Linux Kernel Module System Calls

8.1. System Calls

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 .
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_modulefunction 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.
I can think of two ways to prevent this problem. The first 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. The other solution is to use the reference count to prevent root fromrmmod'ing the module once it is loaded. This is good for production modules, but bad for an educational sample --- which is why I didn't do it here.

Example 8-1. procfs.c

/* syscall.c * * System call "stealing" sample. */ /* Copyright (C) 2001 by Peter Jay Salzman */ /* The necessary header files */ /* Standard in kernel modules */ #include /* We're doing kernel work */ #include /* Specifically, a module */ /* Deal with CONFIG_MODVERSIONS */ #if CONFIG_MODVERSIONS==1 #define MODVERSIONS #include #endif #include /* The list of system calls */ /* For the current (process) structure, we need * this to know who the current user is. */ #include /* In 2.2.3 /usr/include/linux/version.h includes a * macro for this, but 2.0.35 doesn't - so I add it * here if necessary. */ #ifndef KERNEL_VERSION #define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c)) #endif #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0) #include #endif /* 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[]; /* UID we want to spy on - will be filled from the * command line */ int uid; #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0) MODULE_PARM(uid, "i"); #endif /* 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. * * Another reason for this is that we can't get sys_open. * It's a static variable, so it is not exported. */ asmlinkage int (*original_call)(const char *, int, int); /* For some reason, in 2.2.3 current->uid gave me * zero, not the real user ID. I tried to find what went * wrong, but I couldn't do it in a short time, and * I'm lazy - so I'll just use the system call to get the * uid, the way a process would. * * For some reason, after I recompiled the kernel this * problem went away. */ asmlinkage int (*getuid_call)(); /* 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 * processes). */ 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 == getuid_call()) { /* getuid_call is the getuid system call, * which gives the uid of the user who * ran the process which called the system * call we got */ /* Report the file, if relevant */ printk("Opened file by %d: ", uid); do { #if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0) get_user(ch, filename+i); #else ch = get_user(filename+i); #endif 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 "); printk("sync before you insmod'ed me.\n"); printk("My counterpart, cleanup_module(), is even"); printk("more dangerous. If\n"); printk("you value your file system, it will "); printk("be \"sync; rmmod\" \n"); printk("when you remove this module.\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]. */ printk("Spying on UID:%d\n", uid); /* Get the system call for getuid */ getuid_call = sys_call_table[__NR_getuid]; 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 "); printk("open system call\n"); printk("The system may be left in "); printk("an unstable state.\n"); } sys_call_table[__NR_open] = original_call; }

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James Thornton