[PATCH] kprobes: Update Documentation/kprobes.txt

architectures:

- i386

- x86_64 (AMD-64, E64MT)

- x86_64 (AMD-64, EM64T)

- ppc64

- ia64 (Support for probes on certain instruction types is still in progress.)

- ia64 (Does not support probes on instruction slot1.)

- sparc64 (Return probes not yet implemented.)

3. Configuring Kprobes

When configuring the kernel using make menuconfig/xconfig/oldconfig,

ensure that CONFIG_KPROBES is set to "y".  Under "Kernel hacking",

look for "Kprobes".  You may have to enable "Kernel debugging"

(CONFIG_DEBUG_KERNEL) before you can enable Kprobes.

ensure that CONFIG_KPROBES is set to "y".  Under "Instrumentation

Support", look for "Kprobes".

So that you can load and unload Kprobes-based instrumentation modules,

make sure "Loadable module support" (CONFIG_MODULES) and "Module

unloading" (CONFIG_MODULE_UNLOAD) are set to "y".

You may also want to ensure that CONFIG_KALLSYMS and perhaps even

CONFIG_KALLSYMS_ALL are set to "y", since kallsyms_lookup_name()

5. Kprobes Features and Limitations

As of Linux v2.6.12, Kprobes allows multiple probes at the same

address.  Currently, however, there cannot be multiple jprobes on

the same function at the same time.

Kprobes allows multiple probes at the same address.  Currently,

however, there cannot be multiple jprobes on the same function at

the same time.

In general, you can install a probe anywhere in the kernel.

In particular, you can probe interrupt handlers.  Known exceptions

are discussed in this section.

For obvious reasons, it's a bad idea to install a probe in

the code that implements Kprobes (mostly kernel/kprobes.c and

arch/*/kernel/kprobes.c).  A patch in the v2.6.13 timeframe instructs

Kprobes to reject such requests.

The register_*probe functions will return -EINVAL if you attempt

to install a probe in the code that implements Kprobes (mostly

kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such

as do_page_fault and notifier_call_chain).

If you install a probe in an inline-able function, Kprobes makes

no attempt to chase down all inline instances of the function and

Kprobes makes no attempt to prevent probe handlers from stepping on

each other -- e.g., probing printk() and then calling printk() from a

probe handler.  As of Linux v2.6.12, if a probe handler hits a probe,

that second probe's handlers won't be run in that instance.

In Linux v2.6.12 and previous versions, Kprobes' data structures are

protected by a single lock that is held during probe registration and

unregistration and while handlers are run.  Thus, no two handlers

can run simultaneously.  To improve scalability on SMP systems,

this restriction will probably be removed soon, in which case

multiple handlers (or multiple instances of the same handler) may

run concurrently on different CPUs.  Code your handlers accordingly.

Kprobes does not use semaphores or allocate memory except during

probe handler.  If a probe handler hits a probe, that second probe's

handlers won't be run in that instance, and the kprobe.nmissed member

of the second probe will be incremented.

As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of

the same handler) may run concurrently on different CPUs.

Kprobes does not use mutexes or allocate memory except during

registration and unregistration.

Probe handlers are run with preemption disabled.  Depending on the

(As far as we can tell, __builtin_return_address() is used only

for instrumentation and error reporting.)

If the number of times a function is called does not match the

number of times it returns, registering a return probe on that

function may produce undesirable results.  We have the do_exit()

and do_execve() cases covered.  do_fork() is not an issue.  We're

unaware of other specific cases where this could be a problem.

If the number of times a function is called does not match the number

of times it returns, registering a return probe on that function may

produce undesirable results.  We have the do_exit() case covered.

do_execve() and do_fork() are not an issue.  We're unaware of other

specific cases where this could be a problem.

If, upon entry to or exit from a function, the CPU is running on

a stack other than that of the current task, registering a return

probe on that function may produce undesirable results.  For this

reason, Kprobes doesn't support return probes (or kprobes or jprobes)

on the x86_64 version of __switch_to(); the registration functions

return -EINVAL.

6. Probe Overhead

7. TODO

a. SystemTap (http://sourceware.org/systemtap): Work in progress

to provide a simplified programming interface for probe-based

instrumentation.

b. Improved SMP scalability: Currently, work is in progress to handle

multiple kprobes in parallel.

c. Kernel return probes for sparc64.

d. Support for other architectures.

e. User-space probes.

a. SystemTap (http://sourceware.org/systemtap): Provides a simplified

programming interface for probe-based instrumentation.  Try it out.

b. Kernel return probes for sparc64.

c. Support for other architectures.

d. User-space probes.

e. Watchpoint probes (which fire on data references).

8. Kprobes Example

		printk("Couldn't find %s to plant kprobe\n", "do_fork");

		return -1;

	}

	ret = register_kprobe(&kp);

	if (ret < 0) {

	if ((ret = register_kprobe(&kp) < 0)) {

		printk("register_kprobe failed, returned %d\n", ret);

		return -1;

	}