#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/unistd.h>
#include <linux/sched.h>
#include <linux/syscalls.h>
#include <asm/uaccess.h>

extern void* sys_call_table[];

static int uid;
module_param(uid, int, 0644);

//extern asmlinkage long (*sys_open) (const char __user *filename, int flags, int mode);

asmlinkage int (*original_call) (const char*, int, int);

asmlinkage int our_sys_open(const char* filename, int flags, int mode)
{
	int i = 0;
	char ch;

	if (uid == current->cred->uid) {
		printk("Opened file by %d: ", uid);
		do {
			get_user(ch, filename + i++);
			printk("%c", ch);
		} while (ch != 0);
		printk("\n");
	}

	return original_call(filename, flags, mode);
}

//unsigned long** find_sys_call_table(void)
//{
//	unsigned long** sctable;
//	unsigned long ptr;
//
//	extern int loops_per_jiffy;
//
//	sctable = NULL;
//	for (ptr = (unsigned long)&loops_per_jiffy;
//			ptr < (unsigned long)&boot_cpu_data; ptr += sizeof(void*)) {
//		unsigned long *p;
//		p = (unsigned long*)ptr;
//		if (p[__NR_open] == (unsigned long) sys_open) {
//			sctable = (unsigned long**)p;
//			return sctable;
//		}
//	}
//
//	return sctable;
//}

int init_module(void)
{
	original_call = sys_call_table[__NR_open];
	sys_call_table[__NR_open] = our_sys_open;

	printk(KERN_INFO "Spying on uid: %d\n", uid);
//	printk(KERN_INFO "sys_call_table[__NR_open] = %p\n", find_sys_call_table()[__NR_open]);

	return 0;
}

void cleanup_module(void)
{
	if (sys_call_table[__NR_open] != our_sys_open) {
		printk(KERN_ALERT "Somebody else also played with the open syscall\n");
		printk(KERN_ALERT "The system may be left in an unstable state\n");
	}

	sys_call_table[__NR_open] = original_call;
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Guillermo Ramos");