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 - sparc64 (Return probes not yet implemented.) 318 - sparc64 (Return probes not yet implemented.)
319 - arm 319 - arm
320 - ppc 320 - ppc
321 - mips 321 - mips
322 - s390 322 - s390
323 - parisc 323 - parisc
324 - loongarch <<
325 - riscv <<
326 324
327 Configuring Kprobes 325 Configuring Kprobes
328 =================== 326 ===================
329 327
330 When configuring the kernel using make menucon 328 When configuring the kernel using make menuconfig/xconfig/oldconfig,
331 ensure that CONFIG_KPROBES is set to "y", look 329 ensure that CONFIG_KPROBES is set to "y", look for "Kprobes" under
332 "General architecture-dependent options". 330 "General architecture-dependent options".
333 331
334 So that you can load and unload Kprobes-based 332 So that you can load and unload Kprobes-based instrumentation modules,
335 make sure "Loadable module support" (CONFIG_MO 333 make sure "Loadable module support" (CONFIG_MODULES) and "Module
336 unloading" (CONFIG_MODULE_UNLOAD) are set to " 334 unloading" (CONFIG_MODULE_UNLOAD) are set to "y".
337 335
338 Also make sure that CONFIG_KALLSYMS and perhap 336 Also make sure that CONFIG_KALLSYMS and perhaps even CONFIG_KALLSYMS_ALL
339 are set to "y", since kallsyms_lookup_name() i 337 are set to "y", since kallsyms_lookup_name() is used by the in-kernel
340 kprobe address resolution code. 338 kprobe address resolution code.
341 339
342 If you need to insert a probe in the middle of 340 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 341 it useful to "Compile the kernel with debug info" (CONFIG_DEBUG_INFO),
344 so you can use "objdump -d -l vmlinux" to see 342 so you can use "objdump -d -l vmlinux" to see the source-to-object
345 code mapping. 343 code mapping.
346 344
347 API Reference 345 API Reference
348 ============= 346 =============
349 347
350 The Kprobes API includes a "register" function 348 The Kprobes API includes a "register" function and an "unregister"
351 function for each type of probe. The API also 349 function for each type of probe. The API also includes "register_*probes"
352 and "unregister_*probes" functions for (un)reg 350 and "unregister_*probes" functions for (un)registering arrays of probes.
353 Here are terse, mini-man-page specifications f 351 Here are terse, mini-man-page specifications for these functions and
354 the associated probe handlers that you'll writ 352 the associated probe handlers that you'll write. See the files in the
355 samples/kprobes/ sub-directory for examples. 353 samples/kprobes/ sub-directory for examples.
356 354
357 register_kprobe 355 register_kprobe
358 --------------- 356 ---------------
359 357
360 :: 358 ::
361 359
362 #include <linux/kprobes.h> 360 #include <linux/kprobes.h>
363 int register_kprobe(struct kprobe *kp) 361 int register_kprobe(struct kprobe *kp);
364 362
365 Sets a breakpoint at the address kp->addr. Wh 363 Sets a breakpoint at the address kp->addr. When the breakpoint is hit, Kprobes
366 calls kp->pre_handler. After the probed instr 364 calls kp->pre_handler. After the probed instruction is single-stepped, Kprobe
367 calls kp->post_handler. Any or all handlers c 365 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 366 KPROBE_FLAG_DISABLED, that kp will be registered but disabled, so, its handlers
369 aren't hit until calling enable_kprobe(kp). 367 aren't hit until calling enable_kprobe(kp).
370 368
371 .. note:: 369 .. note::
372 370
373 1. With the introduction of the "symbol_nam 371 1. With the introduction of the "symbol_name" field to struct kprobe,
374 the probepoint address resolution will n 372 the probepoint address resolution will now be taken care of by the kernel.
375 The following will now work:: 373 The following will now work::
376 374
377 kp.symbol_name = "symbol_name"; 375 kp.symbol_name = "symbol_name";
378 376
379 (64-bit powerpc intricacies such as func 377 (64-bit powerpc intricacies such as function descriptors are handled
380 transparently) 378 transparently)
381 379
382 2. Use the "offset" field of struct kprobe 380 2. Use the "offset" field of struct kprobe if the offset into the symbol
383 to install a probepoint is known. This f 381 to install a probepoint is known. This field is used to calculate the
384 probepoint. 382 probepoint.
385 383
386 3. Specify either the kprobe "symbol_name" 384 3. Specify either the kprobe "symbol_name" OR the "addr". If both are
387 specified, kprobe registration will fail 385 specified, kprobe registration will fail with -EINVAL.
388 386
389 4. With CISC architectures (such as i386 an 387 4. With CISC architectures (such as i386 and x86_64), the kprobes code
390 does not validate if the kprobe.addr is 388 does not validate if the kprobe.addr is at an instruction boundary.
391 Use "offset" with caution. 389 Use "offset" with caution.
392 390
393 register_kprobe() returns 0 on success, or a n 391 register_kprobe() returns 0 on success, or a negative errno otherwise.
394 392
395 User's pre-handler (kp->pre_handler):: 393 User's pre-handler (kp->pre_handler)::
396 394
397 #include <linux/kprobes.h> 395 #include <linux/kprobes.h>
398 #include <linux/ptrace.h> 396 #include <linux/ptrace.h>
399 int pre_handler(struct kprobe *p, stru 397 int pre_handler(struct kprobe *p, struct pt_regs *regs);
400 398
401 Called with p pointing to the kprobe associate 399 Called with p pointing to the kprobe associated with the breakpoint,
402 and regs pointing to the struct containing the 400 and regs pointing to the struct containing the registers saved when
403 the breakpoint was hit. Return 0 here unless 401 the breakpoint was hit. Return 0 here unless you're a Kprobes geek.
404 402
405 User's post-handler (kp->post_handler):: 403 User's post-handler (kp->post_handler)::
406 404
407 #include <linux/kprobes.h> 405 #include <linux/kprobes.h>
408 #include <linux/ptrace.h> 406 #include <linux/ptrace.h>
409 void post_handler(struct kprobe *p, st 407 void post_handler(struct kprobe *p, struct pt_regs *regs,
410 unsigned long flags) 408 unsigned long flags);
411 409
412 p and regs are as described for the pre_handle 410 p and regs are as described for the pre_handler. flags always seems
413 to be zero. 411 to be zero.
414 412
415 register_kretprobe 413 register_kretprobe
416 ------------------ 414 ------------------
417 415
418 :: 416 ::
419 417
420 #include <linux/kprobes.h> 418 #include <linux/kprobes.h>
421 int register_kretprobe(struct kretprob 419 int register_kretprobe(struct kretprobe *rp);
422 420
423 Establishes a return probe for the function wh 421 Establishes a return probe for the function whose address is
424 rp->kp.addr. When that function returns, Kpro 422 rp->kp.addr. When that function returns, Kprobes calls rp->handler.
425 You must set rp->maxactive appropriately befor 423 You must set rp->maxactive appropriately before you call
426 register_kretprobe(); see "How Does a Return P 424 register_kretprobe(); see "How Does a Return Probe Work?" for details.
427 425
428 register_kretprobe() returns 0 on success, or 426 register_kretprobe() returns 0 on success, or a negative errno
429 otherwise. 427 otherwise.
430 428
431 User's return-probe handler (rp->handler):: 429 User's return-probe handler (rp->handler)::
432 430
433 #include <linux/kprobes.h> 431 #include <linux/kprobes.h>
434 #include <linux/ptrace.h> 432 #include <linux/ptrace.h>
435 int kretprobe_handler(struct kretprobe 433 int kretprobe_handler(struct kretprobe_instance *ri,
436 struct pt_regs * 434 struct pt_regs *regs);
437 435
438 regs is as described for kprobe.pre_handler. 436 regs is as described for kprobe.pre_handler. ri points to the
439 kretprobe_instance object, of which the follow 437 kretprobe_instance object, of which the following fields may be
440 of interest: 438 of interest:
441 439
442 - ret_addr: the return address 440 - ret_addr: the return address
443 - rp: points to the corresponding kretprobe ob 441 - rp: points to the corresponding kretprobe object
444 - task: points to the corresponding task struc 442 - task: points to the corresponding task struct
445 - data: points to per return-instance private 443 - data: points to per return-instance private data; see "Kretprobe
446 entry-handler" for details. 444 entry-handler" for details.
447 445
448 The regs_return_value(regs) macro provides a s 446 The regs_return_value(regs) macro provides a simple abstraction to
449 extract the return value from the appropriate 447 extract the return value from the appropriate register as defined by
450 the architecture's ABI. 448 the architecture's ABI.
451 449
452 The handler's return value is currently ignore 450 The handler's return value is currently ignored.
453 451
454 unregister_*probe 452 unregister_*probe
455 ------------------ 453 ------------------
456 454
457 :: 455 ::
458 456
459 #include <linux/kprobes.h> 457 #include <linux/kprobes.h>
460 void unregister_kprobe(struct kprobe * 458 void unregister_kprobe(struct kprobe *kp);
461 void unregister_kretprobe(struct kretp 459 void unregister_kretprobe(struct kretprobe *rp);
462 460
463 Removes the specified probe. The unregister f 461 Removes the specified probe. The unregister function can be called
464 at any time after the probe has been registere 462 at any time after the probe has been registered.
465 463
466 .. note:: 464 .. note::
467 465
468 If the functions find an incorrect probe (e 466 If the functions find an incorrect probe (ex. an unregistered probe),
469 they clear the addr field of the probe. 467 they clear the addr field of the probe.
470 468
471 register_*probes 469 register_*probes
472 ---------------- 470 ----------------
473 471
474 :: 472 ::
475 473
476 #include <linux/kprobes.h> 474 #include <linux/kprobes.h>
477 int register_kprobes(struct kprobe **k 475 int register_kprobes(struct kprobe **kps, int num);
478 int register_kretprobes(struct kretpro 476 int register_kretprobes(struct kretprobe **rps, int num);
479 477
480 Registers each of the num probes in the specif 478 Registers each of the num probes in the specified array. If any
481 error occurs during registration, all probes i 479 error occurs during registration, all probes in the array, up to
482 the bad probe, are safely unregistered before 480 the bad probe, are safely unregistered before the register_*probes
483 function returns. 481 function returns.
484 482
485 - kps/rps: an array of pointers to ``*probe`` 483 - kps/rps: an array of pointers to ``*probe`` data structures
486 - num: the number of the array entries. 484 - num: the number of the array entries.
487 485
488 .. note:: 486 .. note::
489 487
490 You have to allocate(or define) an array of 488 You have to allocate(or define) an array of pointers and set all
491 of the array entries before using these fun 489 of the array entries before using these functions.
492 490
493 unregister_*probes 491 unregister_*probes
494 ------------------ 492 ------------------
495 493
496 :: 494 ::
497 495
498 #include <linux/kprobes.h> 496 #include <linux/kprobes.h>
499 void unregister_kprobes(struct kprobe 497 void unregister_kprobes(struct kprobe **kps, int num);
500 void unregister_kretprobes(struct kret 498 void unregister_kretprobes(struct kretprobe **rps, int num);
501 499
502 Removes each of the num probes in the specifie 500 Removes each of the num probes in the specified array at once.
503 501
504 .. note:: 502 .. note::
505 503
506 If the functions find some incorrect probes 504 If the functions find some incorrect probes (ex. unregistered
507 probes) in the specified array, they clear 505 probes) in the specified array, they clear the addr field of those
508 incorrect probes. However, other probes in 506 incorrect probes. However, other probes in the array are
509 unregistered correctly. 507 unregistered correctly.
510 508
511 disable_*probe 509 disable_*probe
512 -------------- 510 --------------
513 511
514 :: 512 ::
515 513
516 #include <linux/kprobes.h> 514 #include <linux/kprobes.h>
517 int disable_kprobe(struct kprobe *kp); 515 int disable_kprobe(struct kprobe *kp);
518 int disable_kretprobe(struct kretprobe 516 int disable_kretprobe(struct kretprobe *rp);
519 517
520 Temporarily disables the specified ``*probe``. 518 Temporarily disables the specified ``*probe``. You can enable it again by using
521 enable_*probe(). You must specify the probe wh 519 enable_*probe(). You must specify the probe which has been registered.
522 520
523 enable_*probe 521 enable_*probe
524 ------------- 522 -------------
525 523
526 :: 524 ::
527 525
528 #include <linux/kprobes.h> 526 #include <linux/kprobes.h>
529 int enable_kprobe(struct kprobe *kp); 527 int enable_kprobe(struct kprobe *kp);
530 int enable_kretprobe(struct kretprobe 528 int enable_kretprobe(struct kretprobe *rp);
531 529
532 Enables ``*probe`` which has been disabled by 530 Enables ``*probe`` which has been disabled by disable_*probe(). You must specify
533 the probe which has been registered. 531 the probe which has been registered.
534 532
535 Kprobes Features and Limitations 533 Kprobes Features and Limitations
536 ================================ 534 ================================
537 535
538 Kprobes allows multiple probes at the same add 536 Kprobes allows multiple probes at the same address. Also,
539 a probepoint for which there is a post_handler 537 a probepoint for which there is a post_handler cannot be optimized.
540 So if you install a kprobe with a post_handler 538 So if you install a kprobe with a post_handler, at an optimized
541 probepoint, the probepoint will be unoptimized 539 probepoint, the probepoint will be unoptimized automatically.
542 540
543 In general, you can install a probe anywhere i 541 In general, you can install a probe anywhere in the kernel.
544 In particular, you can probe interrupt handler 542 In particular, you can probe interrupt handlers. Known exceptions
545 are discussed in this section. 543 are discussed in this section.
546 544
547 The register_*probe functions will return -EIN 545 The register_*probe functions will return -EINVAL if you attempt
548 to install a probe in the code that implements 546 to install a probe in the code that implements Kprobes (mostly
549 kernel/kprobes.c and ``arch/*/kernel/kprobes.c 547 kernel/kprobes.c and ``arch/*/kernel/kprobes.c``, but also functions such
550 as do_page_fault and notifier_call_chain). 548 as do_page_fault and notifier_call_chain).
551 549
552 If you install a probe in an inline-able funct 550 If you install a probe in an inline-able function, Kprobes makes
553 no attempt to chase down all inline instances 551 no attempt to chase down all inline instances of the function and
554 install probes there. gcc may inline a functi 552 install probes there. gcc may inline a function without being asked,
555 so keep this in mind if you're not seeing the 553 so keep this in mind if you're not seeing the probe hits you expect.
556 554
557 A probe handler can modify the environment of 555 A probe handler can modify the environment of the probed function
558 -- e.g., by modifying kernel data structures, 556 -- e.g., by modifying kernel data structures, or by modifying the
559 contents of the pt_regs struct (which are rest 557 contents of the pt_regs struct (which are restored to the registers
560 upon return from the breakpoint). So Kprobes 558 upon return from the breakpoint). So Kprobes can be used, for example,
561 to install a bug fix or to inject faults for t 559 to install a bug fix or to inject faults for testing. Kprobes, of
562 course, has no way to distinguish the delibera 560 course, has no way to distinguish the deliberately injected faults
563 from the accidental ones. Don't drink and pro 561 from the accidental ones. Don't drink and probe.
564 562
565 Kprobes makes no attempt to prevent probe hand 563 Kprobes makes no attempt to prevent probe handlers from stepping on
566 each other -- e.g., probing printk() and then 564 each other -- e.g., probing printk() and then calling printk() from a
567 probe handler. If a probe handler hits a prob 565 probe handler. If a probe handler hits a probe, that second probe's
568 handlers won't be run in that instance, and th 566 handlers won't be run in that instance, and the kprobe.nmissed member
569 of the second probe will be incremented. 567 of the second probe will be incremented.
570 568
571 As of Linux v2.6.15-rc1, multiple handlers (or 569 As of Linux v2.6.15-rc1, multiple handlers (or multiple instances of
572 the same handler) may run concurrently on diff 570 the same handler) may run concurrently on different CPUs.
573 571
574 Kprobes does not use mutexes or allocate memor 572 Kprobes does not use mutexes or allocate memory except during
575 registration and unregistration. 573 registration and unregistration.
576 574
577 Probe handlers are run with preemption disable 575 Probe handlers are run with preemption disabled or interrupt disabled,
578 which depends on the architecture and optimiza 576 which depends on the architecture and optimization state. (e.g.,
579 kretprobe handlers and optimized kprobe handle 577 kretprobe handlers and optimized kprobe handlers run without interrupt
580 disabled on x86/x86-64). In any case, your ha 578 disabled on x86/x86-64). In any case, your handler should not yield
581 the CPU (e.g., by attempting to acquire a sema 579 the CPU (e.g., by attempting to acquire a semaphore, or waiting I/O).
582 580
583 Since a return probe is implemented by replaci 581 Since a return probe is implemented by replacing the return
584 address with the trampoline's address, stack b 582 address with the trampoline's address, stack backtraces and calls
585 to __builtin_return_address() will typically y 583 to __builtin_return_address() will typically yield the trampoline's
586 address instead of the real return address for 584 address instead of the real return address for kretprobed functions.
587 (As far as we can tell, __builtin_return_addre 585 (As far as we can tell, __builtin_return_address() is used only
588 for instrumentation and error reporting.) 586 for instrumentation and error reporting.)
589 587
590 If the number of times a function is called do 588 If the number of times a function is called does not match the number
591 of times it returns, registering a return prob 589 of times it returns, registering a return probe on that function may
592 produce undesirable results. In such a case, a 590 produce undesirable results. In such a case, a line:
593 kretprobe BUG!: Processing kretprobe d00000000 591 kretprobe BUG!: Processing kretprobe d000000000041aa8 @ c00000000004f48c
594 gets printed. With this information, one will 592 gets printed. With this information, one will be able to correlate the
595 exact instance of the kretprobe that caused th 593 exact instance of the kretprobe that caused the problem. We have the
596 do_exit() case covered. do_execve() and do_for 594 do_exit() case covered. do_execve() and do_fork() are not an issue.
597 We're unaware of other specific cases where th 595 We're unaware of other specific cases where this could be a problem.
598 596
599 If, upon entry to or exit from a function, the 597 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 598 a stack other than that of the current task, registering a return
601 probe on that function may produce undesirable 599 probe on that function may produce undesirable results. For this
602 reason, Kprobes doesn't support return probes 600 reason, Kprobes doesn't support return probes (or kprobes)
603 on the x86_64 version of __switch_to(); the re 601 on the x86_64 version of __switch_to(); the registration functions
604 return -EINVAL. 602 return -EINVAL.
605 603
606 On x86/x86-64, since the Jump Optimization of 604 On x86/x86-64, since the Jump Optimization of Kprobes modifies
607 instructions widely, there are some limitation 605 instructions widely, there are some limitations to optimization. To
608 explain it, we introduce some terminology. Ima 606 explain it, we introduce some terminology. Imagine a 3-instruction
609 sequence consisting of a two 2-byte instructio 607 sequence consisting of a two 2-byte instructions and one 3-byte
610 instruction. 608 instruction.
611 609
612 :: 610 ::
613 611
614 IA 612 IA
615 | 613 |
616 [-2][-1][0][1][2][3][4][5][6][7] 614 [-2][-1][0][1][2][3][4][5][6][7]
617 [ins1][ins2][ ins3 ] 615 [ins1][ins2][ ins3 ]
618 [<- DCR ->] 616 [<- DCR ->]
619 [<- JTPR ->] 617 [<- JTPR ->]
620 618
621 ins1: 1st Instruction 619 ins1: 1st Instruction
622 ins2: 2nd Instruction 620 ins2: 2nd Instruction
623 ins3: 3rd Instruction 621 ins3: 3rd Instruction
624 IA: Insertion Address 622 IA: Insertion Address
625 JTPR: Jump Target Prohibition Region 623 JTPR: Jump Target Prohibition Region
626 DCR: Detoured Code Region 624 DCR: Detoured Code Region
627 625
628 The instructions in DCR are copied to the out- 626 The instructions in DCR are copied to the out-of-line buffer
629 of the kprobe, because the bytes in DCR are re 627 of the kprobe, because the bytes in DCR are replaced by
630 a 5-byte jump instruction. So there are severa 628 a 5-byte jump instruction. So there are several limitations.
631 629
632 a) The instructions in DCR must be relocatable 630 a) The instructions in DCR must be relocatable.
633 b) The instructions in DCR must not include a 631 b) The instructions in DCR must not include a call instruction.
634 c) JTPR must not be targeted by any jump or ca 632 c) JTPR must not be targeted by any jump or call instruction.
635 d) DCR must not straddle the border between fu 633 d) DCR must not straddle the border between functions.
636 634
637 Anyway, these limitations are checked by the i 635 Anyway, these limitations are checked by the in-kernel instruction
638 decoder, so you don't need to worry about that 636 decoder, so you don't need to worry about that.
639 637
640 Probe Overhead 638 Probe Overhead
641 ============== 639 ==============
642 640
643 On a typical CPU in use in 2005, a kprobe hit 641 On a typical CPU in use in 2005, a kprobe hit takes 0.5 to 1.0
644 microseconds to process. Specifically, a benc 642 microseconds to process. Specifically, a benchmark that hits the same
645 probepoint repeatedly, firing a simple handler 643 probepoint repeatedly, firing a simple handler each time, reports 1-2
646 million hits per second, depending on the arch 644 million hits per second, depending on the architecture. A return-probe
647 hit typically takes 50-75% longer than a kprob 645 hit typically takes 50-75% longer than a kprobe hit.
648 When you have a return probe set on a function 646 When you have a return probe set on a function, adding a kprobe at
649 the entry to that function adds essentially no 647 the entry to that function adds essentially no overhead.
650 648
651 Here are sample overhead figures (in usec) for 649 Here are sample overhead figures (in usec) for different architectures::
652 650
653 k = kprobe; r = return probe; kr = kprobe + 651 k = kprobe; r = return probe; kr = kprobe + return probe
654 on same function 652 on same function
655 653
656 i386: Intel Pentium M, 1495 MHz, 2957.31 bog 654 i386: Intel Pentium M, 1495 MHz, 2957.31 bogomips
657 k = 0.57 usec; r = 0.92; kr = 0.99 655 k = 0.57 usec; r = 0.92; kr = 0.99
658 656
659 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 b 657 x86_64: AMD Opteron 246, 1994 MHz, 3971.48 bogomips
660 k = 0.49 usec; r = 0.80; kr = 0.82 658 k = 0.49 usec; r = 0.80; kr = 0.82
661 659
662 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 660 ppc64: POWER5 (gr), 1656 MHz (SMT disabled, 1 virtual CPU per physical CPU)
663 k = 0.77 usec; r = 1.26; kr = 1.45 661 k = 0.77 usec; r = 1.26; kr = 1.45
664 662
665 Optimized Probe Overhead 663 Optimized Probe Overhead
666 ------------------------ 664 ------------------------
667 665
668 Typically, an optimized kprobe hit takes 0.07 666 Typically, an optimized kprobe hit takes 0.07 to 0.1 microseconds to
669 process. Here are sample overhead figures (in 667 process. Here are sample overhead figures (in usec) for x86 architectures::
670 668
671 k = unoptimized kprobe, b = boosted (single- 669 k = unoptimized kprobe, b = boosted (single-step skipped), o = optimized kprobe,
672 r = unoptimized kretprobe, rb = boosted kret 670 r = unoptimized kretprobe, rb = boosted kretprobe, ro = optimized kretprobe.
673 671
674 i386: Intel(R) Xeon(R) E5410, 2.33GHz, 4656. 672 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; 673 k = 0.80 usec; b = 0.33; o = 0.05; r = 1.10; rb = 0.61; ro = 0.33
676 674
677 x86-64: Intel(R) Xeon(R) E5410, 2.33GHz, 465 675 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; 676 k = 0.99 usec; b = 0.43; o = 0.06; r = 1.24; rb = 0.68; ro = 0.30
679 677
680 TODO 678 TODO
681 ==== 679 ====
682 680
683 a. SystemTap (http://sourceware.org/systemtap) 681 a. SystemTap (http://sourceware.org/systemtap): Provides a simplified
684 programming interface for probe-based instr 682 programming interface for probe-based instrumentation. Try it out.
685 b. Kernel return probes for sparc64. 683 b. Kernel return probes for sparc64.
686 c. Support for other architectures. 684 c. Support for other architectures.
687 d. User-space probes. 685 d. User-space probes.
688 e. Watchpoint probes (which fire on data refer 686 e. Watchpoint probes (which fire on data references).
689 687
690 Kprobes Example 688 Kprobes Example
691 =============== 689 ===============
692 690
693 See samples/kprobes/kprobe_example.c 691 See samples/kprobes/kprobe_example.c
694 692
695 Kretprobes Example 693 Kretprobes Example
696 ================== 694 ==================
697 695
698 See samples/kprobes/kretprobe_example.c 696 See samples/kprobes/kretprobe_example.c
699 697
700 Deprecated Features 698 Deprecated Features
701 =================== 699 ===================
702 700
703 Jprobes is now a deprecated feature. People wh 701 Jprobes is now a deprecated feature. People who are depending on it should
704 migrate to other tracing features or use older 702 migrate to other tracing features or use older kernels. Please consider to
705 migrate your tool to one of the following opti 703 migrate your tool to one of the following options:
706 704
707 - Use trace-event to trace target function wit 705 - Use trace-event to trace target function with arguments.
708 706
709 trace-event is a low-overhead (and almost no 707 trace-event is a low-overhead (and almost no visible overhead if it
710 is off) statically defined event interface. 708 is off) statically defined event interface. You can define new events
711 and trace it via ftrace or any other tracing 709 and trace it via ftrace or any other tracing tools.
712 710
713 See the following urls: 711 See the following urls:
714 712
715 - https://lwn.net/Articles/379903/ 713 - https://lwn.net/Articles/379903/
716 - https://lwn.net/Articles/381064/ 714 - https://lwn.net/Articles/381064/
717 - https://lwn.net/Articles/383362/ 715 - https://lwn.net/Articles/383362/
718 716
719 - Use ftrace dynamic events (kprobe event) wit 717 - Use ftrace dynamic events (kprobe event) with perf-probe.
720 718
721 If you build your kernel with debug info (CO 719 If you build your kernel with debug info (CONFIG_DEBUG_INFO=y), you can
722 find which register/stack is assigned to whi 720 find which register/stack is assigned to which local variable or arguments
723 by using perf-probe and set up new event to 721 by using perf-probe and set up new event to trace it.
724 722
725 See following documents: 723 See following documents:
726 724
727 - Documentation/trace/kprobetrace.rst 725 - Documentation/trace/kprobetrace.rst
728 - Documentation/trace/events.rst 726 - Documentation/trace/events.rst
729 - tools/perf/Documentation/perf-probe.txt 727 - tools/perf/Documentation/perf-probe.txt
730 728
731 729
732 The kprobes debugfs interface 730 The kprobes debugfs interface
733 ============================= 731 =============================
734 732
735 733
736 With recent kernels (> 2.6.20) the list of reg 734 With recent kernels (> 2.6.20) the list of registered kprobes is visible
737 under the /sys/kernel/debug/kprobes/ directory 735 under the /sys/kernel/debug/kprobes/ directory (assuming debugfs is mounted at //sys/kernel/debug).
738 736
739 /sys/kernel/debug/kprobes/list: Lists all regi 737 /sys/kernel/debug/kprobes/list: Lists all registered probes on the system::
740 738
741 c015d71a k vfs_read+0x0 739 c015d71a k vfs_read+0x0
742 c03dedc5 r tcp_v4_rcv+0x0 740 c03dedc5 r tcp_v4_rcv+0x0
743 741
744 The first column provides the kernel address w 742 The first column provides the kernel address where the probe is inserted.
745 The second column identifies the type of probe 743 The second column identifies the type of probe (k - kprobe and r - kretprobe)
746 while the third column specifies the symbol+of 744 while the third column specifies the symbol+offset of the probe.
747 If the probed function belongs to a module, th 745 If the probed function belongs to a module, the module name is also
748 specified. Following columns show probe status 746 specified. Following columns show probe status. If the probe is on
749 a virtual address that is no longer valid (mod 747 a virtual address that is no longer valid (module init sections, module
750 virtual addresses that correspond to modules t 748 virtual addresses that correspond to modules that've been unloaded),
751 such probes are marked with [GONE]. If the pro 749 such probes are marked with [GONE]. If the probe is temporarily disabled,
752 such probes are marked with [DISABLED]. If the 750 such probes are marked with [DISABLED]. If the probe is optimized, it is
753 marked with [OPTIMIZED]. If the probe is ftrac 751 marked with [OPTIMIZED]. If the probe is ftrace-based, it is marked with
754 [FTRACE]. 752 [FTRACE].
755 753
756 /sys/kernel/debug/kprobes/enabled: Turn kprobe 754 /sys/kernel/debug/kprobes/enabled: Turn kprobes ON/OFF forcibly.
757 755
758 Provides a knob to globally and forcibly turn 756 Provides a knob to globally and forcibly turn registered kprobes ON or OFF.
759 By default, all kprobes are enabled. By echoin 757 By default, all kprobes are enabled. By echoing "0" to this file, all
760 registered probes will be disarmed, till such 758 registered probes will be disarmed, till such time a "1" is echoed to this
761 file. Note that this knob just disarms and arm 759 file. Note that this knob just disarms and arms all kprobes and doesn't
762 change each probe's disabling state. This mean 760 change each probe's disabling state. This means that disabled kprobes (marked
763 [DISABLED]) will be not enabled if you turn ON 761 [DISABLED]) will be not enabled if you turn ON all kprobes by this knob.
764 762
765 763
766 The kprobes sysctl interface 764 The kprobes sysctl interface
767 ============================ 765 ============================
768 766
769 /proc/sys/debug/kprobes-optimization: Turn kpr 767 /proc/sys/debug/kprobes-optimization: Turn kprobes optimization ON/OFF.
770 768
771 When CONFIG_OPTPROBES=y, this sysctl interface 769 When CONFIG_OPTPROBES=y, this sysctl interface appears and it provides
772 a knob to globally and forcibly turn jump opti 770 a knob to globally and forcibly turn jump optimization (see section
773 :ref:`kprobes_jump_optimization`) ON or OFF. B 771 :ref:`kprobes_jump_optimization`) ON or OFF. By default, jump optimization
774 is allowed (ON). If you echo "0" to this file 772 is allowed (ON). If you echo "0" to this file or set
775 "debug.kprobes_optimization" to 0 via sysctl, 773 "debug.kprobes_optimization" to 0 via sysctl, all optimized probes will be
776 unoptimized, and any new probes registered aft 774 unoptimized, and any new probes registered after that will not be optimized.
777 775
778 Note that this knob *changes* the optimized st 776 Note that this knob *changes* the optimized state. This means that optimized
779 probes (marked [OPTIMIZED]) will be unoptimize 777 probes (marked [OPTIMIZED]) will be unoptimized ([OPTIMIZED] tag will be
780 removed). If the knob is turned on, they will 778 removed). If the knob is turned on, they will be optimized again.
781 779
782 References 780 References
783 ========== 781 ==========
784 782
785 For additional information on Kprobes, refer t 783 For additional information on Kprobes, refer to the following URLs:
786 784
787 - https://lwn.net/Articles/132196/ 785 - https://lwn.net/Articles/132196/
788 - https://www.kernel.org/doc/ols/2006/ols2006v 786 - https://www.kernel.org/doc/ols/2006/ols2006v2-pages-109-124.pdf
789 787
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