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Jim Kenistond27a4dd2005-08-04 12:53:35 -07001Title : Kernel Probes (Kprobes)
2Authors : Jim Keniston <jkenisto@us.ibm.com>
Masami Hiramatsub26486b2010-02-25 08:35:04 -05003 : Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
4 : Masami Hiramatsu <mhiramat@redhat.com>
Jim Kenistond27a4dd2005-08-04 12:53:35 -07005
6CONTENTS
7
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
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -070018Appendix A: The kprobes debugfs interface
Masami Hiramatsub26486b2010-02-25 08:35:04 -050019Appendix B: The kprobes sysctl interface
Jim Kenistond27a4dd2005-08-04 12:53:35 -070020
211. Concepts: Kprobes, Jprobes, Return Probes
22
23Kprobes enables you to dynamically break into any kernel routine and
24collect debugging and performance information non-disruptively. You
Masami Hiramatsu376e2422014-04-17 17:17:05 +090025can trap at almost any kernel code address(*), specifying a handler
Jim Kenistond27a4dd2005-08-04 12:53:35 -070026routine to be invoked when the breakpoint is hit.
Masami Hiramatsu376e2422014-04-17 17:17:05 +090027(*: some parts of the kernel code can not be trapped, see 1.5 Blacklist)
Jim Kenistond27a4dd2005-08-04 12:53:35 -070028
29There are currently three types of probes: kprobes, jprobes, and
30kretprobes (also called return probes). A kprobe can be inserted
31on virtually any instruction in the kernel. A jprobe is inserted at
32the entry to a kernel function, and provides convenient access to the
33function's arguments. A return probe fires when a specified function
34returns.
35
36In the typical case, Kprobes-based instrumentation is packaged as
37a kernel module. The module's init function installs ("registers")
38one or more probes, and the exit function unregisters them. A
39registration function such as register_kprobe() specifies where
40the probe is to be inserted and what handler is to be called when
41the probe is hit.
42
Masami Hiramatsu3b0cb4c2008-04-28 02:14:30 -070043There are also register_/unregister_*probes() functions for batch
44registration/unregistration of a group of *probes. These functions
45can speed up unregistration process when you have to unregister
46a lot of probes at once.
47
Masami Hiramatsub26486b2010-02-25 08:35:04 -050048The next four subsections explain how the different types of
49probes work and how jump optimization works. They explain certain
50things that you'll need to know in order to make the best use of
51Kprobes -- e.g., the difference between a pre_handler and
52a post_handler, and how to use the maxactive and nmissed fields of
53a kretprobe. But if you're in a hurry to start using Kprobes, you
54can skip ahead to section 2.
Jim Kenistond27a4dd2005-08-04 12:53:35 -070055
561.1 How Does a Kprobe Work?
57
58When a kprobe is registered, Kprobes makes a copy of the probed
59instruction and replaces the first byte(s) of the probed instruction
60with a breakpoint instruction (e.g., int3 on i386 and x86_64).
61
62When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
63registers are saved, and control passes to Kprobes via the
64notifier_call_chain mechanism. Kprobes executes the "pre_handler"
65associated with the kprobe, passing the handler the addresses of the
66kprobe struct and the saved registers.
67
68Next, Kprobes single-steps its copy of the probed instruction.
69(It would be simpler to single-step the actual instruction in place,
70but then Kprobes would have to temporarily remove the breakpoint
71instruction. This would open a small time window when another CPU
72could sail right past the probepoint.)
73
74After the instruction is single-stepped, Kprobes executes the
75"post_handler," if any, that is associated with the kprobe.
76Execution then continues with the instruction following the probepoint.
77
781.2 How Does a Jprobe Work?
79
80A jprobe is implemented using a kprobe that is placed on a function's
81entry point. It employs a simple mirroring principle to allow
82seamless access to the probed function's arguments. The jprobe
83handler routine should have the same signature (arg list and return
84type) as the function being probed, and must always end by calling
85the Kprobes function jprobe_return().
86
87Here's how it works. When the probe is hit, Kprobes makes a copy of
88the saved registers and a generous portion of the stack (see below).
89Kprobes then points the saved instruction pointer at the jprobe's
90handler routine, and returns from the trap. As a result, control
91passes to the handler, which is presented with the same register and
92stack contents as the probed function. When it is done, the handler
93calls jprobe_return(), which traps again to restore the original stack
94contents and processor state and switch to the probed function.
95
96By convention, the callee owns its arguments, so gcc may produce code
97that unexpectedly modifies that portion of the stack. This is why
98Kprobes saves a copy of the stack and restores it after the jprobe
99handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g.,
10064 bytes on i386.
101
102Note that the probed function's args may be passed on the stack
Harvey Harrisonb5606c22008-02-13 15:03:16 -0800103or in registers. The jprobe will work in either case, so long as the
104handler's prototype matches that of the probed function.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700105
Abhishek Sagarf47cd9b2008-02-06 01:38:22 -08001061.3 Return Probes
107
1081.3.1 How Does a Return Probe Work?
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700109
110When you call register_kretprobe(), Kprobes establishes a kprobe at
111the entry to the function. When the probed function is called and this
112probe is hit, Kprobes saves a copy of the return address, and replaces
113the return address with the address of a "trampoline." The trampoline
114is an arbitrary piece of code -- typically just a nop instruction.
115At boot time, Kprobes registers a kprobe at the trampoline.
116
117When the probed function executes its return instruction, control
118passes to the trampoline and that probe is hit. Kprobes' trampoline
Abhishek Sagarf47cd9b2008-02-06 01:38:22 -0800119handler calls the user-specified return handler associated with the
120kretprobe, then sets the saved instruction pointer to the saved return
121address, and that's where execution resumes upon return from the trap.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700122
123While the probed function is executing, its return address is
124stored in an object of type kretprobe_instance. Before calling
125register_kretprobe(), the user sets the maxactive field of the
126kretprobe struct to specify how many instances of the specified
127function can be probed simultaneously. register_kretprobe()
128pre-allocates the indicated number of kretprobe_instance objects.
129
130For example, if the function is non-recursive and is called with a
131spinlock held, maxactive = 1 should be enough. If the function is
132non-recursive and can never relinquish the CPU (e.g., via a semaphore
133or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
134set to a default value. If CONFIG_PREEMPT is enabled, the default
135is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS.
136
137It's not a disaster if you set maxactive too low; you'll just miss
138some probes. In the kretprobe struct, the nmissed field is set to
139zero when the return probe is registered, and is incremented every
140time the probed function is entered but there is no kretprobe_instance
141object available for establishing the return probe.
142
Abhishek Sagarf47cd9b2008-02-06 01:38:22 -08001431.3.2 Kretprobe entry-handler
144
145Kretprobes also provides an optional user-specified handler which runs
146on function entry. This handler is specified by setting the entry_handler
147field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
148function entry is hit, the user-defined entry_handler, if any, is invoked.
149If the entry_handler returns 0 (success) then a corresponding return handler
150is guaranteed to be called upon function return. If the entry_handler
151returns a non-zero error then Kprobes leaves the return address as is, and
152the kretprobe has no further effect for that particular function instance.
153
154Multiple entry and return handler invocations are matched using the unique
155kretprobe_instance object associated with them. Additionally, a user
156may also specify per return-instance private data to be part of each
157kretprobe_instance object. This is especially useful when sharing private
158data between corresponding user entry and return handlers. The size of each
159private data object can be specified at kretprobe registration time by
160setting the data_size field of the kretprobe struct. This data can be
161accessed through the data field of each kretprobe_instance object.
162
163In case probed function is entered but there is no kretprobe_instance
164object available, then in addition to incrementing the nmissed count,
165the user entry_handler invocation is also skipped.
166
Masami Hiramatsub26486b2010-02-25 08:35:04 -05001671.4 How Does Jump Optimization Work?
168
Masami Hiramatsu5cc718b2010-03-15 13:00:54 -0400169If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
170is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500171the "debug.kprobes_optimization" kernel parameter is set to 1 (see
172sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
173instruction instead of a breakpoint instruction at each probepoint.
174
1751.4.1 Init a Kprobe
176
177When a probe is registered, before attempting this optimization,
178Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
179address. So, even if it's not possible to optimize this particular
180probepoint, there'll be a probe there.
181
1821.4.2 Safety Check
183
184Before optimizing a probe, Kprobes performs the following safety checks:
185
186- Kprobes verifies that the region that will be replaced by the jump
187instruction (the "optimized region") lies entirely within one function.
188(A jump instruction is multiple bytes, and so may overlay multiple
189instructions.)
190
191- Kprobes analyzes the entire function and verifies that there is no
192jump into the optimized region. Specifically:
193 - the function contains no indirect jump;
194 - the function contains no instruction that causes an exception (since
195 the fixup code triggered by the exception could jump back into the
196 optimized region -- Kprobes checks the exception tables to verify this);
197 and
198 - there is no near jump to the optimized region (other than to the first
199 byte).
200
201- For each instruction in the optimized region, Kprobes verifies that
202the instruction can be executed out of line.
203
2041.4.3 Preparing Detour Buffer
205
206Next, Kprobes prepares a "detour" buffer, which contains the following
207instruction sequence:
208- code to push the CPU's registers (emulating a breakpoint trap)
209- a call to the trampoline code which calls user's probe handlers.
210- code to restore registers
211- the instructions from the optimized region
212- a jump back to the original execution path.
213
2141.4.4 Pre-optimization
215
216After preparing the detour buffer, Kprobes verifies that none of the
217following situations exist:
218- The probe has either a break_handler (i.e., it's a jprobe) or a
219post_handler.
220- Other instructions in the optimized region are probed.
221- The probe is disabled.
222In any of the above cases, Kprobes won't start optimizing the probe.
223Since these are temporary situations, Kprobes tries to start
224optimizing it again if the situation is changed.
225
226If the kprobe can be optimized, Kprobes enqueues the kprobe to an
227optimizing list, and kicks the kprobe-optimizer workqueue to optimize
228it. If the to-be-optimized probepoint is hit before being optimized,
229Kprobes returns control to the original instruction path by setting
230the CPU's instruction pointer to the copied code in the detour buffer
231-- thus at least avoiding the single-step.
232
2331.4.5 Optimization
234
235The Kprobe-optimizer doesn't insert the jump instruction immediately;
236rather, it calls synchronize_sched() for safety first, because it's
237possible for a CPU to be interrupted in the middle of executing the
238optimized region(*). As you know, synchronize_sched() can ensure
239that all interruptions that were active when synchronize_sched()
240was called are done, but only if CONFIG_PREEMPT=n. So, this version
241of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**)
242
243After that, the Kprobe-optimizer calls stop_machine() to replace
244the optimized region with a jump instruction to the detour buffer,
245using text_poke_smp().
246
2471.4.6 Unoptimization
248
249When an optimized kprobe is unregistered, disabled, or blocked by
250another kprobe, it will be unoptimized. If this happens before
251the optimization is complete, the kprobe is just dequeued from the
252optimized list. If the optimization has been done, the jump is
253replaced with the original code (except for an int3 breakpoint in
254the first byte) by using text_poke_smp().
255
256(*)Please imagine that the 2nd instruction is interrupted and then
257the optimizer replaces the 2nd instruction with the jump *address*
258while the interrupt handler is running. When the interrupt
259returns to original address, there is no valid instruction,
260and it causes an unexpected result.
261
262(**)This optimization-safety checking may be replaced with the
263stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
264kernel.
265
266NOTE for geeks:
267The jump optimization changes the kprobe's pre_handler behavior.
268Without optimization, the pre_handler can change the kernel's execution
269path by changing regs->ip and returning 1. However, when the probe
270is optimized, that modification is ignored. Thus, if you want to
271tweak the kernel's execution path, you need to suppress optimization,
272using one of the following techniques:
273- Specify an empty function for the kprobe's post_handler or break_handler.
274 or
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500275- Execute 'sysctl -w debug.kprobes_optimization=n'
276
Masami Hiramatsu376e2422014-04-17 17:17:05 +09002771.5 Blacklist
278
279Kprobes can probe most of the kernel except itself. This means
280that there are some functions where kprobes cannot probe. Probing
281(trapping) such functions can cause a recursive trap (e.g. double
282fault) or the nested probe handler may never be called.
283Kprobes manages such functions as a blacklist.
284If you want to add a function into the blacklist, you just need
285to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
286to specify a blacklisted function.
287Kprobes checks the given probe address against the blacklist and
288rejects registering it, if the given address is in the blacklist.
289
Jim Kenistond27a4dd2005-08-04 12:53:35 -07002902. Architectures Supported
291
292Kprobes, jprobes, and return probes are implemented on the following
293architectures:
294
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500295- i386 (Supports jump optimization)
296- x86_64 (AMD-64, EM64T) (Supports jump optimization)
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700297- ppc64
Jim Keniston8861da32006-02-14 13:53:06 -0800298- ia64 (Does not support probes on instruction slot1.)
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700299- sparc64 (Return probes not yet implemented.)
Nicolas Pitre5de865b2007-12-03 17:15:52 -0500300- arm
Kumar Galaf8279622008-06-26 02:01:37 -0500301- ppc
David Daney9bb4d9d2010-08-03 11:22:22 -0700302- mips
Heiko Carstens369e8c32014-08-27 07:51:05 +0200303- s390
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700304
3053. Configuring Kprobes
306
307When configuring the kernel using make menuconfig/xconfig/oldconfig,
Jim Keniston8861da32006-02-14 13:53:06 -0800308ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation
309Support", look for "Kprobes".
310
311So that you can load and unload Kprobes-based instrumentation modules,
312make sure "Loadable module support" (CONFIG_MODULES) and "Module
313unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700314
Ananth N Mavinakayanahalli09b18202006-10-02 02:17:32 -0700315Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
316are set to "y", since kallsyms_lookup_name() is used by the in-kernel
317kprobe address resolution code.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700318
319If you need to insert a probe in the middle of a function, you may find
320it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
321so you can use "objdump -d -l vmlinux" to see the source-to-object
322code mapping.
323
3244. API Reference
325
326The Kprobes API includes a "register" function and an "unregister"
Masami Hiramatsu3b0cb4c2008-04-28 02:14:30 -0700327function for each type of probe. The API also includes "register_*probes"
328and "unregister_*probes" functions for (un)registering arrays of probes.
329Here are terse, mini-man-page specifications for these functions and
330the associated probe handlers that you'll write. See the files in the
331samples/kprobes/ sub-directory for examples.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700332
3334.1 register_kprobe
334
335#include <linux/kprobes.h>
336int register_kprobe(struct kprobe *kp);
337
338Sets a breakpoint at the address kp->addr. When the breakpoint is
339hit, Kprobes calls kp->pre_handler. After the probed instruction
340is single-stepped, Kprobe calls kp->post_handler. If a fault
341occurs during execution of kp->pre_handler or kp->post_handler,
342or during single-stepping of the probed instruction, Kprobes calls
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700343kp->fault_handler. Any or all handlers can be NULL. If kp->flags
344is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled,
Francis Galieguea33f3222010-04-23 00:08:02 +0200345so, its handlers aren't hit until calling enable_kprobe(kp).
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700346
Ananth N Mavinakayanahalli09b18202006-10-02 02:17:32 -0700347NOTE:
3481. With the introduction of the "symbol_name" field to struct kprobe,
349the probepoint address resolution will now be taken care of by the kernel.
350The following will now work:
351
352 kp.symbol_name = "symbol_name";
353
354(64-bit powerpc intricacies such as function descriptors are handled
355transparently)
356
3572. Use the "offset" field of struct kprobe if the offset into the symbol
358to install a probepoint is known. This field is used to calculate the
359probepoint.
360
3613. Specify either the kprobe "symbol_name" OR the "addr". If both are
362specified, kprobe registration will fail with -EINVAL.
363
3644. With CISC architectures (such as i386 and x86_64), the kprobes code
365does not validate if the kprobe.addr is at an instruction boundary.
366Use "offset" with caution.
367
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700368register_kprobe() returns 0 on success, or a negative errno otherwise.
369
370User's pre-handler (kp->pre_handler):
371#include <linux/kprobes.h>
372#include <linux/ptrace.h>
373int pre_handler(struct kprobe *p, struct pt_regs *regs);
374
375Called with p pointing to the kprobe associated with the breakpoint,
376and regs pointing to the struct containing the registers saved when
377the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
378
379User's post-handler (kp->post_handler):
380#include <linux/kprobes.h>
381#include <linux/ptrace.h>
382void post_handler(struct kprobe *p, struct pt_regs *regs,
383 unsigned long flags);
384
385p and regs are as described for the pre_handler. flags always seems
386to be zero.
387
388User's fault-handler (kp->fault_handler):
389#include <linux/kprobes.h>
390#include <linux/ptrace.h>
391int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr);
392
393p and regs are as described for the pre_handler. trapnr is the
394architecture-specific trap number associated with the fault (e.g.,
395on i386, 13 for a general protection fault or 14 for a page fault).
396Returns 1 if it successfully handled the exception.
397
3984.2 register_jprobe
399
400#include <linux/kprobes.h>
401int register_jprobe(struct jprobe *jp)
402
403Sets a breakpoint at the address jp->kp.addr, which must be the address
404of the first instruction of a function. When the breakpoint is hit,
405Kprobes runs the handler whose address is jp->entry.
406
407The handler should have the same arg list and return type as the probed
408function; and just before it returns, it must call jprobe_return().
409(The handler never actually returns, since jprobe_return() returns
Harvey Harrisonb5606c22008-02-13 15:03:16 -0800410control to Kprobes.) If the probed function is declared asmlinkage
411or anything else that affects how args are passed, the handler's
412declaration must match.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700413
414register_jprobe() returns 0 on success, or a negative errno otherwise.
415
4164.3 register_kretprobe
417
418#include <linux/kprobes.h>
419int register_kretprobe(struct kretprobe *rp);
420
421Establishes a return probe for the function whose address is
422rp->kp.addr. When that function returns, Kprobes calls rp->handler.
423You must set rp->maxactive appropriately before you call
424register_kretprobe(); see "How Does a Return Probe Work?" for details.
425
426register_kretprobe() returns 0 on success, or a negative errno
427otherwise.
428
429User's return-probe handler (rp->handler):
430#include <linux/kprobes.h>
431#include <linux/ptrace.h>
432int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs);
433
434regs is as described for kprobe.pre_handler. ri points to the
435kretprobe_instance object, of which the following fields may be
436of interest:
437- ret_addr: the return address
438- rp: points to the corresponding kretprobe object
439- task: points to the corresponding task struct
Abhishek Sagarf47cd9b2008-02-06 01:38:22 -0800440- data: points to per return-instance private data; see "Kretprobe
441 entry-handler" for details.
Ananth N Mavinakayanahalli09b18202006-10-02 02:17:32 -0700442
443The regs_return_value(regs) macro provides a simple abstraction to
444extract the return value from the appropriate register as defined by
445the architecture's ABI.
446
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700447The handler's return value is currently ignored.
448
4494.4 unregister_*probe
450
451#include <linux/kprobes.h>
452void unregister_kprobe(struct kprobe *kp);
453void unregister_jprobe(struct jprobe *jp);
454void unregister_kretprobe(struct kretprobe *rp);
455
456Removes the specified probe. The unregister function can be called
457at any time after the probe has been registered.
458
Masami Hiramatsu3b0cb4c2008-04-28 02:14:30 -0700459NOTE:
460If the functions find an incorrect probe (ex. an unregistered probe),
461they clear the addr field of the probe.
462
4634.5 register_*probes
464
465#include <linux/kprobes.h>
466int register_kprobes(struct kprobe **kps, int num);
467int register_kretprobes(struct kretprobe **rps, int num);
468int register_jprobes(struct jprobe **jps, int num);
469
470Registers each of the num probes in the specified array. If any
471error occurs during registration, all probes in the array, up to
472the bad probe, are safely unregistered before the register_*probes
473function returns.
474- kps/rps/jps: an array of pointers to *probe data structures
475- num: the number of the array entries.
476
477NOTE:
478You have to allocate(or define) an array of pointers and set all
479of the array entries before using these functions.
480
4814.6 unregister_*probes
482
483#include <linux/kprobes.h>
484void unregister_kprobes(struct kprobe **kps, int num);
485void unregister_kretprobes(struct kretprobe **rps, int num);
486void unregister_jprobes(struct jprobe **jps, int num);
487
488Removes each of the num probes in the specified array at once.
489
490NOTE:
491If the functions find some incorrect probes (ex. unregistered
492probes) in the specified array, they clear the addr field of those
493incorrect probes. However, other probes in the array are
494unregistered correctly.
495
Masami Hiramatsu8f9b1522009-04-06 19:01:02 -07004964.7 disable_*probe
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700497
498#include <linux/kprobes.h>
499int disable_kprobe(struct kprobe *kp);
Masami Hiramatsu8f9b1522009-04-06 19:01:02 -0700500int disable_kretprobe(struct kretprobe *rp);
501int disable_jprobe(struct jprobe *jp);
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700502
Masami Hiramatsu8f9b1522009-04-06 19:01:02 -0700503Temporarily disables the specified *probe. You can enable it again by using
504enable_*probe(). You must specify the probe which has been registered.
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700505
Masami Hiramatsu8f9b1522009-04-06 19:01:02 -07005064.8 enable_*probe
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700507
508#include <linux/kprobes.h>
509int enable_kprobe(struct kprobe *kp);
Masami Hiramatsu8f9b1522009-04-06 19:01:02 -0700510int enable_kretprobe(struct kretprobe *rp);
511int enable_jprobe(struct jprobe *jp);
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700512
Masami Hiramatsu8f9b1522009-04-06 19:01:02 -0700513Enables *probe which has been disabled by disable_*probe(). You must specify
514the probe which has been registered.
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700515
Jim Kenistond27a4dd2005-08-04 12:53:35 -07005165. Kprobes Features and Limitations
517
Jim Keniston8861da32006-02-14 13:53:06 -0800518Kprobes allows multiple probes at the same address. Currently,
519however, there cannot be multiple jprobes on the same function at
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500520the same time. Also, a probepoint for which there is a jprobe or
521a post_handler cannot be optimized. So if you install a jprobe,
522or a kprobe with a post_handler, at an optimized probepoint, the
523probepoint will be unoptimized automatically.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700524
525In general, you can install a probe anywhere in the kernel.
526In particular, you can probe interrupt handlers. Known exceptions
527are discussed in this section.
528
Jim Keniston8861da32006-02-14 13:53:06 -0800529The register_*probe functions will return -EINVAL if you attempt
530to install a probe in the code that implements Kprobes (mostly
531kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such
532as do_page_fault and notifier_call_chain).
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700533
534If you install a probe in an inline-able function, Kprobes makes
535no attempt to chase down all inline instances of the function and
536install probes there. gcc may inline a function without being asked,
537so keep this in mind if you're not seeing the probe hits you expect.
538
539A probe handler can modify the environment of the probed function
540-- e.g., by modifying kernel data structures, or by modifying the
541contents of the pt_regs struct (which are restored to the registers
542upon return from the breakpoint). So Kprobes can be used, for example,
543to install a bug fix or to inject faults for testing. Kprobes, of
544course, has no way to distinguish the deliberately injected faults
545from the accidental ones. Don't drink and probe.
546
547Kprobes makes no attempt to prevent probe handlers from stepping on
548each other -- e.g., probing printk() and then calling printk() from a
Jim Keniston8861da32006-02-14 13:53:06 -0800549probe handler. If a probe handler hits a probe, that second probe's
550handlers won't be run in that instance, and the kprobe.nmissed member
551of the second probe will be incremented.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700552
Jim Keniston8861da32006-02-14 13:53:06 -0800553As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
554the same handler) may run concurrently on different CPUs.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700555
Jim Keniston8861da32006-02-14 13:53:06 -0800556Kprobes does not use mutexes or allocate memory except during
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700557registration and unregistration.
558
559Probe handlers are run with preemption disabled. Depending on the
Masami Hiramatsu0f55a2f2010-10-14 12:10:18 +0900560architecture and optimization state, handlers may also run with
561interrupts disabled (e.g., kretprobe handlers and optimized kprobe
562handlers run without interrupt disabled on x86/x86-64). In any case,
563your handler should not yield the CPU (e.g., by attempting to acquire
564a semaphore).
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700565
566Since a return probe is implemented by replacing the return
567address with the trampoline's address, stack backtraces and calls
568to __builtin_return_address() will typically yield the trampoline's
569address instead of the real return address for kretprobed functions.
570(As far as we can tell, __builtin_return_address() is used only
571for instrumentation and error reporting.)
572
Jim Keniston8861da32006-02-14 13:53:06 -0800573If the number of times a function is called does not match the number
574of times it returns, registering a return probe on that function may
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -0700575produce undesirable results. In such a case, a line:
576kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
577gets printed. With this information, one will be able to correlate the
578exact instance of the kretprobe that caused the problem. We have the
579do_exit() case covered. do_execve() and do_fork() are not an issue.
580We're unaware of other specific cases where this could be a problem.
Jim Keniston8861da32006-02-14 13:53:06 -0800581
582If, upon entry to or exit from a function, the CPU is running on
583a stack other than that of the current task, registering a return
584probe on that function may produce undesirable results. For this
585reason, Kprobes doesn't support return probes (or kprobes or jprobes)
586on the x86_64 version of __switch_to(); the registration functions
587return -EINVAL.
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700588
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500589On x86/x86-64, since the Jump Optimization of Kprobes modifies
590instructions widely, there are some limitations to optimization. To
591explain it, we introduce some terminology. Imagine a 3-instruction
592sequence consisting of a two 2-byte instructions and one 3-byte
593instruction.
594
595 IA
596 |
597[-2][-1][0][1][2][3][4][5][6][7]
598 [ins1][ins2][ ins3 ]
599 [<- DCR ->]
600 [<- JTPR ->]
601
602ins1: 1st Instruction
603ins2: 2nd Instruction
604ins3: 3rd Instruction
605IA: Insertion Address
606JTPR: Jump Target Prohibition Region
607DCR: Detoured Code Region
608
609The instructions in DCR are copied to the out-of-line buffer
610of the kprobe, because the bytes in DCR are replaced by
611a 5-byte jump instruction. So there are several limitations.
612
613a) The instructions in DCR must be relocatable.
614b) The instructions in DCR must not include a call instruction.
615c) JTPR must not be targeted by any jump or call instruction.
Uwe Kleine-Königb5950762010-11-01 15:38:34 -0400616d) DCR must not straddle the border between functions.
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500617
618Anyway, these limitations are checked by the in-kernel instruction
619decoder, so you don't need to worry about that.
620
Jim Kenistond27a4dd2005-08-04 12:53:35 -07006216. Probe Overhead
622
623On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
624microseconds to process. Specifically, a benchmark that hits the same
625probepoint repeatedly, firing a simple handler each time, reports 1-2
626million hits per second, depending on the architecture. A jprobe or
627return-probe hit typically takes 50-75% longer than a kprobe hit.
628When you have a return probe set on a function, adding a kprobe at
629the entry to that function adds essentially no overhead.
630
631Here are sample overhead figures (in usec) for different architectures.
632k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe
633on same function; jr = jprobe + return probe on same function
634
635i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
636k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40
637
638x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
639k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07
640
641ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
642k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99
643
Masami Hiramatsub26486b2010-02-25 08:35:04 -05006446.1 Optimized Probe Overhead
645
646Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
647process. Here are sample overhead figures (in usec) for x86 architectures.
648k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
649r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
650
651i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
652k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
653
654x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
655k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
656
Jim Kenistond27a4dd2005-08-04 12:53:35 -07006577. TODO
658
Jim Keniston8861da32006-02-14 13:53:06 -0800659a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
660programming interface for probe-based instrumentation. Try it out.
661b. Kernel return probes for sparc64.
662c. Support for other architectures.
663d. User-space probes.
664e. Watchpoint probes (which fire on data references).
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700665
6668. Kprobes Example
667
Ananth N Mavinakayanahalli804defe2008-03-04 14:28:38 -0800668See samples/kprobes/kprobe_example.c
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700669
6709. Jprobes Example
671
Ananth N Mavinakayanahalli804defe2008-03-04 14:28:38 -0800672See samples/kprobes/jprobe_example.c
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700673
67410. Kretprobes Example
675
Ananth N Mavinakayanahalli804defe2008-03-04 14:28:38 -0800676See samples/kprobes/kretprobe_example.c
Jim Kenistond27a4dd2005-08-04 12:53:35 -0700677
678For additional information on Kprobes, refer to the following URLs:
679http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe
680http://www.redhat.com/magazine/005mar05/features/kprobes/
Ananth N Mavinakayanahalli09b18202006-10-02 02:17:32 -0700681http://www-users.cs.umn.edu/~boutcher/kprobes/
682http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115)
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -0700683
684
685Appendix A: The kprobes debugfs interface
686
687With recent kernels (> 2.6.20) the list of registered kprobes is visible
GeunSik Lim156f5a72009-06-02 15:01:37 +0900688under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -0700689
GeunSik Lim156f5a72009-06-02 15:01:37 +0900690/sys/kernel/debug/kprobes/list: Lists all registered probes on the system
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -0700691
692c015d71a k vfs_read+0x0
693c011a316 j do_fork+0x0
694c03dedc5 r tcp_v4_rcv+0x0
695
696The first column provides the kernel address where the probe is inserted.
697The second column identifies the type of probe (k - kprobe, r - kretprobe
698and j - jprobe), while the third column specifies the symbol+offset of
699the probe. If the probed function belongs to a module, the module name
Masami Hiramatsue8386a02009-01-06 14:41:52 -0800700is also specified. Following columns show probe status. If the probe is on
701a virtual address that is no longer valid (module init sections, module
702virtual addresses that correspond to modules that've been unloaded),
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700703such probes are marked with [GONE]. If the probe is temporarily disabled,
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500704such probes are marked with [DISABLED]. If the probe is optimized, it is
705marked with [OPTIMIZED].
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -0700706
GeunSik Lim156f5a72009-06-02 15:01:37 +0900707/sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
Ananth N Mavinakayanahallibf8f6e5b2007-05-08 00:34:16 -0700708
Masami Hiramatsude5bd882009-04-06 19:01:02 -0700709Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
710By default, all kprobes are enabled. By echoing "0" to this file, all
711registered probes will be disarmed, till such time a "1" is echoed to this
712file. Note that this knob just disarms and arms all kprobes and doesn't
713change each probe's disabling state. This means that disabled kprobes (marked
714[DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
Masami Hiramatsub26486b2010-02-25 08:35:04 -0500715
716
717Appendix B: The kprobes sysctl interface
718
719/proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
720
721When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
722a knob to globally and forcibly turn jump optimization (see section
7231.4) ON or OFF. By default, jump optimization is allowed (ON).
724If you echo "0" to this file or set "debug.kprobes_optimization" to
7250 via sysctl, all optimized probes will be unoptimized, and any new
726probes registered after that will not be optimized. Note that this
727knob *changes* the optimized state. This means that optimized probes
728(marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
729removed). If the knob is turned on, they will be optimized again.
730