So far, the only thing we've done was to use well defined kernel mechanisms to register /proc files and device handlers. This is fine if you want to do something the kernel programmers thought you'd want, such as write a device driver. But what if you want to do something unusual, to change the behavior of the system in some way? Then, you're mostly on your own.
This is where kernel programming gets dangerous. While writing the example below, I killed the open system call. This meant I couldn't open any files, I couldn't run any programs, and I couldn't shutdown the computer. I had to pull the power switch. Luckily, no files died. To ensure you won't lose any files either, please run sync right before you do the insmod and the rmmod.
Forget about /proc files, forget about device files. They're just minor details. The real process to kernel communication mechanism, the one used by all processes, is system calls. When a process requests a service from the kernel (such as opening a file, forking to a new process, or requesting more memory), this is the mechanism used. If you want to change the behaviour of the kernel in interesting ways, this is the place to do it. By the way, if you want to see which system calls a program uses, run strace <command> <arguments>.
In general, a process is not supposed to be able to access the kernel. It can't access kernel memory and it can't call kernel functions. The hardware of the CPU enforces this (that's the reason why it's called `protected mode'). System calls are an exception to this general rule. What happens is that the process fills the registers with the appropriate values and then calls a special instruction which jumps to a previously defined location in the kernel (of course, that location is readable by user processes, it is not writable by them). Under Intel CPUs, this is done by means of interrupt 0x80. The hardware knows that once you jump to this location, you are no longer running in restricted user mode, but as the operating system kernel -- and therefore you're allowed to do whatever you want.
The location in the kernel a process can jump to is called system_call. The procedure at that location checks the system call number, which tells the kernel what service the process requested. Then, it looks at the table of system calls (sys_call_table) to see the address of the kernel function to call. Then it calls the function, and after it returns, does a few system checks and then return back to the process (or to a different process, if the process time ran out). If you want to read this code, it's at the source file arch/<architecture>/kernel/entry.S, after the line ENTRY(system_call).
So, if we want to change the way a certain system call works, what we need to do is to write our own function to implement it (usually by adding a bit of our own code, and then calling the original function) and then change the pointer at sys_call_table to point to our function. Because we might be removed later and we don't want to leave the system in an unstable state, it's important for cleanup_module to restore the table to its original state.
The source code here is an example of such a kernel module. We want to `spy' on a certain user, and to printk a message whenever that user opens a file. Towards this end, we replace the system call to open a file with our own function, called our_sys_open. This function checks the uid (user's id) of the current process, and if it's equal to the uid we spy on, it calls printk to display the name of the file to be opened. Then, either way, it calls the original open function with the same parameters, to actually open the file.
The init_module function replaces the appropriate location in sys_call_table and keeps the original pointer in a variable. The cleanup_module function uses that variable to restore everything back to normal. This approach is dangerous, because of the possibility of two kernel modules changing the same system call. Imagine we have two kernel modules, A and B. A's open system call will be A_open and B's will be B_open. Now, when A is inserted into the kernel, the system call is replaced with A_open, which will call the original sys_open when it's done. Next, B is inserted into the kernel, which replaces the system call with B_open, which will call what it thinks is the original system call, A_open, when it's done.
Now, if B is removed first, everything will be well -- it will simply restore the system call to A_open, which calls the original. However, if A is removed and then B is removed, the system will crash. A's removal will restore the system call to the original, sys_open, cutting B out of the loop. Then, when B is removed, it will restore the system call to what it thinks is the original, A_open, which is no longer in memory. At first glance, it appears we could solve this particular problem by checking if the system call is equal to our open function and if so not changing it at all (so that B won't change the system call when it's removed), but that will cause an even worse problem. When A is removed, it sees that the system call was changed to B_open so that it is no longer pointing to A_open, so it won't restore it to sys_open before it is removed from memory. Unfortunately, B_open will still try to call A_open which is no longer there, so that even without removing B the system would crash.
The only way I can think of to prevent this problem is to restore the call to the original value, sys_open. Unfortunately, sys_open is not part of the kernel system table in /proc/ksyms, so we can't access it. If anybody has a better idea, I'd be happy to hear it7.1.
/* syscall.c
*
* System call "stealing" sample
*/
/* Copyright (C) 1998 by Ori Pomerantz */
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
#include <sys/syscall.h> /* The list of system calls */
#include <linux/sched.h> /* For the current (process) structure, we need
* this to know who the current user is. */
/* The system call table (a table of functions). We just define this as
* external, and the kernel will fill it up for us when we are insmod'ed
*/
extern void *sys_call_table[];
int uid; /* UID we want to spy on - will be filled from the command line */
/* A pointer to the original system call. The reason we keep this, rather
* than call the original function (sys_open), is because somebody else might
* have replaced the system call before us. Note that this is not 100% safe,
* because if another module replaced sys_open before us, then when we're
* inserted we'll call the function in that module - and it might be removed
* before we are. */
asmlinkage int (*original_call)(const char *, int, int);
/* The function we'll replace sys_open (the function called when you call
* the open system call) with. To find the exact prototype, with the number
* and type of arguments, we find the original function first (it's at
* fs/open.c.
* In theory, this means that we're tied to the current version of the
* kernel. In practice, the system calls almost never change (it would
* wreck havoc and require programs to be recompiled, since the system
* calls are the interface between the kernel and the rest of the world).
*/
asmlinkage int our_sys_open(const char *filename, int flags, int mode)
{
int i = 0;
char ch;
/* Check if this is the user we're spying on */
if (uid == current->uid) { /* current->uid is the uid of the user who
ran the process which called the system
call we got */
/* Report the file, if relevant */
printk("Opened file: ");
do {
ch = get_user(filename+i);
i++;
printk("%c", ch);
} while (ch != 0);
printk("\n");
}
/* Call the original sys_open - otherwise, we lose the ability to open
* files */
return original_call(filename, flags, mode);
}
/* Initialize the module - replace the system call */
int init_module()
{
/* Warning - too late for it now, but maybe for next time... */
printk("I'm dangerous. I hope you did a sync before you insmod'ed me.\n");
printk("My counterpart, cleapup_module(), is even more dangerous. If\n");
printk("you value your file system, it will be \"sync; rmmod\" \n");
printk("when you remove it.\n");
/* Keep a pointer to the original function in original_call, and
* then replace the system call in the system call table with
* our_sys_open */
original_call = sys_call_table[__NR_open];
sys_call_table[__NR_open] = our_sys_open;
/* To get the address of the function for system call foo, go to
* sys_call_table[__NR_foo]. */
return 0;
}
/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
/* Return the system call back to normal */
if (sys_call_table[__NR_open] != our_sys_open) {
printk("Somebody else also played with the open system call\n");
printk("The system may be left in an unstable state.\n");
}
sys_call_table[__NR_open] = original_call;
}