Hi,

I'm very new to linux kernel development, but I have a few years of programming experience (not much C though).

I'm trying to get a syscall to work in user mode linux (version 2.6.29.1). I found a couple of tutorials on how to do it, but the file- and catalogue-structures doesn't really match, so I've tried to add code as best I could to make it work in my linux version.

The following additions have been made by me:
* In arch/x86/kernel/syscall_table_32.S
long sys_mycall

* In arch/x86/include/asm/unistd_32.h
#define __NR_mycall 333

* In arch/x86/include/asm/syscalls.h
asmlinkage long sys_mycall();

* In kernel\sys_ni.c
cond_syscall(sys_mycall);
(don't know why I would need this line, but it doesn't compile without it)

* In arch/x86/kernel/Makefile
obj-y += mycall.o

* In arch/x86/kernel/mycall.c
#include <asm/linkage.h>

asmlinkage long sys_mycall()
{
kprint("Hello world!\n");
return 1234;
}


I then run the kernel in user mode and try to call my syscall from a c-file:

* testmycall.c
#include<stdio.h>
#include "testmycall.h"

int main(void)
{
printf("Output: %d \n", syscall(333));
}


I get the output -1 and it doesn't print my hello world message :,(
Now I'm stuck and don't really know how to proceed or what could be wrong with my syscall code.
Help would be much appreciated!


Thanks in advance!

/Magnus

PS. I posted this over at the newbie-forum. Maybe some kind person could delete it from there :S

This says it all. Are you compiling a UML kernel or a native 32-bit x86 kernel? The steps you have taken so far will only work in the latter case.

On any other arch (and UML is somewhat like an arch), the following:
will make sys_mycall a weak alias to sys_ni_syscall. Thus, it will enable compilation on an arch for which the syscall has not yet been written, by replacing it with a stub (the “not implemented” system call). If the stub is required for compilation, it means the strong alias is not available (i.e., it is in an object file unused by the current architecture).

So I do not think symbols in arch/x86/kernel will be available to a UML kernel. It would be better to put your file somewhere more universal (especially considering your system call does nothing processor-specific).

Solved it It was exactly what you said. Thanks!

Hi,I have same problem.Please can you tell me which file's place was changed and where did you put it???I realy need these answers