linux-kernel - Re: system call implementation for x86

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Date:	Sun, 20 May 2007 22:46:04 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	"kernel coder" <lhrkernelcoder@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: system call implementation for x86_64

On Sat, 19 May 2007 04:55:12 -0700 kernel coder wrote:

> hi,
> 
> I'm trying to implement a system call for x86_64. Mine processor is
> dual core opetron.There is very little material on web for
> implementing system calls for x86_64 processor for 2.6 series kernel.I
> tried to implement a new system call by observing the existing
> implementation but to no success.Following are files names and changes
> made.

Your example is very CPU-independent, i.e., not x86_64-specific,
so following examples of recently-added syscalls should be good enough.

I used your "patch" below (with a few small modifications) on
2.6.22-rc2 and it worked fine.

Linux unicorn 2.6.22-rc2 #2 SMP Sun May 20 22:22:36 PDT 2007 x86_64 x86_64 x86_64 GNU/Linux
...
[   98.369454] new system call


> //////////////////////////////////////////////////
> file-> include/asm-x86_64/unistd.h
> 
> #define __NR_newcall        273
> __SYSCALL(__NR_newcall, sys_newcall)
> 
> #define __NR_syscall_max __NR_newcall

syscall_max is no longer used.

> 
> //////////////////////////////////////////////////
> file-> include/linux/syscalls.h
> 
> asmlinkage unsigned long sys_newcall(char __user *buf);

not unsigned.

> 
> /////////////////////////////////////////////
> file--> fs/read_write.c
> 
> asmlinkage unsigned long sys_newcall(char __user * buf){

not unsigned.

> 
>      printk("new system call \n");
>      ret 0;

	return 0;

> }
> 
> EXPORT_SYMBOL_GPL(sys_write)

  EXPORT_SYMBOL_GPL(sys_newcall);

> Please let me know where i'm doing wrong .Following is program which
> is calling mine system call
> 
> 
> #include <stdlib.h>
> #include <stdio.h>
> #include <sys/unistd.h>

#include <unistd.h>

> #include <sys/syscall.h>
> 
>   long int ret;
>    int num = 243;
>   char  buffer=[20];

eh?  does not compile.

> 
> int main() {
> 
> 
>   asm ("syscall;"
>        : "=a" (ret)
>        : "0" (num),
>          "D" (buffer),
>       );

I just used the syscall() glibc interface instead of asm:

	ret = syscall(__NR_newcall);

>  return ret;
> }
> 
> When i call this ,nothing gets printed in file /var/log/messages.Am i
> missing something ?

Mostly typos...

> Actually i wana pass a pointer to kernel from user space.Later on data
> will be copied to that memory location .i am thinking of using
> copy_to_user for copying data.Buffer passed through system call will
> be used by kernel function as circular ring.And portions of this ring
> will get updated frequently even after system call has returned.
> 
> Is there any better way to do this?

Sounds mostly OK to me.
Where are the ring head, tail, size, etc. maintained?


---
~Randy
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