[rfc] [patch] kernel hooks

[Vamsi Krishna S .]

The Kernel Hooks Interface is a generalised facility for placing 
hooks in arbitrary kernel locations. A hook is a location in the
kernel that calls out of the kernel to a kernel module routine - 
a hook exit routine. It enables many kernel enhancements, which 
are otherwise self-contained, to become loadable kernel modules 
and retain a substantial degree of independence from the kernel 
source.

Hooks are implemented as fast and slim-line insertions to kernel 
space code. When not active that have practically no overhead to
the kernel. Optimized versions are implemented on ia32, ppc, 
ppc64, s390, s390x archs, where they expand to:
	movl $0, %reg
	     ^x
	testl %reg, %reg
	jz out
	call out exits
	out:

Hooks are armed by editing the number at ^x above to a non-zero 
value so that the exits will be now be called out. 

These hooks are useful especially for invasive facilities like 
security hooks and tracing hooks, which should be compiled in, but 
could be inactive most of the time. The overhead of these hooks 
is very little when they are not armed. These are used by the
Dynamic Probes (http://www.ibm.com/linux/ltc/projects/dprobes/)
and the Linux Kernel State Tracer at (http://lkst.sourceforge.net/)

The patch given below is a slimmed down version of what is at the 
project website http://www.ibm.com/linux/ltc/projects/kernelhooks/. 
I have removed from the full package:
- /proc interface for arming/disarming hooks
- documentation and sample code (could be posted as a seperate patch)
- some extra paranoid checks
- hook exit priorities
- open coded exit lists in favour of using include/linux/list.h
  implementation
- kill typedefs and a number of other cleanups

Todo:
- find some way to eliminate the need for creating a symbol at the 
  hook location so as not to confuse disassembers. We should write 
  the address of the hook location in a seperate section, may be, 
  and set a variable to that location.

Note that the hook structure defined at each hook location is 
exported for use by GPL'ed modules only, just like the security hooks.

Here is an example illustrating how hooks are placed and used:
Defining a hook:
In the source file where you want to place the hook:
DECLARE_HOOK(my_hook);

and at the location where the hook is desired:

	...
	HOOK(my_hook, parm1, parm2);
	...
Now, for this location, exit functions will be called out
when they are registered and enabled.

Before a hook can be used, it needs to be initialised:
	...
	hook_initialise(my_hook);

	...
	hook_terminate(my_hook, force);

To register an exit, say in a different module:

struct hook_rec rec = {
	.hook_exit = exit_fn;
}
	...
	hook_exit_register(my_hook, &rec);

	...
	hook_exit_deregister(&rec);

To enable/disable a hook exit:
	...
	hook_exit_arm(&rec);

	...
	hook_exit_disarm(&rec);
	
Any ideas, suggestions, comments are welcome.

Thanks,
Vamsi.