Hello,
I started to read docs and examples about LKM, and i
have some doubts, if someone could help me. :)
1 - In the nowsday is necessary use get_user(),
put_user(), copy_from_user(),copy_to_user() to
transform from kernel to user memory and vice-versa ?
Or it's automatically actually ? I'm asking it because
i can manipulate normal data received directilly as
argument from syscall without use this functions (as
described in text that i'm reading). :)
2 - I saw that i must use get_fs() and set_fs() to
access DS (data segment) from kernel or User space,
and in article it's describe that it can be used to
"execute" libc functions over kernel. So i tryed it:
int uid;
mm_segment_t old_fs;
old_fs = get_fs();
set_fs(get_ds());
uid = getuid();
printk("\nUID %d",uid);
set_fs(old_fs);
But when i try insmod it:
libc-set_fs.o: unresolved symbol getuid
Ok, i know that i can get my uid via current struct,
but i want to know how to i execute libc functions
over kernel. How to do it ? Can someone give me a
example ? :)
3 - In the article we created a basic device driver
like that:
/*just a dummy for demonstration*/
static int driver_open(struct inode *i, struct file
*f){
printk("<1>Open Function\n");
return 0;
}
/*register every function which will be provided by
our driver*/
static struct file_operations fops = {
NULL, /*lseek*/
NULL, /*read*/
NULL, /*write*/
NULL, /*readdir*/
NULL, /*select*/
NULL, /*ioctl*/
NULL, /*mmap*/
driver_open, /*open, take a look at my dummy
open function*/
NULL, /*release*/
NULL /*fsync...*/
};
int init_module(void){
/*register driver with major 40 and the name driver*/
if(register_chrdev(40, "driver", &fops)) return -EIO;
return 0;
}
void cleanup_module(void){
/*unregister our driver*/
unregister_chrdev(40, "driver");
}
So i created the device with mknod and tryed "open"
with cat, echo, etc, but it never print the "Open
Function" from printk(), why ?
4 - Other intersting thing is that we can re-use
exported symbols, so i tryed to test and import the
vfat_rmdir and do it return 0 to doesn't remove my dir
over fat (only to test, to learn how to work import a
exported symbol), i done like that:
/* Get vfat_rmdir exported function */
extern int *vfat_rmdir_Rc0ad670e;
/* Our faked vfat_mkdir */
int *new_vfat_rmdir(struct inode *dir, struct dentry
*dentry){
return 0;
}
int init_module(void) {
vfat_rmdir_Rc0ad670e=new_vfat_rmdir;
return 0;
}
void cleanup_module(void){
}
And it really can't remove dir under fat with "rm",
but when i type "rm" i receive a dump from kernel, and
i can't access more fat fs! :(
Why ? What i done wrong ?
5 - Relax, this is the last dummy question! :)
I went to define my module over GPL, then i setted:
#include <asm/unistd.h>
#include <stdio.h>
#include <errno.h>
MODULE_LICENSE("GPL");
But, when i insmod it say that i "tainted the kernel".
:/ What is wrong ?
Thkz a lot and sorry for idiot questions.
Regards
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