System Calls: A Kernel Project

Hunter Bell

CS-550-3

Fall 2003

1

Introduction

• System Call Linkage• Generating a New Kernel Function• Defining the System Call Number• Rebuilding the Kernel• Generation and Running of User-space

Program• Problem encountered with programming

exercise2

System Call LinkageMacro (called in user program)

Stub (trap instruction)

Sys_call_table

User space

Kernel Function

Kernel/Supervisor Space

3

What is a Macro?

• In this case, a predefined function in the Operating System that can be called from user space

•Generates a system call stub with zero to five parameters

•For example, generating a system call stub with two parameters:

_syscall2(type,name,type1,arg1,type2,arg2);

•This generates a stub that makes a system call to the kernel function with matching type and name with the argument signature provided 4

Generating a New Kernel Function

• Update System Call Number / Linkage

• Determine location of body for function

• Write code for new Function

• Rebuild Kernel to include new Function

5

Defining the System Call Number (entry.S)

ENTRY(sys_call_table)

.long SYMBOL_NAME(sys_ni_syscall) /* 0 - old "setup()" system call*/

.long SYMBOL_NAME(sys_exit)

.long SYMBOL_NAME(sys_fork)

.long SYMBOL_NAME(sys_read)

.long SYMBOL_NAME(sys_write)

.long SYMBOL_NAME(sys_open) /* 5 */

…

.long SYMBOL_NAME(sys_ni_syscall) /* streams2 */

.long SYMBOL_NAME(sys_vfork) /* 190 */

.long SYMBOL_NAME(sys_pedagogictime) /*191 - Hunter Bell*/

.rept NR_syscalls-191

.long SYMBOL_NAME(sys_ni_syscall)

 

F’n name

# of f’ns

6

Defining the System Call Linkage (unistd.h)

#define __NR_exit 1

#define __NR_fork 2

…

#define __NR_vfork 190

#define __NR_pedagogictime 191

New entry in table - for generating a stub 7

Source Code for Kernel Functionasmlinkage int sys_pedagogictime(int flag,struct timeval *thetime)

{

/*declarations*/

int write_failed;

struct timeval *temp;

 

/*verify we can write to user space*/

write_failed = verify_area(VERIFY_WRITE,thetime,sizeof(thetime));

if(!write_failed)

{

printk("skelcall: Cannot write into user space");

return 0;

}/*end if*/

cli(); /*disable interrupts*/

8

Source Code for Kernel Functioncli(); /*disable interrupts*/

/* write into user space the values in xtime */

__copy_to_user(&thetime->tv_sec,&xtime.tv_sec,sizeof(xtime.tv_sec));

__copy_to_user(&thetime->tv_usec,&xtime.tv_usec,sizeof(xtime.tv_usec));

sti(); /*enable interrupts*/

/* verify we can read from user space*/

write_failed = verify_area(VERIFY_READ,thetime,sizeof(thetime));

if(!write_failed)

{

printk("skelcall: Cannot read from user space");

return 0;

}

/*copy back from user space into temporary variable for printing*/

temp = NULL;

__copy_from_user(&temp->tv_sec,&thetime->tv_sec,sizeof(thetime->tv_sec));

__copy_from_user(&temp->tv_usec,&thetime->tv_usec,sizeof(thetime->tv_usec));

9

Source Code for Kernel Function

/*Print time if flag is true*/

if(flag)

printk("%s",ctime(temp->tv_sec));

return (!(write_failed));

}//end pedagogictime

10

Rebuilding the Kernel5 steps:

• make clean

• make oldconfig

• make depend

• make

• make bzImage11

Rebuilding the Kernel (commands)

Command Functionality

make clean Removes old object files and other temporary files in kernel environment

make oldconfig

Generic form: make <config_opt>

Defines logical environment for new kernel – in this case, the same as the older configuration

make depend Specifies compile-order of kernel files

make Compiles all kernel source code into an executable file

make bzImage

Generic form : make <boot_opt>

Creates a bootable image of the kernel

12

Generation and Running of User-space Program

/* ****************************************

* Hunter Bell - CS550

* exercise-5 (term project)

* this program executes in user space and

* makes a system call to kernel space

***************************************** */

#include <linux/unistd.h>

#include <sys/syscall.h>

#include <stdio.h>

#include <time.h>

int main(int argc, char * argv[])

{

struct timeval *thetime = (struct timeval *) malloc(sizeof(struct timeval *));

int flag = 1;

/* generate a stub for System call for int pedagogictime through a Macro*/

_syscall2(int,pedagogictime,int,flag,struct timeval *,thetime);

return 0;

}//end main

13

Summary – System Call LinkageMacro (called in user program)

Stub (trap instruction)

Sys_call_table

User space

Kernel Function

14

Problem encountered with creating kernel function

• memcpy_fromfs() and memcpy_tofs() did not compile, and a grep produced nothing

15

Summary•System Call Linkage

•Generating a New Kernel Function

•Defining the System Call Number

•Rebuilding the Kernel

•Generation and Running of User-space Program 16