1 ======================= 1 =======================
2 Kernel Probes (Kprobes) 2 Kernel Probes (Kprobes)
3 ======================= 3 =======================
4 4
5 :Author: Jim Keniston <jkenisto@us.ibm.com> 5 :Author: Jim Keniston <jkenisto@us.ibm.com>
6 :Author: Prasanna S Panchamukhi <prasanna.panch 6 :Author: Prasanna S Panchamukhi <prasanna.panchamukhi@gmail.com>
7 :Author: Masami Hiramatsu <mhiramat@kernel.org> !! 7 :Author: Masami Hiramatsu <mhiramat@redhat.com>
8 8
9 .. CONTENTS 9 .. CONTENTS
10 10
11 1. Concepts: Kprobes, and Return Probes 11 1. Concepts: Kprobes, and Return Probes
12 2. Architectures Supported 12 2. Architectures Supported
13 3. Configuring Kprobes 13 3. Configuring Kprobes
14 4. API Reference 14 4. API Reference
15 5. Kprobes Features and Limitations 15 5. Kprobes Features and Limitations
16 6. Probe Overhead 16 6. Probe Overhead
17 7. TODO 17 7. TODO
18 8. Kprobes Example 18 8. Kprobes Example
19 9. Kretprobes Example 19 9. Kretprobes Example
20 10. Deprecated Features 20 10. Deprecated Features
21 Appendix A: The kprobes debugfs interface 21 Appendix A: The kprobes debugfs interface
22 Appendix B: The kprobes sysctl interface 22 Appendix B: The kprobes sysctl interface
23 Appendix C: References 23 Appendix C: References
24 24
25 Concepts: Kprobes and Return Probes 25 Concepts: Kprobes and Return Probes
26 ========================================= 26 =========================================
27 27
28 Kprobes enables you to dynamically break into 28 Kprobes enables you to dynamically break into any kernel routine and
29 collect debugging and performance information 29 collect debugging and performance information non-disruptively. You
30 can trap at almost any kernel code address [1] 30 can trap at almost any kernel code address [1]_, specifying a handler
31 routine to be invoked when the breakpoint is h 31 routine to be invoked when the breakpoint is hit.
32 32
33 .. [1] some parts of the kernel code can not b 33 .. [1] some parts of the kernel code can not be trapped, see
34 :ref:`kprobes_blacklist`) 34 :ref:`kprobes_blacklist`)
35 35
36 There are currently two types of probes: kprob 36 There are currently two types of probes: kprobes, and kretprobes
37 (also called return probes). A kprobe can be 37 (also called return probes). A kprobe can be inserted on virtually
38 any instruction in the kernel. A return probe 38 any instruction in the kernel. A return probe fires when a specified
39 function returns. 39 function returns.
40 40
41 In the typical case, Kprobes-based instrumenta 41 In the typical case, Kprobes-based instrumentation is packaged as
42 a kernel module. The module's init function i 42 a kernel module. The module's init function installs ("registers")
43 one or more probes, and the exit function unre 43 one or more probes, and the exit function unregisters them. A
44 registration function such as register_kprobe( 44 registration function such as register_kprobe() specifies where
45 the probe is to be inserted and what handler i 45 the probe is to be inserted and what handler is to be called when
46 the probe is hit. 46 the probe is hit.
47 47
48 There are also ``register_/unregister_*probes( 48 There are also ``register_/unregister_*probes()`` functions for batch
49 registration/unregistration of a group of ``*p 49 registration/unregistration of a group of ``*probes``. These functions
50 can speed up unregistration process when you h 50 can speed up unregistration process when you have to unregister
51 a lot of probes at once. 51 a lot of probes at once.
52 52
53 The next four subsections explain how the diff 53 The next four subsections explain how the different types of
54 probes work and how jump optimization works. 54 probes work and how jump optimization works. They explain certain
55 things that you'll need to know in order to ma 55 things that you'll need to know in order to make the best use of
56 Kprobes -- e.g., the difference between a pre_ 56 Kprobes -- e.g., the difference between a pre_handler and
57 a post_handler, and how to use the maxactive a 57 a post_handler, and how to use the maxactive and nmissed fields of
58 a kretprobe. But if you're in a hurry to star 58 a kretprobe. But if you're in a hurry to start using Kprobes, you
59 can skip ahead to :ref:`kprobes_archs_supporte 59 can skip ahead to :ref:`kprobes_archs_supported`.
60 60
61 How Does a Kprobe Work? 61 How Does a Kprobe Work?
62 ----------------------- 62 -----------------------
63 63
64 When a kprobe is registered, Kprobes makes a c 64 When a kprobe is registered, Kprobes makes a copy of the probed
65 instruction and replaces the first byte(s) of 65 instruction and replaces the first byte(s) of the probed instruction
66 with a breakpoint instruction (e.g., int3 on i 66 with a breakpoint instruction (e.g., int3 on i386 and x86_64).
67 67
68 When a CPU hits the breakpoint instruction, a 68 When a CPU hits the breakpoint instruction, a trap occurs, the CPU's
69 registers are saved, and control passes to Kpr 69 registers are saved, and control passes to Kprobes via the
70 notifier_call_chain mechanism. Kprobes execut 70 notifier_call_chain mechanism. Kprobes executes the "pre_handler"
71 associated with the kprobe, passing the handle 71 associated with the kprobe, passing the handler the addresses of the
72 kprobe struct and the saved registers. 72 kprobe struct and the saved registers.
73 73
74 Next, Kprobes single-steps its copy of the pro 74 Next, Kprobes single-steps its copy of the probed instruction.
75 (It would be simpler to single-step the actual 75 (It would be simpler to single-step the actual instruction in place,
76 but then Kprobes would have to temporarily rem 76 but then Kprobes would have to temporarily remove the breakpoint
77 instruction. This would open a small time win 77 instruction. This would open a small time window when another CPU
78 could sail right past the probepoint.) 78 could sail right past the probepoint.)
79 79
80 After the instruction is single-stepped, Kprob 80 After the instruction is single-stepped, Kprobes executes the
81 "post_handler," if any, that is associated wit 81 "post_handler," if any, that is associated with the kprobe.
82 Execution then continues with the instruction 82 Execution then continues with the instruction following the probepoint.
83 83
84 Changing Execution Path 84 Changing Execution Path
85 ----------------------- 85 -----------------------
86 86
87 Since kprobes can probe into a running kernel 87 Since kprobes can probe into a running kernel code, it can change the
88 register set, including instruction pointer. T 88 register set, including instruction pointer. This operation requires
89 maximum care, such as keeping the stack frame, 89 maximum care, such as keeping the stack frame, recovering the execution
90 path etc. Since it operates on a running kerne 90 path etc. Since it operates on a running kernel and needs deep knowledge
91 of computer architecture and concurrent comput 91 of computer architecture and concurrent computing, you can easily shoot
92 your foot. 92 your foot.
93 93
94 If you change the instruction pointer (and set 94 If you change the instruction pointer (and set up other related
95 registers) in pre_handler, you must return !0 95 registers) in pre_handler, you must return !0 so that kprobes stops
96 single stepping and just returns to the given 96 single stepping and just returns to the given address.
97 This also means post_handler should not be cal 97 This also means post_handler should not be called anymore.
98 98
99 Note that this operation may be harder on some 99 Note that this operation may be harder on some architectures which use
100 TOC (Table of Contents) for function call, sin 100 TOC (Table of Contents) for function call, since you have to setup a new
101 TOC for your function in your module, and reco 101 TOC for your function in your module, and recover the old one after
102 returning from it. 102 returning from it.
103 103
104 Return Probes 104 Return Probes
105 ------------- 105 -------------
106 106
107 How Does a Return Probe Work? 107 How Does a Return Probe Work?
108 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 108 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
109 109
110 When you call register_kretprobe(), Kprobes es 110 When you call register_kretprobe(), Kprobes establishes a kprobe at
111 the entry to the function. When the probed fu 111 the entry to the function. When the probed function is called and this
112 probe is hit, Kprobes saves a copy of the retu 112 probe is hit, Kprobes saves a copy of the return address, and replaces
113 the return address with the address of a "tram 113 the return address with the address of a "trampoline." The trampoline
114 is an arbitrary piece of code -- typically jus 114 is an arbitrary piece of code -- typically just a nop instruction.
115 At boot time, Kprobes registers a kprobe at th 115 At boot time, Kprobes registers a kprobe at the trampoline.
116 116
117 When the probed function executes its return i 117 When the probed function executes its return instruction, control
118 passes to the trampoline and that probe is hit 118 passes to the trampoline and that probe is hit. Kprobes' trampoline
119 handler calls the user-specified return handle 119 handler calls the user-specified return handler associated with the
120 kretprobe, then sets the saved instruction poi 120 kretprobe, then sets the saved instruction pointer to the saved return
121 address, and that's where execution resumes up 121 address, and that's where execution resumes upon return from the trap.
122 122
123 While the probed function is executing, its re 123 While the probed function is executing, its return address is
124 stored in an object of type kretprobe_instance 124 stored in an object of type kretprobe_instance. Before calling
125 register_kretprobe(), the user sets the maxact 125 register_kretprobe(), the user sets the maxactive field of the
126 kretprobe struct to specify how many instances 126 kretprobe struct to specify how many instances of the specified
127 function can be probed simultaneously. regist 127 function can be probed simultaneously. register_kretprobe()
128 pre-allocates the indicated number of kretprob 128 pre-allocates the indicated number of kretprobe_instance objects.
129 129
130 For example, if the function is non-recursive 130 For example, if the function is non-recursive and is called with a
131 spinlock held, maxactive = 1 should be enough. 131 spinlock held, maxactive = 1 should be enough. If the function is
132 non-recursive and can never relinquish the CPU 132 non-recursive and can never relinquish the CPU (e.g., via a semaphore
133 or preemption), NR_CPUS should be enough. If 133 or preemption), NR_CPUS should be enough. If maxactive <= 0, it is
134 set to a default value: max(10, 2*NR_CPUS). 134 set to a default value: max(10, 2*NR_CPUS).
135 135
136 It's not a disaster if you set maxactive too l 136 It's not a disaster if you set maxactive too low; you'll just miss
137 some probes. In the kretprobe struct, the nmi 137 some probes. In the kretprobe struct, the nmissed field is set to
138 zero when the return probe is registered, and 138 zero when the return probe is registered, and is incremented every
139 time the probed function is entered but there 139 time the probed function is entered but there is no kretprobe_instance
140 object available for establishing the return p 140 object available for establishing the return probe.
141 141
142 Kretprobe entry-handler 142 Kretprobe entry-handler
143 ^^^^^^^^^^^^^^^^^^^^^^^ 143 ^^^^^^^^^^^^^^^^^^^^^^^
144 144
145 Kretprobes also provides an optional user-spec 145 Kretprobes also provides an optional user-specified handler which runs
146 on function entry. This handler is specified b 146 on function entry. This handler is specified by setting the entry_handler
147 field of the kretprobe struct. Whenever the kp 147 field of the kretprobe struct. Whenever the kprobe placed by kretprobe at the
148 function entry is hit, the user-defined entry_ 148 function entry is hit, the user-defined entry_handler, if any, is invoked.
149 If the entry_handler returns 0 (success) then 149 If the entry_handler returns 0 (success) then a corresponding return handler
150 is guaranteed to be called upon function retur 150 is guaranteed to be called upon function return. If the entry_handler
151 returns a non-zero error then Kprobes leaves t 151 returns a non-zero error then Kprobes leaves the return address as is, and
152 the kretprobe has no further effect for that p 152 the kretprobe has no further effect for that particular function instance.
153 153
154 Multiple entry and return handler invocations 154 Multiple entry and return handler invocations are matched using the unique
155 kretprobe_instance object associated with them 155 kretprobe_instance object associated with them. Additionally, a user
156 may also specify per return-instance private d 156 may also specify per return-instance private data to be part of each
157 kretprobe_instance object. This is especially 157 kretprobe_instance object. This is especially useful when sharing private
158 data between corresponding user entry and retu 158 data between corresponding user entry and return handlers. The size of each
159 private data object can be specified at kretpr 159 private data object can be specified at kretprobe registration time by
160 setting the data_size field of the kretprobe s 160 setting the data_size field of the kretprobe struct. This data can be
161 accessed through the data field of each kretpr 161 accessed through the data field of each kretprobe_instance object.
162 162
163 In case probed function is entered but there i 163 In case probed function is entered but there is no kretprobe_instance
164 object available, then in addition to incremen 164 object available, then in addition to incrementing the nmissed count,
165 the user entry_handler invocation is also skip 165 the user entry_handler invocation is also skipped.
166 166
167 .. _kprobes_jump_optimization: 167 .. _kprobes_jump_optimization:
168 168
169 How Does Jump Optimization Work? 169 How Does Jump Optimization Work?
170 -------------------------------- 170 --------------------------------
171 171
172 If your kernel is built with CONFIG_OPTPROBES= 172 If your kernel is built with CONFIG_OPTPROBES=y (currently this flag
173 is automatically set 'y' on x86/x86-64, non-pr 173 is automatically set 'y' on x86/x86-64, non-preemptive kernel) and
174 the "debug.kprobes_optimization" kernel parame 174 the "debug.kprobes_optimization" kernel parameter is set to 1 (see
175 sysctl(8)), Kprobes tries to reduce probe-hit 175 sysctl(8)), Kprobes tries to reduce probe-hit overhead by using a jump
176 instruction instead of a breakpoint instructio 176 instruction instead of a breakpoint instruction at each probepoint.
177 177
178 Init a Kprobe 178 Init a Kprobe
179 ^^^^^^^^^^^^^ 179 ^^^^^^^^^^^^^
180 180
181 When a probe is registered, before attempting 181 When a probe is registered, before attempting this optimization,
182 Kprobes inserts an ordinary, breakpoint-based 182 Kprobes inserts an ordinary, breakpoint-based kprobe at the specified
183 address. So, even if it's not possible to opti 183 address. So, even if it's not possible to optimize this particular
184 probepoint, there'll be a probe there. 184 probepoint, there'll be a probe there.
185 185
186 Safety Check 186 Safety Check
187 ^^^^^^^^^^^^ 187 ^^^^^^^^^^^^
188 188
189 Before optimizing a probe, Kprobes performs th 189 Before optimizing a probe, Kprobes performs the following safety checks:
190 190
191 - Kprobes verifies that the region that will b 191 - Kprobes verifies that the region that will be replaced by the jump
192 instruction (the "optimized region") lies en 192 instruction (the "optimized region") lies entirely within one function.
193 (A jump instruction is multiple bytes, and s 193 (A jump instruction is multiple bytes, and so may overlay multiple
194 instructions.) 194 instructions.)
195 195
196 - Kprobes analyzes the entire function and ver 196 - Kprobes analyzes the entire function and verifies that there is no
197 jump into the optimized region. Specificall 197 jump into the optimized region. Specifically:
198 198
199 - the function contains no indirect jump; 199 - the function contains no indirect jump;
200 - the function contains no instruction that 200 - the function contains no instruction that causes an exception (since
201 the fixup code triggered by the exception 201 the fixup code triggered by the exception could jump back into the
202 optimized region -- Kprobes checks the exc 202 optimized region -- Kprobes checks the exception tables to verify this);
203 - there is no near jump to the optimized reg 203 - there is no near jump to the optimized region (other than to the first
204 byte). 204 byte).
205 205
206 - For each instruction in the optimized region 206 - For each instruction in the optimized region, Kprobes verifies that
207 the instruction can be executed out of line. 207 the instruction can be executed out of line.
208 208
209 Preparing Detour Buffer 209 Preparing Detour Buffer
210 ^^^^^^^^^^^^^^^^^^^^^^^ 210 ^^^^^^^^^^^^^^^^^^^^^^^
211 211
212 Next, Kprobes prepares a "detour" buffer, whic 212 Next, Kprobes prepares a "detour" buffer, which contains the following
213 instruction sequence: 213 instruction sequence:
214 214
215 - code to push the CPU's registers (emulating 215 - code to push the CPU's registers (emulating a breakpoint trap)
216 - a call to the trampoline code which calls us 216 - a call to the trampoline code which calls user's probe handlers.
217 - code to restore registers 217 - code to restore registers
218 - the instructions from the optimized region 218 - the instructions from the optimized region
219 - a jump back to the original execution path. 219 - a jump back to the original execution path.
220 220
221 Pre-optimization 221 Pre-optimization
222 ^^^^^^^^^^^^^^^^ 222 ^^^^^^^^^^^^^^^^
223 223
224 After preparing the detour buffer, Kprobes ver 224 After preparing the detour buffer, Kprobes verifies that none of the
225 following situations exist: 225 following situations exist:
226 226
227 - The probe has a post_handler. 227 - The probe has a post_handler.
228 - Other instructions in the optimized region a 228 - Other instructions in the optimized region are probed.
229 - The probe is disabled. 229 - The probe is disabled.
230 230
231 In any of the above cases, Kprobes won't start 231 In any of the above cases, Kprobes won't start optimizing the probe.
232 Since these are temporary situations, Kprobes 232 Since these are temporary situations, Kprobes tries to start
233 optimizing it again if the situation is change 233 optimizing it again if the situation is changed.
234 234
235 If the kprobe can be optimized, Kprobes enqueu 235 If the kprobe can be optimized, Kprobes enqueues the kprobe to an
236 optimizing list, and kicks the kprobe-optimize 236 optimizing list, and kicks the kprobe-optimizer workqueue to optimize
237 it. If the to-be-optimized probepoint is hit 237 it. If the to-be-optimized probepoint is hit before being optimized,
238 Kprobes returns control to the original instru 238 Kprobes returns control to the original instruction path by setting
239 the CPU's instruction pointer to the copied co 239 the CPU's instruction pointer to the copied code in the detour buffer
240 -- thus at least avoiding the single-step. 240 -- thus at least avoiding the single-step.
241 241
242 Optimization 242 Optimization
243 ^^^^^^^^^^^^ 243 ^^^^^^^^^^^^
244 244
245 The Kprobe-optimizer doesn't insert the jump i 245 The Kprobe-optimizer doesn't insert the jump instruction immediately;
246 rather, it calls synchronize_rcu() for safety 246 rather, it calls synchronize_rcu() for safety first, because it's
247 possible for a CPU to be interrupted in the mi 247 possible for a CPU to be interrupted in the middle of executing the
248 optimized region [3]_. As you know, synchroni 248 optimized region [3]_. As you know, synchronize_rcu() can ensure
249 that all interruptions that were active when s 249 that all interruptions that were active when synchronize_rcu()
250 was called are done, but only if CONFIG_PREEMP 250 was called are done, but only if CONFIG_PREEMPT=n. So, this version
251 of kprobe optimization supports only kernels w 251 of kprobe optimization supports only kernels with CONFIG_PREEMPT=n [4]_.
252 252
253 After that, the Kprobe-optimizer calls stop_ma 253 After that, the Kprobe-optimizer calls stop_machine() to replace
254 the optimized region with a jump instruction t 254 the optimized region with a jump instruction to the detour buffer,
255 using text_poke_smp(). 255 using text_poke_smp().
256 256
257 Unoptimization 257 Unoptimization
258 ^^^^^^^^^^^^^^ 258 ^^^^^^^^^^^^^^
259 259
260 When an optimized kprobe is unregistered, disa 260 When an optimized kprobe is unregistered, disabled, or blocked by
261 another kprobe, it will be unoptimized. If th 261 another kprobe, it will be unoptimized. If this happens before
262 the optimization is complete, the kprobe is ju 262 the optimization is complete, the kprobe is just dequeued from the
263 optimized list. If the optimization has been 263 optimized list. If the optimization has been done, the jump is
264 replaced with the original code (except for an 264 replaced with the original code (except for an int3 breakpoint in
265 the first byte) by using text_poke_smp(). 265 the first byte) by using text_poke_smp().
266 266
267 .. [3] Please imagine that the 2nd instruction 267 .. [3] Please imagine that the 2nd instruction is interrupted and then
268 the optimizer replaces the 2nd instruction 268 the optimizer replaces the 2nd instruction with the jump *address*
269 while the interrupt handler is running. Whe 269 while the interrupt handler is running. When the interrupt
270 returns to original address, there is no va 270 returns to original address, there is no valid instruction,
271 and it causes an unexpected result. 271 and it causes an unexpected result.
272 272
273 .. [4] This optimization-safety checking may b 273 .. [4] This optimization-safety checking may be replaced with the
274 stop-machine method that ksplice uses for s 274 stop-machine method that ksplice uses for supporting a CONFIG_PREEMPT=y
275 kernel. 275 kernel.
276 276
277 NOTE for geeks: 277 NOTE for geeks:
278 The jump optimization changes the kprobe's pre 278 The jump optimization changes the kprobe's pre_handler behavior.
279 Without optimization, the pre_handler can chan 279 Without optimization, the pre_handler can change the kernel's execution
280 path by changing regs->ip and returning 1. Ho 280 path by changing regs->ip and returning 1. However, when the probe
281 is optimized, that modification is ignored. T 281 is optimized, that modification is ignored. Thus, if you want to
282 tweak the kernel's execution path, you need to 282 tweak the kernel's execution path, you need to suppress optimization,
283 using one of the following techniques: 283 using one of the following techniques:
284 284
285 - Specify an empty function for the kprobe's p 285 - Specify an empty function for the kprobe's post_handler.
286 286
287 or 287 or
288 288
289 - Execute 'sysctl -w debug.kprobes_optimizatio 289 - Execute 'sysctl -w debug.kprobes_optimization=n'
290 290
291 .. _kprobes_blacklist: 291 .. _kprobes_blacklist:
292 292
293 Blacklist 293 Blacklist
294 --------- 294 ---------
295 295
296 Kprobes can probe most of the kernel except it 296 Kprobes can probe most of the kernel except itself. This means
297 that there are some functions where kprobes ca 297 that there are some functions where kprobes cannot probe. Probing
298 (trapping) such functions can cause a recursiv 298 (trapping) such functions can cause a recursive trap (e.g. double
299 fault) or the nested probe handler may never b 299 fault) or the nested probe handler may never be called.
300 Kprobes manages such functions as a blacklist. 300 Kprobes manages such functions as a blacklist.
301 If you want to add a function into the blackli 301 If you want to add a function into the blacklist, you just need
302 to (1) include linux/kprobes.h and (2) use NOK 302 to (1) include linux/kprobes.h and (2) use NOKPROBE_SYMBOL() macro
303 to specify a blacklisted function. 303 to specify a blacklisted function.
304 Kprobes checks the given probe address against 304 Kprobes checks the given probe address against the blacklist and
305 rejects registering it, if the given address i 305 rejects registering it, if the given address is in the blacklist.
306 306
307 .. _kprobes_archs_supported: 307 .. _kprobes_archs_supported:
308 308
309 Architectures Supported 309 Architectures Supported
310 ======================= 310 =======================
311 311
312 Kprobes and return probes are implemented on t 312 Kprobes and return probes are implemented on the following
313 architectures: 313 architectures:
314 314
315 - i386 (Supports jump optimization) 315 - i386 (Supports jump optimization)
316 - x86_64 (AMD-64, EM64T) (Supports jump optimi 316 - x86_64 (AMD-64, EM64T) (Supports jump optimization)
317 - ppc64 317 - ppc64
>> 318 - ia64 (Does not support probes on instruction slot1.)
318 - sparc64 (Return probes not yet implemented.) 319 - sparc64 (Return probes not yet implemented.)
319 - arm 320 - arm
320 - ppc 321 - ppc
321 - mips 322 - mips
322 - s390 323 - s390
323 - parisc 324 - parisc
324 - loongarch <<
325 - riscv <<
326 325
327 Configuring Kprobes 326 Configuring Kprobes
328 =================== 327 ===================
329 328
330 When configuring the kernel using make menucon 329 When configuring the kernel using make menuconfig/xconfig/oldconfig,
331 ensure that CONFIG_KPROBES is set to "y", look !! 330 ensure that CONFIG_KPROBES is set to "y". Under "General setup", look
332 "General architecture-dependent options". !! 331 for "Kprobes".
333 332
334 So that you can load and unload Kprobes-based 333 So that you can load and unload Kprobes-based instrumentation modules,
335 make sure "Loadable module support" (CONFIG_MO 334 make sure "Loadable module support" (CONFIG_MODULES) and "Module
336 unloading" (CONFIG_MODULE_UNLOAD) are set to " 335 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
337 336
338 Also make sure that CONFIG_KALLSYMS and perhap 337 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
339 are set to "y", since kallsyms_lookup_name() i 338 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
340 kprobe address resolution code. 339 kprobe address resolution code.
341 340
342 If you need to insert a probe in the middle of 341 If you need to insert a probe in the middle of a function, you may find
343 it useful to "Compile the kernel with debug in 342 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
344 so you can use "objdump -d -l vmlinux" to see 343 so you can use "objdump -d -l vmlinux" to see the source-to-object
345 code mapping. 344 code mapping.
346 345
347 API Reference 346 API Reference
348 ============= 347 =============
349 348
350 The Kprobes API includes a "register" function 349 The Kprobes API includes a "register" function and an "unregister"
351 function for each type of probe. The API also 350 function for each type of probe. The API also includes "register_*probes"
352 and "unregister_*probes" functions for (un)reg 351 and "unregister_*probes" functions for (un)registering arrays of probes.
353 Here are terse, mini-man-page specifications f 352 Here are terse, mini-man-page specifications for these functions and
354 the associated probe handlers that you'll writ 353 the associated probe handlers that you'll write. See the files in the
355 samples/kprobes/ sub-directory for examples. 354 samples/kprobes/ sub-directory for examples.
356 355
357 register_kprobe 356 register_kprobe
358 --------------- 357 ---------------
359 358
360 :: 359 ::
361 360
362 #include <linux/kprobes.h> 361 #include <linux/kprobes.h>
363 int register_kprobe(struct kprobe *kp) 362 int register_kprobe(struct kprobe *kp);
364 363
365 Sets a breakpoint at the address kp->addr. Wh 364 Sets a breakpoint at the address kp->addr. When the breakpoint is hit, Kprobes
366 calls kp->pre_handler. After the probed instr 365 calls kp->pre_handler. After the probed instruction is single-stepped, Kprobe
367 calls kp->post_handler. Any or all handlers c 366 calls kp->post_handler. Any or all handlers can be NULL. If kp->flags is set
368 KPROBE_FLAG_DISABLED, that kp will be register 367 KPROBE_FLAG_DISABLED, that kp will be registered but disabled, so, its handlers
369 aren't hit until calling enable_kprobe(kp). 368 aren't hit until calling enable_kprobe(kp).
370 369
371 .. note:: 370 .. note::
372 371
373 1. With the introduction of the "symbol_nam 372 1. With the introduction of the "symbol_name" field to struct kprobe,
374 the probepoint address resolution will n 373 the probepoint address resolution will now be taken care of by the kernel.
375 The following will now work:: 374 The following will now work::
376 375
377 kp.symbol_name = "symbol_name"; 376 kp.symbol_name = "symbol_name";
378 377
379 (64-bit powerpc intricacies such as func 378 (64-bit powerpc intricacies such as function descriptors are handled
380 transparently) 379 transparently)
381 380
382 2. Use the "offset" field of struct kprobe 381 2. Use the "offset" field of struct kprobe if the offset into the symbol
383 to install a probepoint is known. This f 382 to install a probepoint is known. This field is used to calculate the
384 probepoint. 383 probepoint.
385 384
386 3. Specify either the kprobe "symbol_name" 385 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
387 specified, kprobe registration will fail 386 specified, kprobe registration will fail with -EINVAL.
388 387
389 4. With CISC architectures (such as i386 an 388 4. With CISC architectures (such as i386 and x86_64), the kprobes code
390 does not validate if the kprobe.addr is 389 does not validate if the kprobe.addr is at an instruction boundary.
391 Use "offset" with caution. 390 Use "offset" with caution.
392 391
393 register_kprobe() returns 0 on success, or a n 392 register_kprobe() returns 0 on success, or a negative errno otherwise.
394 393
395 User's pre-handler (kp->pre_handler):: 394 User's pre-handler (kp->pre_handler)::
396 395
397 #include <linux/kprobes.h> 396 #include <linux/kprobes.h>
398 #include <linux/ptrace.h> 397 #include <linux/ptrace.h>
399 int pre_handler(struct kprobe *p, stru 398 int pre_handler(struct kprobe *p, struct pt_regs *regs);
400 399
401 Called with p pointing to the kprobe associate 400 Called with p pointing to the kprobe associated with the breakpoint,
402 and regs pointing to the struct containing the 401 and regs pointing to the struct containing the registers saved when
403 the breakpoint was hit. Return 0 here unless 402 the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
404 403
405 User's post-handler (kp->post_handler):: 404 User's post-handler (kp->post_handler)::
406 405
407 #include <linux/kprobes.h> 406 #include <linux/kprobes.h>
408 #include <linux/ptrace.h> 407 #include <linux/ptrace.h>
409 void post_handler(struct kprobe *p, st 408 void post_handler(struct kprobe *p, struct pt_regs *regs,
410 unsigned long flags) 409 unsigned long flags);
411 410
412 p and regs are as described for the pre_handle 411 p and regs are as described for the pre_handler. flags always seems
413 to be zero. 412 to be zero.
414 413
415 register_kretprobe 414 register_kretprobe
416 ------------------ 415 ------------------
417 416
418 :: 417 ::
419 418
420 #include <linux/kprobes.h> 419 #include <linux/kprobes.h>
421 int register_kretprobe(struct kretprob 420 int register_kretprobe(struct kretprobe *rp);
422 421
423 Establishes a return probe for the function wh 422 Establishes a return probe for the function whose address is
424 rp->kp.addr. When that function returns, Kpro 423 rp->kp.addr. When that function returns, Kprobes calls rp->handler.
425 You must set rp->maxactive appropriately befor 424 You must set rp->maxactive appropriately before you call
426 register_kretprobe(); see "How Does a Return P 425 register_kretprobe(); see "How Does a Return Probe Work?" for details.
427 426
428 register_kretprobe() returns 0 on success, or 427 register_kretprobe() returns 0 on success, or a negative errno
429 otherwise. 428 otherwise.
430 429
431 User's return-probe handler (rp->handler):: 430 User's return-probe handler (rp->handler)::
432 431
433 #include <linux/kprobes.h> 432 #include <linux/kprobes.h>
434 #include <linux/ptrace.h> 433 #include <linux/ptrace.h>
435 int kretprobe_handler(struct kretprobe 434 int kretprobe_handler(struct kretprobe_instance *ri,
436 struct pt_regs * 435 struct pt_regs *regs);
437 436
438 regs is as described for kprobe.pre_handler. 437 regs is as described for kprobe.pre_handler. ri points to the
439 kretprobe_instance object, of which the follow 438 kretprobe_instance object, of which the following fields may be
440 of interest: 439 of interest:
441 440
442 - ret_addr: the return address 441 - ret_addr: the return address
443 - rp: points to the corresponding kretprobe ob 442 - rp: points to the corresponding kretprobe object
444 - task: points to the corresponding task struc 443 - task: points to the corresponding task struct
445 - data: points to per return-instance private 444 - data: points to per return-instance private data; see "Kretprobe
446 entry-handler" for details. 445 entry-handler" for details.
447 446
448 The regs_return_value(regs) macro provides a s 447 The regs_return_value(regs) macro provides a simple abstraction to
449 extract the return value from the appropriate 448 extract the return value from the appropriate register as defined by
450 the architecture's ABI. 449 the architecture's ABI.
451 450
452 The handler's return value is currently ignore 451 The handler's return value is currently ignored.
453 452
454 unregister_*probe 453 unregister_*probe
455 ------------------ 454 ------------------
456 455
457 :: 456 ::
458 457
459 #include <linux/kprobes.h> 458 #include <linux/kprobes.h>
460 void unregister_kprobe(struct kprobe * 459 void unregister_kprobe(struct kprobe *kp);
461 void unregister_kretprobe(struct kretp 460 void unregister_kretprobe(struct kretprobe *rp);
462 461
463 Removes the specified probe. The unregister f 462 Removes the specified probe. The unregister function can be called
464 at any time after the probe has been registere 463 at any time after the probe has been registered.
465 464
466 .. note:: 465 .. note::
467 466
468 If the functions find an incorrect probe (e 467 If the functions find an incorrect probe (ex. an unregistered probe),
469 they clear the addr field of the probe. 468 they clear the addr field of the probe.
470 469
471 register_*probes 470 register_*probes
472 ---------------- 471 ----------------
473 472
474 :: 473 ::
475 474
476 #include <linux/kprobes.h> 475 #include <linux/kprobes.h>
477 int register_kprobes(struct kprobe **k 476 int register_kprobes(struct kprobe **kps, int num);
478 int register_kretprobes(struct kretpro 477 int register_kretprobes(struct kretprobe **rps, int num);
479 478
480 Registers each of the num probes in the specif 479 Registers each of the num probes in the specified array. If any
481 error occurs during registration, all probes i 480 error occurs during registration, all probes in the array, up to
482 the bad probe, are safely unregistered before 481 the bad probe, are safely unregistered before the register_*probes
483 function returns. 482 function returns.
484 483
485 - kps/rps: an array of pointers to ``*probe`` 484 - kps/rps: an array of pointers to ``*probe`` data structures
486 - num: the number of the array entries. 485 - num: the number of the array entries.
487 486
488 .. note:: 487 .. note::
489 488
490 You have to allocate(or define) an array of 489 You have to allocate(or define) an array of pointers and set all
491 of the array entries before using these fun 490 of the array entries before using these functions.
492 491
493 unregister_*probes 492 unregister_*probes
494 ------------------ 493 ------------------
495 494
496 :: 495 ::
497 496
498 #include <linux/kprobes.h> 497 #include <linux/kprobes.h>
499 void unregister_kprobes(struct kprobe 498 void unregister_kprobes(struct kprobe **kps, int num);
500 void unregister_kretprobes(struct kret 499 void unregister_kretprobes(struct kretprobe **rps, int num);
501 500
502 Removes each of the num probes in the specifie 501 Removes each of the num probes in the specified array at once.
503 502
504 .. note:: 503 .. note::
505 504
506 If the functions find some incorrect probes 505 If the functions find some incorrect probes (ex. unregistered
507 probes) in the specified array, they clear 506 probes) in the specified array, they clear the addr field of those
508 incorrect probes. However, other probes in 507 incorrect probes. However, other probes in the array are
509 unregistered correctly. 508 unregistered correctly.
510 509
511 disable_*probe 510 disable_*probe
512 -------------- 511 --------------
513 512
514 :: 513 ::
515 514
516 #include <linux/kprobes.h> 515 #include <linux/kprobes.h>
517 int disable_kprobe(struct kprobe *kp); 516 int disable_kprobe(struct kprobe *kp);
518 int disable_kretprobe(struct kretprobe 517 int disable_kretprobe(struct kretprobe *rp);
519 518
520 Temporarily disables the specified ``*probe``. 519 Temporarily disables the specified ``*probe``. You can enable it again by using
521 enable_*probe(). You must specify the probe wh 520 enable_*probe(). You must specify the probe which has been registered.
522 521
523 enable_*probe 522 enable_*probe
524 ------------- 523 -------------
525 524
526 :: 525 ::
527 526
528 #include <linux/kprobes.h> 527 #include <linux/kprobes.h>
529 int enable_kprobe(struct kprobe *kp); 528 int enable_kprobe(struct kprobe *kp);
530 int enable_kretprobe(struct kretprobe 529 int enable_kretprobe(struct kretprobe *rp);
531 530
532 Enables ``*probe`` which has been disabled by 531 Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
533 the probe which has been registered. 532 the probe which has been registered.
534 533
535 Kprobes Features and Limitations 534 Kprobes Features and Limitations
536 ================================ 535 ================================
537 536
538 Kprobes allows multiple probes at the same add 537 Kprobes allows multiple probes at the same address. Also,
539 a probepoint for which there is a post_handler 538 a probepoint for which there is a post_handler cannot be optimized.
540 So if you install a kprobe with a post_handler 539 So if you install a kprobe with a post_handler, at an optimized
541 probepoint, the probepoint will be unoptimized 540 probepoint, the probepoint will be unoptimized automatically.
542 541
543 In general, you can install a probe anywhere i 542 In general, you can install a probe anywhere in the kernel.
544 In particular, you can probe interrupt handler 543 In particular, you can probe interrupt handlers. Known exceptions
545 are discussed in this section. 544 are discussed in this section.
546 545
547 The register_*probe functions will return -EIN 546 The register_*probe functions will return -EINVAL if you attempt
548 to install a probe in the code that implements 547 to install a probe in the code that implements Kprobes (mostly
549 kernel/kprobes.c and ``arch/*/kernel/kprobes.c 548 kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
550 as do_page_fault and notifier_call_chain). 549 as do_page_fault and notifier_call_chain).
551 550
552 If you install a probe in an inline-able funct 551 If you install a probe in an inline-able function, Kprobes makes
553 no attempt to chase down all inline instances 552 no attempt to chase down all inline instances of the function and
554 install probes there. gcc may inline a functi 553 install probes there. gcc may inline a function without being asked,
555 so keep this in mind if you're not seeing the 554 so keep this in mind if you're not seeing the probe hits you expect.
556 555
557 A probe handler can modify the environment of 556 A probe handler can modify the environment of the probed function
558 -- e.g., by modifying kernel data structures, 557 -- e.g., by modifying kernel data structures, or by modifying the
559 contents of the pt_regs struct (which are rest 558 contents of the pt_regs struct (which are restored to the registers
560 upon return from the breakpoint). So Kprobes 559 upon return from the breakpoint). So Kprobes can be used, for example,
561 to install a bug fix or to inject faults for t 560 to install a bug fix or to inject faults for testing. Kprobes, of
562 course, has no way to distinguish the delibera 561 course, has no way to distinguish the deliberately injected faults
563 from the accidental ones. Don't drink and pro 562 from the accidental ones. Don't drink and probe.
564 563
565 Kprobes makes no attempt to prevent probe hand 564 Kprobes makes no attempt to prevent probe handlers from stepping on
566 each other -- e.g., probing printk() and then 565 each other -- e.g., probing printk() and then calling printk() from a
567 probe handler. If a probe handler hits a prob 566 probe handler. If a probe handler hits a probe, that second probe's
568 handlers won't be run in that instance, and th 567 handlers won't be run in that instance, and the kprobe.nmissed member
569 of the second probe will be incremented. 568 of the second probe will be incremented.
570 569
571 As of Linux v2.6.15-rc1, multiple handlers (or 570 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
572 the same handler) may run concurrently on diff 571 the same handler) may run concurrently on different CPUs.
573 572
574 Kprobes does not use mutexes or allocate memor 573 Kprobes does not use mutexes or allocate memory except during
575 registration and unregistration. 574 registration and unregistration.
576 575
577 Probe handlers are run with preemption disable 576 Probe handlers are run with preemption disabled or interrupt disabled,
578 which depends on the architecture and optimiza 577 which depends on the architecture and optimization state. (e.g.,
579 kretprobe handlers and optimized kprobe handle 578 kretprobe handlers and optimized kprobe handlers run without interrupt
580 disabled on x86/x86-64). In any case, your ha 579 disabled on x86/x86-64). In any case, your handler should not yield
581 the CPU (e.g., by attempting to acquire a sema 580 the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).
582 581
583 Since a return probe is implemented by replaci 582 Since a return probe is implemented by replacing the return
584 address with the trampoline's address, stack b 583 address with the trampoline's address, stack backtraces and calls
585 to __builtin_return_address() will typically y 584 to __builtin_return_address() will typically yield the trampoline's
586 address instead of the real return address for 585 address instead of the real return address for kretprobed functions.
587 (As far as we can tell, __builtin_return_addre 586 (As far as we can tell, __builtin_return_address() is used only
588 for instrumentation and error reporting.) 587 for instrumentation and error reporting.)
589 588
590 If the number of times a function is called do 589 If the number of times a function is called does not match the number
591 of times it returns, registering a return prob 590 of times it returns, registering a return probe on that function may
592 produce undesirable results. In such a case, a 591 produce undesirable results. In such a case, a line:
593 kretprobe BUG!: Processing kretprobe d00000000 592 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
594 gets printed. With this information, one will 593 gets printed. With this information, one will be able to correlate the
595 exact instance of the kretprobe that caused th 594 exact instance of the kretprobe that caused the problem. We have the
596 do_exit() case covered. do_execve() and do_for 595 do_exit() case covered. do_execve() and do_fork() are not an issue.
597 We're unaware of other specific cases where th 596 We're unaware of other specific cases where this could be a problem.
598 597
599 If, upon entry to or exit from a function, the 598 If, upon entry to or exit from a function, the CPU is running on
600 a stack other than that of the current task, r 599 a stack other than that of the current task, registering a return
601 probe on that function may produce undesirable 600 probe on that function may produce undesirable results. For this
602 reason, Kprobes doesn't support return probes 601 reason, Kprobes doesn't support return probes (or kprobes)
603 on the x86_64 version of __switch_to(); the re 602 on the x86_64 version of __switch_to(); the registration functions
604 return -EINVAL. 603 return -EINVAL.
605 604
606 On x86/x86-64, since the Jump Optimization of 605 On x86/x86-64, since the Jump Optimization of Kprobes modifies
607 instructions widely, there are some limitation 606 instructions widely, there are some limitations to optimization. To
608 explain it, we introduce some terminology. Ima 607 explain it, we introduce some terminology. Imagine a 3-instruction
609 sequence consisting of a two 2-byte instructio 608 sequence consisting of a two 2-byte instructions and one 3-byte
610 instruction. 609 instruction.
611 610
612 :: 611 ::
613 612
614 IA 613 IA
615 | 614 |
616 [-2][-1][0][1][2][3][4][5][6][7] 615 [-2][-1][0][1][2][3][4][5][6][7]
617 [ins1][ins2][ ins3 ] 616 [ins1][ins2][ ins3 ]
618 [<- DCR ->] 617 [<- DCR ->]
619 [<- JTPR ->] 618 [<- JTPR ->]
620 619
621 ins1: 1st Instruction 620 ins1: 1st Instruction
622 ins2: 2nd Instruction 621 ins2: 2nd Instruction
623 ins3: 3rd Instruction 622 ins3: 3rd Instruction
624 IA: Insertion Address 623 IA: Insertion Address
625 JTPR: Jump Target Prohibition Region 624 JTPR: Jump Target Prohibition Region
626 DCR: Detoured Code Region 625 DCR: Detoured Code Region
627 626
628 The instructions in DCR are copied to the out- 627 The instructions in DCR are copied to the out-of-line buffer
629 of the kprobe, because the bytes in DCR are re 628 of the kprobe, because the bytes in DCR are replaced by
630 a 5-byte jump instruction. So there are severa 629 a 5-byte jump instruction. So there are several limitations.
631 630
632 a) The instructions in DCR must be relocatable 631 a) The instructions in DCR must be relocatable.
633 b) The instructions in DCR must not include a 632 b) The instructions in DCR must not include a call instruction.
634 c) JTPR must not be targeted by any jump or ca 633 c) JTPR must not be targeted by any jump or call instruction.
635 d) DCR must not straddle the border between fu 634 d) DCR must not straddle the border between functions.
636 635
637 Anyway, these limitations are checked by the i 636 Anyway, these limitations are checked by the in-kernel instruction
638 decoder, so you don't need to worry about that 637 decoder, so you don't need to worry about that.
639 638
640 Probe Overhead 639 Probe Overhead
641 ============== 640 ==============
642 641
643 On a typical CPU in use in 2005, a kprobe hit 642 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
644 microseconds to process. Specifically, a benc 643 microseconds to process. Specifically, a benchmark that hits the same
645 probepoint repeatedly, firing a simple handler 644 probepoint repeatedly, firing a simple handler each time, reports 1-2
646 million hits per second, depending on the arch 645 million hits per second, depending on the architecture. A return-probe
647 hit typically takes 50-75% longer than a kprob 646 hit typically takes 50-75% longer than a kprobe hit.
648 When you have a return probe set on a function 647 When you have a return probe set on a function, adding a kprobe at
649 the entry to that function adds essentially no 648 the entry to that function adds essentially no overhead.
650 649
651 Here are sample overhead figures (in usec) for 650 Here are sample overhead figures (in usec) for different architectures::
652 651
653 k = kprobe; r = return probe; kr = kprobe + 652 k = kprobe; r = return probe; kr = kprobe + return probe
654 on same function 653 on same function
655 654
656 i386: Intel Pentium M, 1495 MHz, 2957.31 bog 655 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
657 k = 0.57 usec; r = 0.92; kr = 0.99 656 k = 0.57 usec; r = 0.92; kr = 0.99
658 657
659 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 b 658 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
660 k = 0.49 usec; r = 0.80; kr = 0.82 659 k = 0.49 usec; r = 0.80; kr = 0.82
661 660
662 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 661 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
663 k = 0.77 usec; r = 1.26; kr = 1.45 662 k = 0.77 usec; r = 1.26; kr = 1.45
664 663
665 Optimized Probe Overhead 664 Optimized Probe Overhead
666 ------------------------ 665 ------------------------
667 666
668 Typically, an optimized kprobe hit takes 0.07 667 Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
669 process. Here are sample overhead figures (in 668 process. Here are sample overhead figures (in usec) for x86 architectures::
670 669
671 k = unoptimized kprobe, b = boosted (single- 670 k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
672 r = unoptimized kretprobe, rb = boosted kret 671 r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
673 672
674 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656. 673 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
675 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; 674 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
676 675
677 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 465 676 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 4656.90 bogomips
678 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; 677 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
679 678
680 TODO 679 TODO
681 ==== 680 ====
682 681
683 a. SystemTap (http://sourceware.org/systemtap) 682 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
684 programming interface for probe-based instr 683 programming interface for probe-based instrumentation. Try it out.
685 b. Kernel return probes for sparc64. 684 b. Kernel return probes for sparc64.
686 c. Support for other architectures. 685 c. Support for other architectures.
687 d. User-space probes. 686 d. User-space probes.
688 e. Watchpoint probes (which fire on data refer 687 e. Watchpoint probes (which fire on data references).
689 688
690 Kprobes Example 689 Kprobes Example
691 =============== 690 ===============
692 691
693 See samples/kprobes/kprobe_example.c 692 See samples/kprobes/kprobe_example.c
694 693
695 Kretprobes Example 694 Kretprobes Example
696 ================== 695 ==================
697 696
698 See samples/kprobes/kretprobe_example.c 697 See samples/kprobes/kretprobe_example.c
699 698
700 Deprecated Features 699 Deprecated Features
701 =================== 700 ===================
702 701
703 Jprobes is now a deprecated feature. People wh 702 Jprobes is now a deprecated feature. People who are depending on it should
704 migrate to other tracing features or use older 703 migrate to other tracing features or use older kernels. Please consider to
705 migrate your tool to one of the following opti 704 migrate your tool to one of the following options:
706 705
707 - Use trace-event to trace target function wit 706 - Use trace-event to trace target function with arguments.
708 707
709 trace-event is a low-overhead (and almost no 708 trace-event is a low-overhead (and almost no visible overhead if it
710 is off) statically defined event interface. 709 is off) statically defined event interface. You can define new events
711 and trace it via ftrace or any other tracing 710 and trace it via ftrace or any other tracing tools.
712 711
713 See the following urls: 712 See the following urls:
714 713
715 - https://lwn.net/Articles/379903/ 714 - https://lwn.net/Articles/379903/
716 - https://lwn.net/Articles/381064/ 715 - https://lwn.net/Articles/381064/
717 - https://lwn.net/Articles/383362/ 716 - https://lwn.net/Articles/383362/
718 717
719 - Use ftrace dynamic events (kprobe event) wit 718 - Use ftrace dynamic events (kprobe event) with perf-probe.
720 719
721 If you build your kernel with debug info (CO 720 If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
722 find which register/stack is assigned to whi 721 find which register/stack is assigned to which local variable or arguments
723 by using perf-probe and set up new event to 722 by using perf-probe and set up new event to trace it.
724 723
725 See following documents: 724 See following documents:
726 725
727 - Documentation/trace/kprobetrace.rst 726 - Documentation/trace/kprobetrace.rst
728 - Documentation/trace/events.rst 727 - Documentation/trace/events.rst
729 - tools/perf/Documentation/perf-probe.txt 728 - tools/perf/Documentation/perf-probe.txt
730 729
731 730
732 The kprobes debugfs interface 731 The kprobes debugfs interface
733 ============================= 732 =============================
734 733
735 734
736 With recent kernels (> 2.6.20) the list of reg 735 With recent kernels (> 2.6.20) the list of registered kprobes is visible
737 under the /sys/kernel/debug/kprobes/ directory 736 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
738 737
739 /sys/kernel/debug/kprobes/list: Lists all regi 738 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
740 739
741 c015d71a k vfs_read+0x0 740 c015d71a k vfs_read+0x0
742 c03dedc5 r tcp_v4_rcv+0x0 741 c03dedc5 r tcp_v4_rcv+0x0
743 742
744 The first column provides the kernel address w 743 The first column provides the kernel address where the probe is inserted.
745 The second column identifies the type of probe 744 The second column identifies the type of probe (k - kprobe and r - kretprobe)
746 while the third column specifies the symbol+of 745 while the third column specifies the symbol+offset of the probe.
747 If the probed function belongs to a module, th 746 If the probed function belongs to a module, the module name is also
748 specified. Following columns show probe status 747 specified. Following columns show probe status. If the probe is on
749 a virtual address that is no longer valid (mod 748 a virtual address that is no longer valid (module init sections, module
750 virtual addresses that correspond to modules t 749 virtual addresses that correspond to modules that've been unloaded),
751 such probes are marked with [GONE]. If the pro 750 such probes are marked with [GONE]. If the probe is temporarily disabled,
752 such probes are marked with [DISABLED]. If the 751 such probes are marked with [DISABLED]. If the probe is optimized, it is
753 marked with [OPTIMIZED]. If the probe is ftrac 752 marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
754 [FTRACE]. 753 [FTRACE].
755 754
756 /sys/kernel/debug/kprobes/enabled: Turn kprobe 755 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
757 756
758 Provides a knob to globally and forcibly turn 757 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
759 By default, all kprobes are enabled. By echoin 758 By default, all kprobes are enabled. By echoing "0" to this file, all
760 registered probes will be disarmed, till such 759 registered probes will be disarmed, till such time a "1" is echoed to this
761 file. Note that this knob just disarms and arm 760 file. Note that this knob just disarms and arms all kprobes and doesn't
762 change each probe's disabling state. This mean 761 change each probe's disabling state. This means that disabled kprobes (marked
763 [DISABLED]) will be not enabled if you turn ON 762 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
764 763
765 764
766 The kprobes sysctl interface 765 The kprobes sysctl interface
767 ============================ 766 ============================
768 767
769 /proc/sys/debug/kprobes-optimization: Turn kpr 768 /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
770 769
771 When CONFIG_OPTPROBES=y, this sysctl interface 770 When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
772 a knob to globally and forcibly turn jump opti 771 a knob to globally and forcibly turn jump optimization (see section
773 :ref:`kprobes_jump_optimization`) ON or OFF. B 772 :ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
774 is allowed (ON). If you echo "0" to this file 773 is allowed (ON). If you echo "0" to this file or set
775 "debug.kprobes_optimization" to 0 via sysctl, 774 "debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
776 unoptimized, and any new probes registered aft 775 unoptimized, and any new probes registered after that will not be optimized.
777 776
778 Note that this knob *changes* the optimized st 777 Note that this knob *changes* the optimized state. This means that optimized
779 probes (marked [OPTIMIZED]) will be unoptimize 778 probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
780 removed). If the knob is turned on, they will 779 removed). If the knob is turned on, they will be optimized again.
781 780
782 References 781 References
783 ========== 782 ==========
784 783
785 For additional information on Kprobes, refer t 784 For additional information on Kprobes, refer to the following URLs:
786 785
787 - https://lwn.net/Articles/132196/ 786 - https://lwn.net/Articles/132196/
788 - https://www.kernel.org/doc/ols/2006/ols2006v 787 - https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf
789 788
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