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1 | Title : Kernel Probes (Kprobes) |
2 | Authors : Jim Keniston <jkenisto@us.ibm.com> | |
b26486bf MH |
3 | : Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com> |
4 | : Masami Hiramatsu <mhiramat@redhat.com> | |
d27a4ddd JK |
5 | |
6 | CONTENTS | |
7 | ||
8 | 1. Concepts: Kprobes, Jprobes, Return Probes | |
9 | 2. Architectures Supported | |
10 | 3. Configuring Kprobes | |
11 | 4. API Reference | |
12 | 5. Kprobes Features and Limitations | |
13 | 6. Probe Overhead | |
14 | 7. TODO | |
15 | 8. Kprobes Example | |
16 | 9. Jprobes Example | |
17 | 10. Kretprobes Example | |
bf8f6e5b | 18 | Appendix A: The kprobes debugfs interface |
b26486bf | 19 | Appendix B: The kprobes sysctl interface |
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20 | |
21 | 1. Concepts: Kprobes, Jprobes, Return Probes | |
22 | ||
23 | Kprobes enables you to dynamically break into any kernel routine and | |
24 | collect debugging and performance information non-disruptively. You | |
376e2424 | 25 | can trap at almost any kernel code address(*), specifying a handler |
d27a4ddd | 26 | routine to be invoked when the breakpoint is hit. |
376e2424 | 27 | (*: some parts of the kernel code can not be trapped, see 1.5 Blacklist) |
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28 | |
29 | There are currently three types of probes: kprobes, jprobes, and | |
30 | kretprobes (also called return probes). A kprobe can be inserted | |
31 | on virtually any instruction in the kernel. A jprobe is inserted at | |
32 | the entry to a kernel function, and provides convenient access to the | |
33 | function's arguments. A return probe fires when a specified function | |
34 | returns. | |
35 | ||
36 | In the typical case, Kprobes-based instrumentation is packaged as | |
37 | a kernel module. The module's init function installs ("registers") | |
38 | one or more probes, and the exit function unregisters them. A | |
39 | registration function such as register_kprobe() specifies where | |
40 | the probe is to be inserted and what handler is to be called when | |
41 | the probe is hit. | |
42 | ||
3b0cb4ca MH |
43 | There are also register_/unregister_*probes() functions for batch |
44 | registration/unregistration of a group of *probes. These functions | |
45 | can speed up unregistration process when you have to unregister | |
46 | a lot of probes at once. | |
47 | ||
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48 | The next four subsections explain how the different types of |
49 | probes work and how jump optimization works. They explain certain | |
50 | things that you'll need to know in order to make the best use of | |
51 | Kprobes -- e.g., the difference between a pre_handler and | |
52 | a post_handler, and how to use the maxactive and nmissed fields of | |
53 | a kretprobe. But if you're in a hurry to start using Kprobes, you | |
54 | can skip ahead to section 2. | |
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55 | |
56 | 1.1 How Does a Kprobe Work? | |
57 | ||
58 | When a kprobe is registered, Kprobes makes a copy of the probed | |
59 | instruction and replaces the first byte(s) of the probed instruction | |
60 | with a breakpoint instruction (e.g., int3 on i386 and x86_64). | |
61 | ||
62 | When a CPU hits the breakpoint instruction, a trap occurs, the CPU's | |
63 | registers are saved, and control passes to Kprobes via the | |
64 | notifier_call_chain mechanism. Kprobes executes the "pre_handler" | |
65 | associated with the kprobe, passing the handler the addresses of the | |
66 | kprobe struct and the saved registers. | |
67 | ||
68 | Next, Kprobes single-steps its copy of the probed instruction. | |
69 | (It would be simpler to single-step the actual instruction in place, | |
70 | but then Kprobes would have to temporarily remove the breakpoint | |
71 | instruction. This would open a small time window when another CPU | |
72 | could sail right past the probepoint.) | |
73 | ||
74 | After the instruction is single-stepped, Kprobes executes the | |
75 | "post_handler," if any, that is associated with the kprobe. | |
76 | Execution then continues with the instruction following the probepoint. | |
77 | ||
78 | 1.2 How Does a Jprobe Work? | |
79 | ||
80 | A jprobe is implemented using a kprobe that is placed on a function's | |
81 | entry point. It employs a simple mirroring principle to allow | |
82 | seamless access to the probed function's arguments. The jprobe | |
83 | handler routine should have the same signature (arg list and return | |
84 | type) as the function being probed, and must always end by calling | |
85 | the Kprobes function jprobe_return(). | |
86 | ||
87 | Here's how it works. When the probe is hit, Kprobes makes a copy of | |
88 | the saved registers and a generous portion of the stack (see below). | |
89 | Kprobes then points the saved instruction pointer at the jprobe's | |
90 | handler routine, and returns from the trap. As a result, control | |
91 | passes to the handler, which is presented with the same register and | |
92 | stack contents as the probed function. When it is done, the handler | |
93 | calls jprobe_return(), which traps again to restore the original stack | |
94 | contents and processor state and switch to the probed function. | |
95 | ||
96 | By convention, the callee owns its arguments, so gcc may produce code | |
97 | that unexpectedly modifies that portion of the stack. This is why | |
98 | Kprobes saves a copy of the stack and restores it after the jprobe | |
99 | handler has run. Up to MAX_STACK_SIZE bytes are copied -- e.g., | |
100 | 64 bytes on i386. | |
101 | ||
102 | Note that the probed function's args may be passed on the stack | |
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103 | or in registers. The jprobe will work in either case, so long as the |
104 | handler's prototype matches that of the probed function. | |
d27a4ddd | 105 | |
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106 | 1.3 Return Probes |
107 | ||
108 | 1.3.1 How Does a Return Probe Work? | |
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109 | |
110 | When you call register_kretprobe(), Kprobes establishes a kprobe at | |
111 | the entry to the function. When the probed function is called and this | |
112 | probe is hit, Kprobes saves a copy of the return address, and replaces | |
113 | the return address with the address of a "trampoline." The trampoline | |
114 | is an arbitrary piece of code -- typically just a nop instruction. | |
115 | At boot time, Kprobes registers a kprobe at the trampoline. | |
116 | ||
117 | When the probed function executes its return instruction, control | |
118 | passes to the trampoline and that probe is hit. Kprobes' trampoline | |
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119 | handler calls the user-specified return handler associated with the |
120 | kretprobe, then sets the saved instruction pointer to the saved return | |
121 | address, and that's where execution resumes upon return from the trap. | |
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122 | |
123 | While the probed function is executing, its return address is | |
124 | stored in an object of type kretprobe_instance. Before calling | |
125 | register_kretprobe(), the user sets the maxactive field of the | |
126 | kretprobe struct to specify how many instances of the specified | |
127 | function can be probed simultaneously. register_kretprobe() | |
128 | pre-allocates the indicated number of kretprobe_instance objects. | |
129 | ||
130 | For example, if the function is non-recursive and is called with a | |
131 | spinlock held, maxactive = 1 should be enough. If the function is | |
132 | non-recursive and can never relinquish the CPU (e.g., via a semaphore | |
133 | or preemption), NR_CPUS should be enough. If maxactive <= 0, it is | |
134 | set to a default value. If CONFIG_PREEMPT is enabled, the default | |
135 | is max(10, 2*NR_CPUS). Otherwise, the default is NR_CPUS. | |
136 | ||
137 | It's not a disaster if you set maxactive too low; you'll just miss | |
138 | some probes. In the kretprobe struct, the nmissed field is set to | |
139 | zero when the return probe is registered, and is incremented every | |
140 | time the probed function is entered but there is no kretprobe_instance | |
141 | object available for establishing the return probe. | |
142 | ||
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143 | 1.3.2 Kretprobe entry-handler |
144 | ||
145 | Kretprobes also provides an optional user-specified handler which runs | |
146 | on function entry. This handler is specified by setting the entry_handler | |
147 | field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the | |
148 | function entry is hit, the user-defined entry_handler, if any, is invoked. | |
149 | If the entry_handler returns 0 (success) then a corresponding return handler | |
150 | is guaranteed to be called upon function return. If the entry_handler | |
151 | returns a non-zero error then Kprobes leaves the return address as is, and | |
152 | the kretprobe has no further effect for that particular function instance. | |
153 | ||
154 | Multiple entry and return handler invocations are matched using the unique | |
155 | kretprobe_instance object associated with them. Additionally, a user | |
156 | may also specify per return-instance private data to be part of each | |
157 | kretprobe_instance object. This is especially useful when sharing private | |
158 | data between corresponding user entry and return handlers. The size of each | |
159 | private data object can be specified at kretprobe registration time by | |
160 | setting the data_size field of the kretprobe struct. This data can be | |
161 | accessed through the data field of each kretprobe_instance object. | |
162 | ||
163 | In case probed function is entered but there is no kretprobe_instance | |
164 | object available, then in addition to incrementing the nmissed count, | |
165 | the user entry_handler invocation is also skipped. | |
166 | ||
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167 | 1.4 How Does Jump Optimization Work? |
168 | ||
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169 | If your kernel is built with CONFIG_OPTPROBES=y (currently this flag |
170 | is automatically set 'y' on x86/x86-64, non-preemptive kernel) and | |
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171 | the "debug.kprobes_optimization" kernel parameter is set to 1 (see |
172 | sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump | |
173 | instruction instead of a breakpoint instruction at each probepoint. | |
174 | ||
175 | 1.4.1 Init a Kprobe | |
176 | ||
177 | When a probe is registered, before attempting this optimization, | |
178 | Kprobes inserts an ordinary, breakpoint-based kprobe at the specified | |
179 | address. So, even if it's not possible to optimize this particular | |
180 | probepoint, there'll be a probe there. | |
181 | ||
182 | 1.4.2 Safety Check | |
183 | ||
184 | Before optimizing a probe, Kprobes performs the following safety checks: | |
185 | ||
186 | - Kprobes verifies that the region that will be replaced by the jump | |
187 | instruction (the "optimized region") lies entirely within one function. | |
188 | (A jump instruction is multiple bytes, and so may overlay multiple | |
189 | instructions.) | |
190 | ||
191 | - Kprobes analyzes the entire function and verifies that there is no | |
192 | jump 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 | |
202 | the instruction can be executed out of line. | |
203 | ||
204 | 1.4.3 Preparing Detour Buffer | |
205 | ||
206 | Next, Kprobes prepares a "detour" buffer, which contains the following | |
207 | instruction 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 | ||
214 | 1.4.4 Pre-optimization | |
215 | ||
216 | After preparing the detour buffer, Kprobes verifies that none of the | |
217 | following situations exist: | |
218 | - The probe has either a break_handler (i.e., it's a jprobe) or a | |
219 | post_handler. | |
220 | - Other instructions in the optimized region are probed. | |
221 | - The probe is disabled. | |
222 | In any of the above cases, Kprobes won't start optimizing the probe. | |
223 | Since these are temporary situations, Kprobes tries to start | |
224 | optimizing it again if the situation is changed. | |
225 | ||
226 | If the kprobe can be optimized, Kprobes enqueues the kprobe to an | |
227 | optimizing list, and kicks the kprobe-optimizer workqueue to optimize | |
228 | it. If the to-be-optimized probepoint is hit before being optimized, | |
229 | Kprobes returns control to the original instruction path by setting | |
230 | the CPU's instruction pointer to the copied code in the detour buffer | |
231 | -- thus at least avoiding the single-step. | |
232 | ||
233 | 1.4.5 Optimization | |
234 | ||
235 | The Kprobe-optimizer doesn't insert the jump instruction immediately; | |
236 | rather, it calls synchronize_sched() for safety first, because it's | |
237 | possible for a CPU to be interrupted in the middle of executing the | |
238 | optimized region(*). As you know, synchronize_sched() can ensure | |
239 | that all interruptions that were active when synchronize_sched() | |
240 | was called are done, but only if CONFIG_PREEMPT=n. So, this version | |
241 | of kprobe optimization supports only kernels with CONFIG_PREEMPT=n.(**) | |
242 | ||
243 | After that, the Kprobe-optimizer calls stop_machine() to replace | |
244 | the optimized region with a jump instruction to the detour buffer, | |
245 | using text_poke_smp(). | |
246 | ||
247 | 1.4.6 Unoptimization | |
248 | ||
249 | When an optimized kprobe is unregistered, disabled, or blocked by | |
250 | another kprobe, it will be unoptimized. If this happens before | |
251 | the optimization is complete, the kprobe is just dequeued from the | |
252 | optimized list. If the optimization has been done, the jump is | |
253 | replaced with the original code (except for an int3 breakpoint in | |
254 | the first byte) by using text_poke_smp(). | |
255 | ||
256 | (*)Please imagine that the 2nd instruction is interrupted and then | |
257 | the optimizer replaces the 2nd instruction with the jump *address* | |
258 | while the interrupt handler is running. When the interrupt | |
259 | returns to original address, there is no valid instruction, | |
260 | and it causes an unexpected result. | |
261 | ||
262 | (**)This optimization-safety checking may be replaced with the | |
263 | stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y | |
264 | kernel. | |
265 | ||
266 | NOTE for geeks: | |
267 | The jump optimization changes the kprobe's pre_handler behavior. | |
268 | Without optimization, the pre_handler can change the kernel's execution | |
269 | path by changing regs->ip and returning 1. However, when the probe | |
270 | is optimized, that modification is ignored. Thus, if you want to | |
271 | tweak the kernel's execution path, you need to suppress optimization, | |
272 | using one of the following techniques: | |
273 | - Specify an empty function for the kprobe's post_handler or break_handler. | |
274 | or | |
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275 | - Execute 'sysctl -w debug.kprobes_optimization=n' |
276 | ||
376e2424 MH |
277 | 1.5 Blacklist |
278 | ||
279 | Kprobes can probe most of the kernel except itself. This means | |
280 | that there are some functions where kprobes cannot probe. Probing | |
281 | (trapping) such functions can cause a recursive trap (e.g. double | |
282 | fault) or the nested probe handler may never be called. | |
283 | Kprobes manages such functions as a blacklist. | |
284 | If you want to add a function into the blacklist, you just need | |
285 | to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro | |
286 | to specify a blacklisted function. | |
287 | Kprobes checks the given probe address against the blacklist and | |
288 | rejects registering it, if the given address is in the blacklist. | |
289 | ||
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290 | 2. Architectures Supported |
291 | ||
292 | Kprobes, jprobes, and return probes are implemented on the following | |
293 | architectures: | |
294 | ||
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295 | - i386 (Supports jump optimization) |
296 | - x86_64 (AMD-64, EM64T) (Supports jump optimization) | |
d27a4ddd | 297 | - ppc64 |
8861da31 | 298 | - ia64 (Does not support probes on instruction slot1.) |
d27a4ddd | 299 | - sparc64 (Return probes not yet implemented.) |
5de865b4 | 300 | - arm |
f8279621 | 301 | - ppc |
9bb4d9df | 302 | - mips |
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303 | |
304 | 3. Configuring Kprobes | |
305 | ||
306 | When configuring the kernel using make menuconfig/xconfig/oldconfig, | |
8861da31 JK |
307 | ensure that CONFIG_KPROBES is set to "y". Under "Instrumentation |
308 | Support", look for "Kprobes". | |
309 | ||
310 | So that you can load and unload Kprobes-based instrumentation modules, | |
311 | make sure "Loadable module support" (CONFIG_MODULES) and "Module | |
312 | unloading" (CONFIG_MODULE_UNLOAD) are set to "y". | |
d27a4ddd | 313 | |
09b18203 AM |
314 | Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL |
315 | are set to "y", since kallsyms_lookup_name() is used by the in-kernel | |
316 | kprobe address resolution code. | |
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317 | |
318 | If you need to insert a probe in the middle of a function, you may find | |
319 | it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO), | |
320 | so you can use "objdump -d -l vmlinux" to see the source-to-object | |
321 | code mapping. | |
322 | ||
323 | 4. API Reference | |
324 | ||
325 | The Kprobes API includes a "register" function and an "unregister" | |
3b0cb4ca MH |
326 | function for each type of probe. The API also includes "register_*probes" |
327 | and "unregister_*probes" functions for (un)registering arrays of probes. | |
328 | Here are terse, mini-man-page specifications for these functions and | |
329 | the associated probe handlers that you'll write. See the files in the | |
330 | samples/kprobes/ sub-directory for examples. | |
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331 | |
332 | 4.1 register_kprobe | |
333 | ||
334 | #include <linux/kprobes.h> | |
335 | int register_kprobe(struct kprobe *kp); | |
336 | ||
337 | Sets a breakpoint at the address kp->addr. When the breakpoint is | |
338 | hit, Kprobes calls kp->pre_handler. After the probed instruction | |
339 | is single-stepped, Kprobe calls kp->post_handler. If a fault | |
340 | occurs during execution of kp->pre_handler or kp->post_handler, | |
341 | or during single-stepping of the probed instruction, Kprobes calls | |
de5bd88d MH |
342 | kp->fault_handler. Any or all handlers can be NULL. If kp->flags |
343 | is set KPROBE_FLAG_DISABLED, that kp will be registered but disabled, | |
a33f3224 | 344 | so, its handlers aren't hit until calling enable_kprobe(kp). |
d27a4ddd | 345 | |
09b18203 AM |
346 | NOTE: |
347 | 1. With the introduction of the "symbol_name" field to struct kprobe, | |
348 | the probepoint address resolution will now be taken care of by the kernel. | |
349 | The following will now work: | |
350 | ||
351 | kp.symbol_name = "symbol_name"; | |
352 | ||
353 | (64-bit powerpc intricacies such as function descriptors are handled | |
354 | transparently) | |
355 | ||
356 | 2. Use the "offset" field of struct kprobe if the offset into the symbol | |
357 | to install a probepoint is known. This field is used to calculate the | |
358 | probepoint. | |
359 | ||
360 | 3. Specify either the kprobe "symbol_name" OR the "addr". If both are | |
361 | specified, kprobe registration will fail with -EINVAL. | |
362 | ||
363 | 4. With CISC architectures (such as i386 and x86_64), the kprobes code | |
364 | does not validate if the kprobe.addr is at an instruction boundary. | |
365 | Use "offset" with caution. | |
366 | ||
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367 | register_kprobe() returns 0 on success, or a negative errno otherwise. |
368 | ||
369 | User's pre-handler (kp->pre_handler): | |
370 | #include <linux/kprobes.h> | |
371 | #include <linux/ptrace.h> | |
372 | int pre_handler(struct kprobe *p, struct pt_regs *regs); | |
373 | ||
374 | Called with p pointing to the kprobe associated with the breakpoint, | |
375 | and regs pointing to the struct containing the registers saved when | |
376 | the breakpoint was hit. Return 0 here unless you're a Kprobes geek. | |
377 | ||
378 | User's post-handler (kp->post_handler): | |
379 | #include <linux/kprobes.h> | |
380 | #include <linux/ptrace.h> | |
381 | void post_handler(struct kprobe *p, struct pt_regs *regs, | |
382 | unsigned long flags); | |
383 | ||
384 | p and regs are as described for the pre_handler. flags always seems | |
385 | to be zero. | |
386 | ||
387 | User's fault-handler (kp->fault_handler): | |
388 | #include <linux/kprobes.h> | |
389 | #include <linux/ptrace.h> | |
390 | int fault_handler(struct kprobe *p, struct pt_regs *regs, int trapnr); | |
391 | ||
392 | p and regs are as described for the pre_handler. trapnr is the | |
393 | architecture-specific trap number associated with the fault (e.g., | |
394 | on i386, 13 for a general protection fault or 14 for a page fault). | |
395 | Returns 1 if it successfully handled the exception. | |
396 | ||
397 | 4.2 register_jprobe | |
398 | ||
399 | #include <linux/kprobes.h> | |
400 | int register_jprobe(struct jprobe *jp) | |
401 | ||
402 | Sets a breakpoint at the address jp->kp.addr, which must be the address | |
403 | of the first instruction of a function. When the breakpoint is hit, | |
404 | Kprobes runs the handler whose address is jp->entry. | |
405 | ||
406 | The handler should have the same arg list and return type as the probed | |
407 | function; and just before it returns, it must call jprobe_return(). | |
408 | (The handler never actually returns, since jprobe_return() returns | |
b5606c2d HH |
409 | control to Kprobes.) If the probed function is declared asmlinkage |
410 | or anything else that affects how args are passed, the handler's | |
411 | declaration must match. | |
d27a4ddd JK |
412 | |
413 | register_jprobe() returns 0 on success, or a negative errno otherwise. | |
414 | ||
415 | 4.3 register_kretprobe | |
416 | ||
417 | #include <linux/kprobes.h> | |
418 | int register_kretprobe(struct kretprobe *rp); | |
419 | ||
420 | Establishes a return probe for the function whose address is | |
421 | rp->kp.addr. When that function returns, Kprobes calls rp->handler. | |
422 | You must set rp->maxactive appropriately before you call | |
423 | register_kretprobe(); see "How Does a Return Probe Work?" for details. | |
424 | ||
425 | register_kretprobe() returns 0 on success, or a negative errno | |
426 | otherwise. | |
427 | ||
428 | User's return-probe handler (rp->handler): | |
429 | #include <linux/kprobes.h> | |
430 | #include <linux/ptrace.h> | |
431 | int kretprobe_handler(struct kretprobe_instance *ri, struct pt_regs *regs); | |
432 | ||
433 | regs is as described for kprobe.pre_handler. ri points to the | |
434 | kretprobe_instance object, of which the following fields may be | |
435 | of interest: | |
436 | - ret_addr: the return address | |
437 | - rp: points to the corresponding kretprobe object | |
438 | - task: points to the corresponding task struct | |
f47cd9b5 AS |
439 | - data: points to per return-instance private data; see "Kretprobe |
440 | entry-handler" for details. | |
09b18203 AM |
441 | |
442 | The regs_return_value(regs) macro provides a simple abstraction to | |
443 | extract the return value from the appropriate register as defined by | |
444 | the architecture's ABI. | |
445 | ||
d27a4ddd JK |
446 | The handler's return value is currently ignored. |
447 | ||
448 | 4.4 unregister_*probe | |
449 | ||
450 | #include <linux/kprobes.h> | |
451 | void unregister_kprobe(struct kprobe *kp); | |
452 | void unregister_jprobe(struct jprobe *jp); | |
453 | void unregister_kretprobe(struct kretprobe *rp); | |
454 | ||
455 | Removes the specified probe. The unregister function can be called | |
456 | at any time after the probe has been registered. | |
457 | ||
3b0cb4ca MH |
458 | NOTE: |
459 | If the functions find an incorrect probe (ex. an unregistered probe), | |
460 | they clear the addr field of the probe. | |
461 | ||
462 | 4.5 register_*probes | |
463 | ||
464 | #include <linux/kprobes.h> | |
465 | int register_kprobes(struct kprobe **kps, int num); | |
466 | int register_kretprobes(struct kretprobe **rps, int num); | |
467 | int register_jprobes(struct jprobe **jps, int num); | |
468 | ||
469 | Registers each of the num probes in the specified array. If any | |
470 | error occurs during registration, all probes in the array, up to | |
471 | the bad probe, are safely unregistered before the register_*probes | |
472 | function returns. | |
473 | - kps/rps/jps: an array of pointers to *probe data structures | |
474 | - num: the number of the array entries. | |
475 | ||
476 | NOTE: | |
477 | You have to allocate(or define) an array of pointers and set all | |
478 | of the array entries before using these functions. | |
479 | ||
480 | 4.6 unregister_*probes | |
481 | ||
482 | #include <linux/kprobes.h> | |
483 | void unregister_kprobes(struct kprobe **kps, int num); | |
484 | void unregister_kretprobes(struct kretprobe **rps, int num); | |
485 | void unregister_jprobes(struct jprobe **jps, int num); | |
486 | ||
487 | Removes each of the num probes in the specified array at once. | |
488 | ||
489 | NOTE: | |
490 | If the functions find some incorrect probes (ex. unregistered | |
491 | probes) in the specified array, they clear the addr field of those | |
492 | incorrect probes. However, other probes in the array are | |
493 | unregistered correctly. | |
494 | ||
8f9b1528 | 495 | 4.7 disable_*probe |
de5bd88d MH |
496 | |
497 | #include <linux/kprobes.h> | |
498 | int disable_kprobe(struct kprobe *kp); | |
8f9b1528 MH |
499 | int disable_kretprobe(struct kretprobe *rp); |
500 | int disable_jprobe(struct jprobe *jp); | |
de5bd88d | 501 | |
8f9b1528 MH |
502 | Temporarily disables the specified *probe. You can enable it again by using |
503 | enable_*probe(). You must specify the probe which has been registered. | |
de5bd88d | 504 | |
8f9b1528 | 505 | 4.8 enable_*probe |
de5bd88d MH |
506 | |
507 | #include <linux/kprobes.h> | |
508 | int enable_kprobe(struct kprobe *kp); | |
8f9b1528 MH |
509 | int enable_kretprobe(struct kretprobe *rp); |
510 | int enable_jprobe(struct jprobe *jp); | |
de5bd88d | 511 | |
8f9b1528 MH |
512 | Enables *probe which has been disabled by disable_*probe(). You must specify |
513 | the probe which has been registered. | |
de5bd88d | 514 | |
d27a4ddd JK |
515 | 5. Kprobes Features and Limitations |
516 | ||
8861da31 JK |
517 | Kprobes allows multiple probes at the same address. Currently, |
518 | however, there cannot be multiple jprobes on the same function at | |
b26486bf MH |
519 | the same time. Also, a probepoint for which there is a jprobe or |
520 | a post_handler cannot be optimized. So if you install a jprobe, | |
521 | or a kprobe with a post_handler, at an optimized probepoint, the | |
522 | probepoint will be unoptimized automatically. | |
d27a4ddd JK |
523 | |
524 | In general, you can install a probe anywhere in the kernel. | |
525 | In particular, you can probe interrupt handlers. Known exceptions | |
526 | are discussed in this section. | |
527 | ||
8861da31 JK |
528 | The register_*probe functions will return -EINVAL if you attempt |
529 | to install a probe in the code that implements Kprobes (mostly | |
530 | kernel/kprobes.c and arch/*/kernel/kprobes.c, but also functions such | |
531 | as do_page_fault and notifier_call_chain). | |
d27a4ddd JK |
532 | |
533 | If you install a probe in an inline-able function, Kprobes makes | |
534 | no attempt to chase down all inline instances of the function and | |
535 | install probes there. gcc may inline a function without being asked, | |
536 | so keep this in mind if you're not seeing the probe hits you expect. | |
537 | ||
538 | A probe handler can modify the environment of the probed function | |
539 | -- e.g., by modifying kernel data structures, or by modifying the | |
540 | contents of the pt_regs struct (which are restored to the registers | |
541 | upon return from the breakpoint). So Kprobes can be used, for example, | |
542 | to install a bug fix or to inject faults for testing. Kprobes, of | |
543 | course, has no way to distinguish the deliberately injected faults | |
544 | from the accidental ones. Don't drink and probe. | |
545 | ||
546 | Kprobes makes no attempt to prevent probe handlers from stepping on | |
547 | each other -- e.g., probing printk() and then calling printk() from a | |
8861da31 JK |
548 | probe handler. If a probe handler hits a probe, that second probe's |
549 | handlers won't be run in that instance, and the kprobe.nmissed member | |
550 | of the second probe will be incremented. | |
551 | ||
552 | As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of | |
553 | the same handler) may run concurrently on different CPUs. | |
554 | ||
555 | Kprobes does not use mutexes or allocate memory except during | |
d27a4ddd JK |
556 | registration and unregistration. |
557 | ||
558 | Probe handlers are run with preemption disabled. Depending on the | |
0f55a2f3 MH |
559 | architecture and optimization state, handlers may also run with |
560 | interrupts disabled (e.g., kretprobe handlers and optimized kprobe | |
561 | handlers run without interrupt disabled on x86/x86-64). In any case, | |
562 | your handler should not yield the CPU (e.g., by attempting to acquire | |
563 | a semaphore). | |
d27a4ddd JK |
564 | |
565 | Since a return probe is implemented by replacing the return | |
566 | address with the trampoline's address, stack backtraces and calls | |
567 | to __builtin_return_address() will typically yield the trampoline's | |
568 | address instead of the real return address for kretprobed functions. | |
569 | (As far as we can tell, __builtin_return_address() is used only | |
570 | for instrumentation and error reporting.) | |
571 | ||
8861da31 JK |
572 | If the number of times a function is called does not match the number |
573 | of times it returns, registering a return probe on that function may | |
bf8f6e5b AM |
574 | produce undesirable results. In such a case, a line: |
575 | kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c | |
576 | gets printed. With this information, one will be able to correlate the | |
577 | exact instance of the kretprobe that caused the problem. We have the | |
578 | do_exit() case covered. do_execve() and do_fork() are not an issue. | |
579 | We're unaware of other specific cases where this could be a problem. | |
8861da31 JK |
580 | |
581 | If, upon entry to or exit from a function, the CPU is running on | |
582 | a stack other than that of the current task, registering a return | |
583 | probe on that function may produce undesirable results. For this | |
584 | reason, Kprobes doesn't support return probes (or kprobes or jprobes) | |
585 | on the x86_64 version of __switch_to(); the registration functions | |
586 | return -EINVAL. | |
d27a4ddd | 587 | |
b26486bf MH |
588 | On x86/x86-64, since the Jump Optimization of Kprobes modifies |
589 | instructions widely, there are some limitations to optimization. To | |
590 | explain it, we introduce some terminology. Imagine a 3-instruction | |
591 | sequence consisting of a two 2-byte instructions and one 3-byte | |
592 | instruction. | |
593 | ||
594 | IA | |
595 | | | |
596 | [-2][-1][0][1][2][3][4][5][6][7] | |
597 | [ins1][ins2][ ins3 ] | |
598 | [<- DCR ->] | |
599 | [<- JTPR ->] | |
600 | ||
601 | ins1: 1st Instruction | |
602 | ins2: 2nd Instruction | |
603 | ins3: 3rd Instruction | |
604 | IA: Insertion Address | |
605 | JTPR: Jump Target Prohibition Region | |
606 | DCR: Detoured Code Region | |
607 | ||
608 | The instructions in DCR are copied to the out-of-line buffer | |
609 | of the kprobe, because the bytes in DCR are replaced by | |
610 | a 5-byte jump instruction. So there are several limitations. | |
611 | ||
612 | a) The instructions in DCR must be relocatable. | |
613 | b) The instructions in DCR must not include a call instruction. | |
614 | c) JTPR must not be targeted by any jump or call instruction. | |
b595076a | 615 | d) DCR must not straddle the border between functions. |
b26486bf MH |
616 | |
617 | Anyway, these limitations are checked by the in-kernel instruction | |
618 | decoder, so you don't need to worry about that. | |
619 | ||
d27a4ddd JK |
620 | 6. Probe Overhead |
621 | ||
622 | On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0 | |
623 | microseconds to process. Specifically, a benchmark that hits the same | |
624 | probepoint repeatedly, firing a simple handler each time, reports 1-2 | |
625 | million hits per second, depending on the architecture. A jprobe or | |
626 | return-probe hit typically takes 50-75% longer than a kprobe hit. | |
627 | When you have a return probe set on a function, adding a kprobe at | |
628 | the entry to that function adds essentially no overhead. | |
629 | ||
630 | Here are sample overhead figures (in usec) for different architectures. | |
631 | k = kprobe; j = jprobe; r = return probe; kr = kprobe + return probe | |
632 | on same function; jr = jprobe + return probe on same function | |
633 | ||
634 | i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips | |
635 | k = 0.57 usec; j = 1.00; r = 0.92; kr = 0.99; jr = 1.40 | |
636 | ||
637 | x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips | |
638 | k = 0.49 usec; j = 0.76; r = 0.80; kr = 0.82; jr = 1.07 | |
639 | ||
640 | ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU) | |
641 | k = 0.77 usec; j = 1.31; r = 1.26; kr = 1.45; jr = 1.99 | |
642 | ||
b26486bf MH |
643 | 6.1 Optimized Probe Overhead |
644 | ||
645 | Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to | |
646 | process. Here are sample overhead figures (in usec) for x86 architectures. | |
647 | k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe, | |
648 | r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe. | |
649 | ||
650 | i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips | |
651 | k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33 | |
652 | ||
653 | x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips | |
654 | k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30 | |
655 | ||
d27a4ddd JK |
656 | 7. TODO |
657 | ||
8861da31 JK |
658 | a. SystemTap (http://sourceware.org/systemtap): Provides a simplified |
659 | programming interface for probe-based instrumentation. Try it out. | |
660 | b. Kernel return probes for sparc64. | |
661 | c. Support for other architectures. | |
662 | d. User-space probes. | |
663 | e. Watchpoint probes (which fire on data references). | |
d27a4ddd JK |
664 | |
665 | 8. Kprobes Example | |
666 | ||
804defea | 667 | See samples/kprobes/kprobe_example.c |
d27a4ddd JK |
668 | |
669 | 9. Jprobes Example | |
670 | ||
804defea | 671 | See samples/kprobes/jprobe_example.c |
d27a4ddd JK |
672 | |
673 | 10. Kretprobes Example | |
674 | ||
804defea | 675 | See samples/kprobes/kretprobe_example.c |
d27a4ddd JK |
676 | |
677 | For additional information on Kprobes, refer to the following URLs: | |
678 | http://www-106.ibm.com/developerworks/library/l-kprobes.html?ca=dgr-lnxw42Kprobe | |
679 | http://www.redhat.com/magazine/005mar05/features/kprobes/ | |
09b18203 AM |
680 | http://www-users.cs.umn.edu/~boutcher/kprobes/ |
681 | http://www.linuxsymposium.org/2006/linuxsymposium_procv2.pdf (pages 101-115) | |
bf8f6e5b AM |
682 | |
683 | ||
684 | Appendix A: The kprobes debugfs interface | |
685 | ||
686 | With recent kernels (> 2.6.20) the list of registered kprobes is visible | |
156f5a78 | 687 | under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug). |
bf8f6e5b | 688 | |
156f5a78 | 689 | /sys/kernel/debug/kprobes/list: Lists all registered probes on the system |
bf8f6e5b AM |
690 | |
691 | c015d71a k vfs_read+0x0 | |
692 | c011a316 j do_fork+0x0 | |
693 | c03dedc5 r tcp_v4_rcv+0x0 | |
694 | ||
695 | The first column provides the kernel address where the probe is inserted. | |
696 | The second column identifies the type of probe (k - kprobe, r - kretprobe | |
697 | and j - jprobe), while the third column specifies the symbol+offset of | |
698 | the probe. If the probed function belongs to a module, the module name | |
e8386a0c MH |
699 | is also specified. Following columns show probe status. If the probe is on |
700 | a virtual address that is no longer valid (module init sections, module | |
701 | virtual addresses that correspond to modules that've been unloaded), | |
de5bd88d | 702 | such probes are marked with [GONE]. If the probe is temporarily disabled, |
b26486bf MH |
703 | such probes are marked with [DISABLED]. If the probe is optimized, it is |
704 | marked with [OPTIMIZED]. | |
bf8f6e5b | 705 | |
156f5a78 | 706 | /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly. |
bf8f6e5b | 707 | |
de5bd88d MH |
708 | Provides a knob to globally and forcibly turn registered kprobes ON or OFF. |
709 | By default, all kprobes are enabled. By echoing "0" to this file, all | |
710 | registered probes will be disarmed, till such time a "1" is echoed to this | |
711 | file. Note that this knob just disarms and arms all kprobes and doesn't | |
712 | change each probe's disabling state. This means that disabled kprobes (marked | |
713 | [DISABLED]) will be not enabled if you turn ON all kprobes by this knob. | |
b26486bf MH |
714 | |
715 | ||
716 | Appendix B: The kprobes sysctl interface | |
717 | ||
718 | /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF. | |
719 | ||
720 | When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides | |
721 | a knob to globally and forcibly turn jump optimization (see section | |
722 | 1.4) ON or OFF. By default, jump optimization is allowed (ON). | |
723 | If you echo "0" to this file or set "debug.kprobes_optimization" to | |
724 | 0 via sysctl, all optimized probes will be unoptimized, and any new | |
725 | probes registered after that will not be optimized. Note that this | |
726 | knob *changes* the optimized state. This means that optimized probes | |
727 | (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be | |
728 | removed). If the knob is turned on, they will be optimized again. | |
729 |