {
>  unsigned long a[6];
>  int err;
>
>  if (call < 1 || call > SYS_ACCEPT4) {
>       return -EINVAL;
>  }
>
>  if (err = copy_from_user(a, args, nargs[call])) {
>      return -EFAULT;
>  }
>
>  switch (call) {
>     case 0: /*some work*/ break;
>     case 1: /*some work*/ break;
>     default: break;
>  }
>
>  return err;
> }
>
>> You don't say what kernel version.
>
> My kernel version is 2.6.29
>
>> As 312 is already taken (we're now up to 369), and you're using .long, I
>> can only assume this is quite an old kernel...  Modern kernels use CALL()
>> in calls.S to ensure that the number of syscalls are properly counted.
>
> call.S:
>
> CALL(sys_mycall)
>
>> So it's really not a good idea to create new syscalls for production code.
>> It's usually safer to write something like a char device driver and use
>> the read/write/ioctl methods instead.
>
> I understood. I just want to understand "how it works"
>
>> Have you checked the value of 'call' and nargs[call] ?  Note that
>> copy_from_user() takes the number of _bytes_ to copy, not the number
>> of arguments.
>>
>
> Yes, i checked "call", as i show in code before. nargs i found in
> net/socket.c and it defined as:
> #define AL(x) ((x) * sizeof(unsigned long))
> static const unsigned char nargs[19]={
>        AL(0),AL(3),AL(3),AL(3),AL(2),AL(3),
>        AL(3),AL(3),AL(4),AL(4),AL(4),AL(6),
>        AL(6),AL(2),AL(5),AL(5),AL(3),AL(3),
>        AL(4)
> };
>
> I looking on SYSCALL_DEFINE2(socketcall, int, call, unsigned long
> __user *, args) and here is: if (copy_from_user(a, args,
> nargs[call])).
> I used that just as an example that i found..
>