Merge branch 'merge' of git://git.kernel.org/pub/scm/linux/kernel/git/benh/powerpc

[safe/jmp/linux-2.6] / Documentation / kprobes.txt

diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
index 0541fe1..48b3de9 100644 (file)
--- a/Documentation/kprobes.txt
+++ b/Documentation/kprobes.txt
@@ -14,6 +14,7 @@ CONTENTS
 8. Kprobes Example
 9. Jprobes Example
 10. Kretprobes Example
 8. Kprobes Example
 9. Jprobes Example
 10. Kretprobes Example

+Appendix A: The kprobes debugfs interface

 
 1. Concepts: Kprobes, Jprobes, Return Probes
 
 
 1. Concepts: Kprobes, Jprobes, Return Probes
 


@@ -36,6 +37,11 @@ registration function such as register_kprobe() specifies where
 the probe is to be inserted and what handler is to be called when
 the probe is hit.
 
 the probe is to be inserted and what handler is to be called when
 the probe is hit.
 

+There are also register_/unregister_*probes() functions for batch
+registration/unregistration of a group of *probes. These functions
+can speed up unregistration process when you have to unregister
+a lot of probes at once.
+

 The next three subsections explain how the different types of
 probes work.  They explain certain things that you'll need to
 know in order to make the best use of Kprobes -- e.g., the
 The next three subsections explain how the different types of
 probes work.  They explain certain things that you'll need to
 know in order to make the best use of Kprobes -- e.g., the


@@ -91,11 +97,12 @@ handler has run.  Up to MAX_STACK_SIZE bytes are copied -- e.g.,
 64 bytes on i386.
 
 Note that the probed function's args may be passed on the stack
 64 bytes on i386.
 
 Note that the probed function's args may be passed on the stack

-or in registers (e.g., for x86_64 or for an i386 fastcall function).
-The jprobe will work in either case, so long as the handler's
-prototype matches that of the probed function.
+or in registers.  The jprobe will work in either case, so long as the
+handler's prototype matches that of the probed function.
+
+1.3 Return Probes

 
 

-1.3 How Does a Return Probe Work?
+1.3.1 How Does a Return Probe Work?

 
 When you call register_kretprobe(), Kprobes establishes a kprobe at
 the entry to the function.  When the probed function is called and this
 
 When you call register_kretprobe(), Kprobes establishes a kprobe at
 the entry to the function.  When the probed function is called and this


@@ -106,9 +113,9 @@ At boot time, Kprobes registers a kprobe at the trampoline.
 
 When the probed function executes its return instruction, control
 passes to the trampoline and that probe is hit.  Kprobes' trampoline
 
 When the probed function executes its return instruction, control
 passes to the trampoline and that probe is hit.  Kprobes' trampoline

-handler calls the user-specified handler associated with the kretprobe,
-then sets the saved instruction pointer to the saved return address,
-and that's where execution resumes upon return from the trap.
+handler calls the user-specified return handler associated with the
+kretprobe, then sets the saved instruction pointer to the saved return
+address, and that's where execution resumes upon return from the trap.

 
 While the probed function is executing, its return address is
 stored in an object of type kretprobe_instance.  Before calling
 
 While the probed function is executing, its return address is
 stored in an object of type kretprobe_instance.  Before calling


@@ -130,27 +137,56 @@ zero when the return probe is registered, and is incremented every
 time the probed function is entered but there is no kretprobe_instance
 object available for establishing the return probe.
 
 time the probed function is entered but there is no kretprobe_instance
 object available for establishing the return probe.
 

+1.3.2 Kretprobe entry-handler
+
+Kretprobes also provides an optional user-specified handler which runs
+on function entry. This handler is specified by setting the entry_handler
+field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
+function entry is hit, the user-defined entry_handler, if any, is invoked.
+If the entry_handler returns 0 (success) then a corresponding return handler
+is guaranteed to be called upon function return. If the entry_handler
+returns a non-zero error then Kprobes leaves the return address as is, and
+the kretprobe has no further effect for that particular function instance.
+
+Multiple entry and return handler invocations are matched using the unique
+kretprobe_instance object associated with them. Additionally, a user
+may also specify per return-instance private data to be part of each
+kretprobe_instance object. This is especially useful when sharing private
+data between corresponding user entry and return handlers. The size of each
+private data object can be specified at kretprobe registration time by
+setting the data_size field of the kretprobe struct. This data can be
+accessed through the data field of each kretprobe_instance object.
+
+In case probed function is entered but there is no kretprobe_instance
+object available, then in addition to incrementing the nmissed count,
+the user entry_handler invocation is also skipped.
+

 2. Architectures Supported
 
 Kprobes, jprobes, and return probes are implemented on the following
 architectures:
 
 - i386
 2. Architectures Supported
 
 Kprobes, jprobes, and return probes are implemented on the following
 architectures:
 
 - i386

-- x86_64 (AMD-64, E64MT)
+- x86_64 (AMD-64, EM64T)

 - ppc64
 - 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.)
 - sparc64 (Return probes not yet implemented.)

+- arm
+- ppc

 
 3. Configuring Kprobes
 
 When configuring the kernel using make menuconfig/xconfig/oldconfig,
 
 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".

 
 

-You may also want to ensure that CONFIG_KALLSYMS and perhaps even
-CONFIG_KALLSYMS_ALL are set to "y", since kallsyms_lookup_name()
-is a handy, version-independent way to find a function's address.
+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".
+
+Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
+are set to "y", since kallsyms_lookup_name() is used by the in-kernel
+kprobe address resolution code.

 
 If you need to insert a probe in the middle of a function, you may find
 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
 
 If you need to insert a probe in the middle of a function, you may find
 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),


@@ -160,9 +196,11 @@ code mapping.
 4. API Reference
 
 The Kprobes API includes a "register" function and an "unregister"
 4. API Reference
 
 The Kprobes API includes a "register" function and an "unregister"

-function for each type of probe.  Here are terse, mini-man-page
-specifications for these functions and the associated probe handlers
-that you'll write.  See the latter half of this document for examples.
+function for each type of probe. The API also includes "register_*probes"
+and "unregister_*probes" functions for (un)registering arrays of probes.
+Here are terse, mini-man-page specifications for these functions and
+the associated probe handlers that you'll write. See the files in the
+samples/kprobes/ sub-directory for examples.

 
 4.1 register_kprobe
 
 
 4.1 register_kprobe
 


@@ -176,6 +214,27 @@ occurs during execution of kp->pre_handler or kp->post_handler,
 or during single-stepping of the probed instruction, Kprobes calls
 kp->fault_handler.  Any or all handlers can be NULL.
 
 or during single-stepping of the probed instruction, Kprobes calls
 kp->fault_handler.  Any or all handlers can be NULL.
 

+NOTE:
+1. With the introduction of the "symbol_name" field to struct kprobe,
+the probepoint address resolution will now be taken care of by the kernel.
+The following will now work:
+
+       kp.symbol_name = "symbol_name";
+
+(64-bit powerpc intricacies such as function descriptors are handled
+transparently)
+
+2. Use the "offset" field of struct kprobe if the offset into the symbol
+to install a probepoint is known. This field is used to calculate the
+probepoint.
+
+3. Specify either the kprobe "symbol_name" OR the "addr". If both are
+specified, kprobe registration will fail with -EINVAL.
+
+4. With CISC architectures (such as i386 and x86_64), the kprobes code
+does not validate if the kprobe.addr is at an instruction boundary.
+Use "offset" with caution.
+

 register_kprobe() returns 0 on success, or a negative errno otherwise.
 
 User's pre-handler (kp->pre_handler):
 register_kprobe() returns 0 on success, or a negative errno otherwise.
 
 User's pre-handler (kp->pre_handler):


@@ -218,9 +277,9 @@ Kprobes runs the handler whose address is jp->entry.
 The handler should have the same arg list and return type as the probed
 function; and just before it returns, it must call jprobe_return().
 (The handler never actually returns, since jprobe_return() returns
 The handler should have the same arg list and return type as the probed
 function; and just before it returns, it must call jprobe_return().
 (The handler never actually returns, since jprobe_return() returns

-control to Kprobes.)  If the probed function is declared asmlinkage,
-fastcall, or anything else that affects how args are passed, the
-handler's declaration must match.
+control to Kprobes.)  If the probed function is declared asmlinkage
+or anything else that affects how args are passed, the handler's
+declaration must match.

 
 register_jprobe() returns 0 on success, or a negative errno otherwise.
 
 
 register_jprobe() returns 0 on success, or a negative errno otherwise.
 


@@ -248,6 +307,13 @@ of interest:
 - ret_addr: the return address
 - rp: points to the corresponding kretprobe object
 - task: points to the corresponding task struct
 - ret_addr: the return address
 - rp: points to the corresponding kretprobe object
 - task: points to the corresponding task struct

+- data: points to per return-instance private data; see "Kretprobe
+       entry-handler" for details.
+
+The regs_return_value(regs) macro provides a simple abstraction to
+extract the return value from the appropriate register as defined by
+the architecture's ABI.
+

 The handler's return value is currently ignored.
 
 4.4 unregister_*probe
 The handler's return value is currently ignored.
 
 4.4 unregister_*probe


@@ -260,20 +326,57 @@ void unregister_kretprobe(struct kretprobe *rp);
 Removes the specified probe.  The unregister function can be called
 at any time after the probe has been registered.
 
 Removes the specified probe.  The unregister function can be called
 at any time after the probe has been registered.
 

+NOTE:
+If the functions find an incorrect probe (ex. an unregistered probe),
+they clear the addr field of the probe.
+
+4.5 register_*probes
+
+#include <linux/kprobes.h>
+int register_kprobes(struct kprobe **kps, int num);
+int register_kretprobes(struct kretprobe **rps, int num);
+int register_jprobes(struct jprobe **jps, int num);
+
+Registers each of the num probes in the specified array.  If any
+error occurs during registration, all probes in the array, up to
+the bad probe, are safely unregistered before the register_*probes
+function returns.
+- kps/rps/jps: an array of pointers to *probe data structures
+- num: the number of the array entries.
+
+NOTE:
+You have to allocate(or define) an array of pointers and set all
+of the array entries before using these functions.
+
+4.6 unregister_*probes
+
+#include <linux/kprobes.h>
+void unregister_kprobes(struct kprobe **kps, int num);
+void unregister_kretprobes(struct kretprobe **rps, int num);
+void unregister_jprobes(struct jprobe **jps, int num);
+
+Removes each of the num probes in the specified array at once.
+
+NOTE:
+If the functions find some incorrect probes (ex. unregistered
+probes) in the specified array, they clear the addr field of those
+incorrect probes. However, other probes in the array are
+unregistered correctly.
+

 5. Kprobes Features and Limitations
 
 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.
 
 
 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
 
 If you install a probe in an inline-able function, Kprobes makes
 no attempt to chase down all inline instances of the function and


@@ -290,18 +393,14 @@ from the accidental ones.  Don't drink and probe.
 
 Kprobes makes no attempt to prevent probe handlers from stepping on
 each other -- e.g., probing printk() and then calling printk() from a
 
 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
 registration and unregistration.
 
 Probe handlers are run with preemption disabled.  Depending on the


@@ -316,11 +415,21 @@ address instead of the real return address for kretprobed functions.
 (As far as we can tell, __builtin_return_address() is used only
 for instrumentation and error reporting.)
 
 (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. In such a case, a line:
+kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
+gets printed. With this information, one will be able to correlate the
+exact instance of the kretprobe that caused the problem. 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
 
 
 6. Probe Overhead
 


@@ -347,242 +456,54 @@ k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
 
 7. TODO
 
 
 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
 
 
 8. Kprobes Example
 

-Here's a sample kernel module showing the use of kprobes to dump a
-stack trace and selected i386 registers when do_fork() is called.
------ cut here -----
-/*kprobe_example.c*/
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/kprobes.h>
-#include <linux/kallsyms.h>
-#include <linux/sched.h>
-
-/*For each probe you need to allocate a kprobe structure*/
-static struct kprobe kp;
-
-/*kprobe pre_handler: called just before the probed instruction is executed*/
-int handler_pre(struct kprobe *p, struct pt_regs *regs)
-{
-       printk("pre_handler: p->addr=0x%p, eip=%lx, eflags=0x%lx\n",
-               p->addr, regs->eip, regs->eflags);
-       dump_stack();
-       return 0;
-}
-
-/*kprobe post_handler: called after the probed instruction is executed*/
-void handler_post(struct kprobe *p, struct pt_regs *regs, unsigned long flags)
-{
-       printk("post_handler: p->addr=0x%p, eflags=0x%lx\n",
-               p->addr, regs->eflags);
-}
-
-/* fault_handler: this is called if an exception is generated for any
- * instruction within the pre- or post-handler, or when Kprobes
- * single-steps the probed instruction.
- */
-int handler_fault(struct kprobe *p, struct pt_regs *regs, int trapnr)
-{
-       printk("fault_handler: p->addr=0x%p, trap #%dn",
-               p->addr, trapnr);
-       /* Return 0 because we don't handle the fault. */
-       return 0;
-}
-
-int init_module(void)
-{
-       int ret;
-       kp.pre_handler = handler_pre;
-       kp.post_handler = handler_post;
-       kp.fault_handler = handler_fault;
-       kp.addr = (kprobe_opcode_t*) kallsyms_lookup_name("do_fork");
-       /* register the kprobe now */
-       if (!kp.addr) {
-               printk("Couldn't find %s to plant kprobe\n", "do_fork");
-               return -1;
-       }
-       if ((ret = register_kprobe(&kp) < 0)) {
-               printk("register_kprobe failed, returned %d\n", ret);
-               return -1;
-       }
-       printk("kprobe registered\n");
-       return 0;
-}
-
-void cleanup_module(void)
-{
-       unregister_kprobe(&kp);
-       printk("kprobe unregistered\n");
-}
-
-MODULE_LICENSE("GPL");
------ cut here -----
-
-You can build the kernel module, kprobe-example.ko, using the following
-Makefile:
------ cut here -----
-obj-m := kprobe-example.o
-KDIR := /lib/modules/$(shell uname -r)/build
-PWD := $(shell pwd)
-default:
-       $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
-clean:
-       rm -f *.mod.c *.ko *.o
------ cut here -----
-
-$ make
-$ su -
-...
-# insmod kprobe-example.ko
-
-You will see the trace data in /var/log/messages and on the console
-whenever do_fork() is invoked to create a new process.
+See samples/kprobes/kprobe_example.c

 
 9. Jprobes Example
 
 
 9. Jprobes Example
 

-Here's a sample kernel module showing the use of jprobes to dump
-the arguments of do_fork().
------ cut here -----
-/*jprobe-example.c */
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/fs.h>
-#include <linux/uio.h>
-#include <linux/kprobes.h>
-#include <linux/kallsyms.h>
-
-/*
- * Jumper probe for do_fork.
- * Mirror principle enables access to arguments of the probed routine
- * from the probe handler.
- */
-
-/* Proxy routine having the same arguments as actual do_fork() routine */
-long jdo_fork(unsigned long clone_flags, unsigned long stack_start,
-             struct pt_regs *regs, unsigned long stack_size,
-             int __user * parent_tidptr, int __user * child_tidptr)
-{
-       printk("jprobe: clone_flags=0x%lx, stack_size=0x%lx, regs=0x%p\n",
-              clone_flags, stack_size, regs);
-       /* Always end with a call to jprobe_return(). */
-       jprobe_return();
-       /*NOTREACHED*/
-       return 0;
-}
-
-static struct jprobe my_jprobe = {
-       .entry = (kprobe_opcode_t *) jdo_fork
-};
-
-int init_module(void)
-{
-       int ret;
-       my_jprobe.kp.addr = (kprobe_opcode_t *) kallsyms_lookup_name("do_fork");
-       if (!my_jprobe.kp.addr) {
-               printk("Couldn't find %s to plant jprobe\n", "do_fork");
-               return -1;
-       }
-
-       if ((ret = register_jprobe(&my_jprobe)) <0) {
-               printk("register_jprobe failed, returned %d\n", ret);
-               return -1;
-       }
-       printk("Planted jprobe at %p, handler addr %p\n",
-              my_jprobe.kp.addr, my_jprobe.entry);
-       return 0;
-}
-
-void cleanup_module(void)
-{
-       unregister_jprobe(&my_jprobe);
-       printk("jprobe unregistered\n");
-}
-
-MODULE_LICENSE("GPL");
------ cut here -----
-
-Build and insert the kernel module as shown in the above kprobe
-example.  You will see the trace data in /var/log/messages and on
-the console whenever do_fork() is invoked to create a new process.
-(Some messages may be suppressed if syslogd is configured to
-eliminate duplicate messages.)
+See samples/kprobes/jprobe_example.c

 
 10. Kretprobes Example
 
 
 10. Kretprobes Example
 

-Here's a sample kernel module showing the use of return probes to
-report failed calls to sys_open().
------ cut here -----
-/*kretprobe-example.c*/
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/kprobes.h>
-#include <linux/kallsyms.h>
-
-static const char *probed_func = "sys_open";
-
-/* Return-probe handler: If the probed function fails, log the return value. */
-static int ret_handler(struct kretprobe_instance *ri, struct pt_regs *regs)
-{
-       // Substitute the appropriate register name for your architecture --
-       // e.g., regs->rax for x86_64, regs->gpr[3] for ppc64.
-       int retval = (int) regs->eax;
-       if (retval < 0) {
-               printk("%s returns %d\n", probed_func, retval);
-       }
-       return 0;
-}
-
-static struct kretprobe my_kretprobe = {
-       .handler = ret_handler,
-       /* Probe up to 20 instances concurrently. */
-       .maxactive = 20
-};
-
-int init_module(void)
-{
-       int ret;
-       my_kretprobe.kp.addr =
-               (kprobe_opcode_t *) kallsyms_lookup_name(probed_func);
-       if (!my_kretprobe.kp.addr) {
-               printk("Couldn't find %s to plant return probe\n", probed_func);
-               return -1;
-       }
-       if ((ret = register_kretprobe(&my_kretprobe)) < 0) {
-               printk("register_kretprobe failed, returned %d\n", ret);
-               return -1;
-       }
-       printk("Planted return probe at %p\n", my_kretprobe.kp.addr);
-       return 0;
-}
-
-void cleanup_module(void)
-{
-       unregister_kretprobe(&my_kretprobe);
-       printk("kretprobe unregistered\n");
-       /* nmissed > 0 suggests that maxactive was set too low. */
-       printk("Missed probing %d instances of %s\n",
-               my_kretprobe.nmissed, probed_func);
-}
-
-MODULE_LICENSE("GPL");
------ cut here -----
-
-Build and insert the kernel module as shown in the above kprobe
-example.  You will see the trace data in /var/log/messages and on the
-console whenever sys_open() returns a negative value.  (Some messages
-may be suppressed if syslogd is configured to eliminate duplicate
-messages.)
+See samples/kprobes/kretprobe_example.c

 
 For additional information on Kprobes, refer to the following URLs:
 http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
 http://www.redhat.com/magazine/005mar05/features/kprobes/
 
 For additional information on Kprobes, refer to the following URLs:
 http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
 http://www.redhat.com/magazine/005mar05/features/kprobes/

+http://www-users.cs.umn.edu/~boutcher/kprobes/
+http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
+
+
+Appendix A: The kprobes debugfs interface
+
+With recent kernels (> 2.6.20) the list of registered kprobes is visible
+under the /debug/kprobes/ directory (assuming debugfs is mounted at /debug).
+
+/debug/kprobes/list: Lists all registered probes on the system
+
+c015d71a  k  vfs_read+0x0
+c011a316  j  do_fork+0x0
+c03dedc5  r  tcp_v4_rcv+0x0
+
+The first column provides the kernel address where the probe is inserted.
+The second column identifies the type of probe (k - kprobe, r - kretprobe
+and j - jprobe), while the third column specifies the symbol+offset of
+the probe. If the probed function belongs to a module, the module name
+is also specified. Following columns show probe status. If the probe is on
+a virtual address that is no longer valid (module init sections, module
+virtual addresses that correspond to modules that've been unloaded),
+such probes are marked with [GONE].
+
+/debug/kprobes/enabled: Turn kprobes ON/OFF
+
+Provides a knob to globally turn registered kprobes ON or OFF. By default,
+all kprobes are enabled. By echoing "0" to this file, all registered probes
+will be disarmed, till such time a "1" is echoed to this file.