1Title : Kernel Probes (Kprobes)
2Authors : Jim Keniston <>
3    : Prasanna S Panchamukhi <>
4    : Masami Hiramatsu <>
81. Concepts: Kprobes, Jprobes, Return Probes
92. Architectures Supported
103. Configuring Kprobes
114. API Reference
125. Kprobes Features and Limitations
136. Probe Overhead
147. TODO
158. Kprobes Example
169. Jprobes Example
1710. Kretprobes Example
18Appendix A: The kprobes debugfs interface
19Appendix B: The kprobes sysctl interface
211. Concepts: Kprobes, Jprobes, Return Probes
23Kprobes enables you to dynamically break into any kernel routine and
24collect debugging and performance information non-disruptively. You
25can trap at almost any kernel code address, specifying a handler
26routine to be invoked when the breakpoint is hit.
28There are currently three types of probes: kprobes, jprobes, and
29kretprobes (also called return probes). A kprobe can be inserted
30on virtually any instruction in the kernel. A jprobe is inserted at
31the entry to a kernel function, and provides convenient access to the
32function's arguments. A return probe fires when a specified function
35In the typical case, Kprobes-based instrumentation is packaged as
36a kernel module. The module's init function installs ("registers")
37one or more probes, and the exit function unregisters them. A
38registration function such as register_kprobe() specifies where
39the probe is to be inserted and what handler is to be called when
40the probe is hit.
42There are also register_/unregister_*probes() functions for batch
43registration/unregistration of a group of *probes. These functions
44can speed up unregistration process when you have to unregister
45a lot of probes at once.
47The next four subsections explain how the different types of
48probes work and how jump optimization works. They explain certain
49things that you'll need to know in order to make the best use of
50Kprobes -- e.g., the difference between a pre_handler and
51a post_handler, and how to use the maxactive and nmissed fields of
52a kretprobe. But if you're in a hurry to start using Kprobes, you
53can skip ahead to section 2.
551.1 How Does a Kprobe Work?
57When a kprobe is registered, Kprobes makes a copy of the probed
58instruction and replaces the first byte(s) of the probed instruction
59with a breakpoint instruction (e.g., int3 on i386 and x86_64).
61When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
62registers are saved, and control passes to Kprobes via the
63notifier_call_chain mechanism. Kprobes executes the "pre_handler"
64associated with the kprobe, passing the handler the addresses of the
65kprobe struct and the saved registers.
67Next, Kprobes single-steps its copy of the probed instruction.
68(It would be simpler to single-step the actual instruction in place,
69but then Kprobes would have to temporarily remove the breakpoint
70instruction. This would open a small time window when another CPU
71could sail right past the probepoint.)
73After the instruction is single-stepped, Kprobes executes the
74"post_handler," if any, that is associated with the kprobe.
75Execution then continues with the instruction following the probepoint.
771.2 How Does a Jprobe Work?
79A jprobe is implemented using a kprobe that is placed on a function's
80entry point. It employs a simple mirroring principle to allow
81seamless access to the probed function's arguments. The jprobe
82handler routine should have the same signature (arg list and return
83type) as the function being probed, and must always end by calling
84the Kprobes function jprobe_return().
86Here's how it works. When the probe is hit, Kprobes makes a copy of
87the saved registers and a generous portion of the stack (see below).
88Kprobes then points the saved instruction pointer at the jprobe's
89handler routine, and returns from the trap. As a result, control
90passes to the handler, which is presented with the same register and
91stack contents as the probed function. When it is done, the handler
92calls jprobe_return(), which traps again to restore the original stack
93contents and processor state and switch to the probed function.
95By convention, the callee owns its arguments, so gcc may produce code
96that unexpectedly modifies that portion of the stack. This is why
97Kprobes saves a copy of the stack and restores it after the jprobe
98handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
9964 bytes on i386.
101Note that the probed function's args may be passed on the stack
102or in registers. The jprobe will work in either case, so long as the
103handler's prototype matches that of the probed function.
1051.3 Return Probes
1071.3.1 How Does a Return Probe Work?
109When you call register_kretprobe(), Kprobes establishes a kprobe at
110the entry to the function. When the probed function is called and this
111probe is hit, Kprobes saves a copy of the return address, and replaces
112the return address with the address of a "trampoline." The trampoline
113is an arbitrary piece of code -- typically just a nop instruction.
114At boot time, Kprobes registers a kprobe at the trampoline.
116When the probed function executes its return instruction, control
117passes to the trampoline and that probe is hit. Kprobes' trampoline
118handler calls the user-specified return handler associated with the
119kretprobe, then sets the saved instruction pointer to the saved return
120address, and that's where execution resumes upon return from the trap.
122While the probed function is executing, its return address is
123stored in an object of type kretprobe_instance. Before calling
124register_kretprobe(), the user sets the maxactive field of the
125kretprobe struct to specify how many instances of the specified
126function can be probed simultaneously. register_kretprobe()
127pre-allocates the indicated number of kretprobe_instance objects.
129For example, if the function is non-recursive and is called with a
130spinlock held, maxactive = 1 should be enough. If the function is
131non-recursive and can never relinquish the CPU (e.g., via a semaphore
132or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
133set to a default value. If CONFIG_PREEMPT is enabled, the default
134is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
136It's not a disaster if you set maxactive too low; you'll just miss
137some probes. In the kretprobe struct, the nmissed field is set to
138zero when the return probe is registered, and is incremented every
139time the probed function is entered but there is no kretprobe_instance
140object available for establishing the return probe.
1421.3.2 Kretprobe entry-handler
144Kretprobes also provides an optional user-specified handler which runs
145on function entry. This handler is specified by setting the entry_handler
146field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
147function entry is hit, the user-defined entry_handler, if any, is invoked.
148If the entry_handler returns 0 (success) then a corresponding return handler
149is guaranteed to be called upon function return. If the entry_handler
150returns a non-zero error then Kprobes leaves the return address as is, and
151the kretprobe has no further effect for that particular function instance.
153Multiple entry and return handler invocations are matched using the unique
154kretprobe_instance object associated with them. Additionally, a user
155may also specify per return-instance private data to be part of each
156kretprobe_instance object. This is especially useful when sharing private
157data between corresponding user entry and return handlers. The size of each
158private data object can be specified at kretprobe registration time by
159setting the data_size field of the kretprobe struct. This data can be
160accessed through the data field of each kretprobe_instance object.
162In case probed function is entered but there is no kretprobe_instance
163object available, then in addition to incrementing the nmissed count,
164the user entry_handler invocation is also skipped.
1661.4 How Does Jump Optimization Work?
168If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
169is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
170the "debug.kprobes_optimization" kernel parameter is set to 1 (see
171sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
172instruction instead of a breakpoint instruction at each probepoint.
1741.4.1 Init a Kprobe
176When a probe is registered, before attempting this optimization,
177Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
178address. So, even if it's not possible to optimize this particular
179probepoint, there'll be a probe there.
1811.4.2 Safety Check
183Before optimizing a probe, Kprobes performs the following safety checks:
185- Kprobes verifies that the region that will be replaced by the jump
186instruction (the "optimized region") lies entirely within one function.
187(A jump instruction is multiple bytes, and so may overlay multiple
190- Kprobes analyzes the entire function and verifies that there is no
191jump into the optimized region. Specifically:
192  - the function contains no indirect jump;
193  - the function contains no instruction that causes an exception (since
194  the fixup code triggered by the exception could jump back into the
195  optimized region -- Kprobes checks the exception tables to verify this);
196  and
197  - there is no near jump to the optimized region (other than to the first
198  byte).
200- For each instruction in the optimized region, Kprobes verifies that
201the instruction can be executed out of line.
2031.4.3 Preparing Detour Buffer
205Next, Kprobes prepares a "detour" buffer, which contains the following
206instruction sequence:
207- code to push the CPU's registers (emulating a breakpoint trap)
208- a call to the trampoline code which calls user's probe handlers.
209- code to restore registers
210- the instructions from the optimized region
211- a jump back to the original execution path.
2131.4.4 Pre-optimization
215After preparing the detour buffer, Kprobes verifies that none of the
216following situations exist:
217- The probe has either a break_handler (i.e., it's a jprobe) or a
219- Other instructions in the optimized region are probed.
220- The probe is disabled.
221In any of the above cases, Kprobes won't start optimizing the probe.
222Since these are temporary situations, Kprobes tries to start
223optimizing it again if the situation is changed.
225If the kprobe can be optimized, Kprobes enqueues the kprobe to an
226optimizing list, and kicks the kprobe-optimizer workqueue to optimize
227it. If the to-be-optimized probepoint is hit before being optimized,
228Kprobes returns control to the original instruction path by setting
229the CPU's instruction pointer to the copied code in the detour buffer
230-- thus at least avoiding the single-step.
2321.4.5 Optimization
234The Kprobe-optimizer doesn't insert the jump instruction immediately;
235rather, it calls synchronize_sched() for safety first, because it's
236possible for a CPU to be interrupted in the middle of executing the
237optimized region(*). As you know, synchronize_sched() can ensure
238that all interruptions that were active when synchronize_sched()
239was called are done, but only if CONFIG_PREEMPT=n. So, this version
240of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
242After that, the Kprobe-optimizer calls stop_machine() to replace
243the optimized region with a jump instruction to the detour buffer,
244using text_poke_smp().
2461.4.6 Unoptimization
248When an optimized kprobe is unregistered, disabled, or blocked by
249another kprobe, it will be unoptimized. If this happens before
250the optimization is complete, the kprobe is just dequeued from the
251optimized list. If the optimization has been done, the jump is
252replaced with the original code (except for an int3 breakpoint in
253the first byte) by using text_poke_smp().
255(*)Please imagine that the 2nd instruction is interrupted and then
256the optimizer replaces the 2nd instruction with the jump *address*
257while the interrupt handler is running. When the interrupt
258returns to original address, there is no valid instruction,
259and it causes an unexpected result.
261(**)This optimization-safety checking may be replaced with the
262stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
265NOTE for geeks:
266The jump optimization changes the kprobe's pre_handler behavior.
267Without optimization, the pre_handler can change the kernel's execution
268path by changing regs->ip and returning 1. However, when the probe
269is optimized, that modification is ignored. Thus, if you want to
270tweak the kernel's execution path, you need to suppress optimization,
271using one of the following techniques:
272- Specify an empty function for the kprobe's post_handler or break_handler.
273 or
274- Execute 'sysctl -w debug.kprobes_optimization=n'
2762. Architectures Supported
278Kprobes, jprobes, and return probes are implemented on the following
281- i386 (Supports jump optimization)
282- x86_64 (AMD-64, EM64T) (Supports jump optimization)
283- ppc64
284- ia64 (Does not support probes on instruction slot1.)
285- sparc64 (Return probes not yet implemented.)
286- arm
287- ppc
288- mips
2903. Configuring Kprobes
292When configuring the kernel using make menuconfig/xconfig/oldconfig,
293ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation
294Support", look for "Kprobes".
296So that you can load and unload Kprobes-based instrumentation modules,
297make sure "Loadable module support" (CONFIG_MODULES) and "Module
298unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
300Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
301are set to "y", since kallsyms_lookup_name() is used by the in-kernel
302kprobe address resolution code.
304If you need to insert a probe in the middle of a function, you may find
305it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
306so you can use "objdump -d -l vmlinux" to see the source-to-object
307code mapping.
3094. API Reference
311The Kprobes API includes a "register" function and an "unregister"
312function for each type of probe. The API also includes "register_*probes"
313and "unregister_*probes" functions for (un)registering arrays of probes.
314Here are terse, mini-man-page specifications for these functions and
315the associated probe handlers that you'll write. See the files in the
316samples/kprobes/ sub-directory for examples.
3184.1 register_kprobe
320#include <linux/kprobes.h>
321int register_kprobe(struct kprobe *kp);
323Sets a breakpoint at the address kp->addr. When the breakpoint is
324hit, Kprobes calls kp->pre_handler. After the probed instruction
325is single-stepped, Kprobe calls kp->post_handler. If a fault
326occurs during execution of kp->pre_handler or kp->post_handler,
327or during single-stepping of the probed instruction, Kprobes calls
328kp->fault_handler. Any or all handlers can be NULL. If kp->flags
329is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
330so, its handlers aren't hit until calling enable_kprobe(kp).
3331. With the introduction of the "symbol_name" field to struct kprobe,
334the probepoint address resolution will now be taken care of by the kernel.
335The following will now work:
337    kp.symbol_name = "symbol_name";
339(64-bit powerpc intricacies such as function descriptors are handled
3422. Use the "offset" field of struct kprobe if the offset into the symbol
343to install a probepoint is known. This field is used to calculate the
3463. Specify either the kprobe "symbol_name" OR the "addr". If both are
347specified, kprobe registration will fail with -EINVAL.
3494. With CISC architectures (such as i386 and x86_64), the kprobes code
350does not validate if the kprobe.addr is at an instruction boundary.
351Use "offset" with caution.
353register_kprobe() returns 0 on success, or a negative errno otherwise.
355User's pre-handler (kp->pre_handler):
356#include <linux/kprobes.h>
357#include <linux/ptrace.h>
358int pre_handler(struct kprobe *p, struct pt_regs *regs);
360Called with p pointing to the kprobe associated with the breakpoint,
361and regs pointing to the struct containing the registers saved when
362the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
364User's post-handler (kp->post_handler):
365#include <linux/kprobes.h>
366#include <linux/ptrace.h>
367void post_handler(struct kprobe *p, struct pt_regs *regs,
368    unsigned long flags);
370p and regs are as described for the pre_handler. flags always seems
371to be zero.
373User's fault-handler (kp->fault_handler):
374#include <linux/kprobes.h>
375#include <linux/ptrace.h>
376int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
378p and regs are as described for the pre_handler. trapnr is the
379architecture-specific trap number associated with the fault (e.g.,
380on i386, 13 for a general protection fault or 14 for a page fault).
381Returns 1 if it successfully handled the exception.
3834.2 register_jprobe
385#include <linux/kprobes.h>
386int register_jprobe(struct jprobe *jp)
388Sets a breakpoint at the address jp->kp.addr, which must be the address
389of the first instruction of a function. When the breakpoint is hit,
390Kprobes runs the handler whose address is jp->entry.
392The handler should have the same arg list and return type as the probed
393function; and just before it returns, it must call jprobe_return().
394(The handler never actually returns, since jprobe_return() returns
395control to Kprobes.) If the probed function is declared asmlinkage
396or anything else that affects how args are passed, the handler's
397declaration must match.
399register_jprobe() returns 0 on success, or a negative errno otherwise.
4014.3 register_kretprobe
403#include <linux/kprobes.h>
404int register_kretprobe(struct kretprobe *rp);
406Establishes a return probe for the function whose address is
407rp->kp.addr. When that function returns, Kprobes calls rp->handler.
408You must set rp->maxactive appropriately before you call
409register_kretprobe(); see "How Does a Return Probe Work?" for details.
411register_kretprobe() returns 0 on success, or a negative errno
414User's return-probe handler (rp->handler):
415#include <linux/kprobes.h>
416#include <linux/ptrace.h>
417int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
419regs is as described for kprobe.pre_handler. ri points to the
420kretprobe_instance object, of which the following fields may be
421of interest:
422- ret_addr: the return address
423- rp: points to the corresponding kretprobe object
424- task: points to the corresponding task struct
425- data: points to per return-instance private data; see "Kretprobe
426    entry-handler" for details.
428The regs_return_value(regs) macro provides a simple abstraction to
429extract the return value from the appropriate register as defined by
430the architecture's ABI.
432The handler's return value is currently ignored.
4344.4 unregister_*probe
436#include <linux/kprobes.h>
437void unregister_kprobe(struct kprobe *kp);
438void unregister_jprobe(struct jprobe *jp);
439void unregister_kretprobe(struct kretprobe *rp);
441Removes the specified probe. The unregister function can be called
442at any time after the probe has been registered.
445If the functions find an incorrect probe (ex. an unregistered probe),
446they clear the addr field of the probe.
4484.5 register_*probes
450#include <linux/kprobes.h>
451int register_kprobes(struct kprobe **kps, int num);
452int register_kretprobes(struct kretprobe **rps, int num);
453int register_jprobes(struct jprobe **jps, int num);
455Registers each of the num probes in the specified array. If any
456error occurs during registration, all probes in the array, up to
457the bad probe, are safely unregistered before the register_*probes
458function returns.
459- kps/rps/jps: an array of pointers to *probe data structures
460- num: the number of the array entries.
463You have to allocate(or define) an array of pointers and set all
464of the array entries before using these functions.
4664.6 unregister_*probes
468#include <linux/kprobes.h>
469void unregister_kprobes(struct kprobe **kps, int num);
470void unregister_kretprobes(struct kretprobe **rps, int num);
471void unregister_jprobes(struct jprobe **jps, int num);
473Removes each of the num probes in the specified array at once.
476If the functions find some incorrect probes (ex. unregistered
477probes) in the specified array, they clear the addr field of those
478incorrect probes. However, other probes in the array are
479unregistered correctly.
4814.7 disable_*probe
483#include <linux/kprobes.h>
484int disable_kprobe(struct kprobe *kp);
485int disable_kretprobe(struct kretprobe *rp);
486int disable_jprobe(struct jprobe *jp);
488Temporarily disables the specified *probe. You can enable it again by using
489enable_*probe(). You must specify the probe which has been registered.
4914.8 enable_*probe
493#include <linux/kprobes.h>
494int enable_kprobe(struct kprobe *kp);
495int enable_kretprobe(struct kretprobe *rp);
496int enable_jprobe(struct jprobe *jp);
498Enables *probe which has been disabled by disable_*probe(). You must specify
499the probe which has been registered.
5015. Kprobes Features and Limitations
503Kprobes allows multiple probes at the same address. Currently,
504however, there cannot be multiple jprobes on the same function at
505the same time. Also, a probepoint for which there is a jprobe or
506a post_handler cannot be optimized. So if you install a jprobe,
507or a kprobe with a post_handler, at an optimized probepoint, the
508probepoint will be unoptimized automatically.
510In general, you can install a probe anywhere in the kernel.
511In particular, you can probe interrupt handlers. Known exceptions
512are discussed in this section.
514The register_*probe functions will return -EINVAL if you attempt
515to install a probe in the code that implements Kprobes (mostly
516kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
517as do_page_fault and notifier_call_chain).
519If you install a probe in an inline-able function, Kprobes makes
520no attempt to chase down all inline instances of the function and
521install probes there. gcc may inline a function without being asked,
522so keep this in mind if you're not seeing the probe hits you expect.
524A probe handler can modify the environment of the probed function
525-- e.g., by modifying kernel data structures, or by modifying the
526contents of the pt_regs struct (which are restored to the registers
527upon return from the breakpoint). So Kprobes can be used, for example,
528to install a bug fix or to inject faults for testing. Kprobes, of
529course, has no way to distinguish the deliberately injected faults
530from the accidental ones. Don't drink and probe.
532Kprobes makes no attempt to prevent probe handlers from stepping on
533each other -- e.g., probing printk() and then calling printk() from a
534probe handler. If a probe handler hits a probe, that second probe's
535handlers won't be run in that instance, and the kprobe.nmissed member
536of the second probe will be incremented.
538As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
539the same handler) may run concurrently on different CPUs.
541Kprobes does not use mutexes or allocate memory except during
542registration and unregistration.
544Probe handlers are run with preemption disabled. Depending on the
545architecture and optimization state, handlers may also run with
546interrupts disabled (e.g., kretprobe handlers and optimized kprobe
547handlers run without interrupt disabled on x86/x86-64). In any case,
548your handler should not yield the CPU (e.g., by attempting to acquire
549a semaphore).
551Since a return probe is implemented by replacing the return
552address with the trampoline's address, stack backtraces and calls
553to __builtin_return_address() will typically yield the trampoline's
554address instead of the real return address for kretprobed functions.
555(As far as we can tell, __builtin_return_address() is used only
556for instrumentation and error reporting.)
558If the number of times a function is called does not match the number
559of times it returns, registering a return probe on that function may
560produce undesirable results. In such a case, a line:
561kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
562gets printed. With this information, one will be able to correlate the
563exact instance of the kretprobe that caused the problem. We have the
564do_exit() case covered. do_execve() and do_fork() are not an issue.
565We're unaware of other specific cases where this could be a problem.
567If, upon entry to or exit from a function, the CPU is running on
568a stack other than that of the current task, registering a return
569probe on that function may produce undesirable results. For this
570reason, Kprobes doesn't support return probes (or kprobes or jprobes)
571on the x86_64 version of __switch_to(); the registration functions
572return -EINVAL.
574On x86/x86-64, since the Jump Optimization of Kprobes modifies
575instructions widely, there are some limitations to optimization. To
576explain it, we introduce some terminology. Imagine a 3-instruction
577sequence consisting of a two 2-byte instructions and one 3-byte
580        IA
581         |
583        [ins1][ins2][ ins3 ]
584    [<- DCR ->]
585       [<- JTPR ->]
587ins1: 1st Instruction
588ins2: 2nd Instruction
589ins3: 3rd Instruction
590IA: Insertion Address
591JTPR: Jump Target Prohibition Region
592DCR: Detoured Code Region
594The instructions in DCR are copied to the out-of-line buffer
595of the kprobe, because the bytes in DCR are replaced by
596a 5-byte jump instruction. So there are several limitations.
598a) The instructions in DCR must be relocatable.
599b) The instructions in DCR must not include a call instruction.
600c) JTPR must not be targeted by any jump or call instruction.
601d) DCR must not straddle the border between functions.
603Anyway, these limitations are checked by the in-kernel instruction
604decoder, so you don't need to worry about that.
6066. Probe Overhead
608On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
609microseconds to process. Specifically, a benchmark that hits the same
610probepoint repeatedly, firing a simple handler each time, reports 1-2
611million hits per second, depending on the architecture. A jprobe or
612return-probe hit typically takes 50-75% longer than a kprobe hit.
613When you have a return probe set on a function, adding a kprobe at
614the entry to that function adds essentially no overhead.
616Here are sample overhead figures (in usec) for different architectures.
617k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
618on same function; jr = jprobe + return probe on same function
620i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
621k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
623x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
624k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
626ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
627k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
6296.1 Optimized Probe Overhead
631Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
632process. Here are sample overhead figures (in usec) for x86 architectures.
633k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
634r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
636i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
637k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
639x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
640k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
6427. TODO
644a. SystemTap ( Provides a simplified
645programming interface for probe-based instrumentation. Try it out.
646b. Kernel return probes for sparc64.
647c. Support for other architectures.
648d. User-space probes.
649e. Watchpoint probes (which fire on data references).
6518. Kprobes Example
653See samples/kprobes/kprobe_example.c
6559. Jprobes Example
657See samples/kprobes/jprobe_example.c
65910. Kretprobes Example
661See samples/kprobes/kretprobe_example.c
663For additional information on Kprobes, refer to the following URLs:
667 (pages 101-115)
670Appendix A: The kprobes debugfs interface
672With recent kernels (> 2.6.20) the list of registered kprobes is visible
673under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
675/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
677c015d71a k vfs_read+0x0
678c011a316 j do_fork+0x0
679c03dedc5 r tcp_v4_rcv+0x0
681The first column provides the kernel address where the probe is inserted.
682The second column identifies the type of probe (k - kprobe, r - kretprobe
683and j - jprobe), while the third column specifies the symbol+offset of
684the probe. If the probed function belongs to a module, the module name
685is also specified. Following columns show probe status. If the probe is on
686a virtual address that is no longer valid (module init sections, module
687virtual addresses that correspond to modules that've been unloaded),
688such probes are marked with [GONE]. If the probe is temporarily disabled,
689such probes are marked with [DISABLED]. If the probe is optimized, it is
690marked with [OPTIMIZED].
692/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
694Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
695By default, all kprobes are enabled. By echoing "0" to this file, all
696registered probes will be disarmed, till such time a "1" is echoed to this
697file. Note that this knob just disarms and arms all kprobes and doesn't
698change each probe's disabling state. This means that disabled kprobes (marked
699[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
702Appendix B: The kprobes sysctl interface
704/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
706When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
707a knob to globally and forcibly turn jump optimization (see section
7081.4) ON or OFF. By default, jump optimization is allowed (ON).
709If you echo "0" to this file or set "debug.kprobes_optimization" to
7100 via sysctl, all optimized probes will be unoptimized, and any new
711probes registered after that will not be optimized. Note that this
712knob *changes* the optimized state. This means that optimized probes
713(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
714removed). If the knob is turned on, they will be optimized again.

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