1 ================================= 1 =================================
2 Using ftrace to hook to functions 2 Using ftrace to hook to functions
3 ================================= 3 =================================
4 4
5 .. Copyright 2017 VMware Inc. 5 .. Copyright 2017 VMware Inc.
6 .. Author: Steven Rostedt <srostedt@goodmis 6 .. Author: Steven Rostedt <srostedt@goodmis.org>
7 .. License: The GNU Free Documentation Lice 7 .. License: The GNU Free Documentation License, Version 1.2
8 .. (dual licensed under the GPL 8 .. (dual licensed under the GPL v2)
9 9
10 Written for: 4.14 10 Written for: 4.14
11 11
12 Introduction 12 Introduction
13 ============ 13 ============
14 14
15 The ftrace infrastructure was originally creat 15 The ftrace infrastructure was originally created to attach callbacks to the
16 beginning of functions in order to record and 16 beginning of functions in order to record and trace the flow of the kernel.
17 But callbacks to the start of a function can h 17 But callbacks to the start of a function can have other use cases. Either
18 for live kernel patching, or for security moni 18 for live kernel patching, or for security monitoring. This document describes
19 how to use ftrace to implement your own functi 19 how to use ftrace to implement your own function callbacks.
20 20
21 21
22 The ftrace context 22 The ftrace context
23 ================== 23 ==================
24 .. warning:: 24 .. warning::
25 25
26 The ability to add a callback to almost any 26 The ability to add a callback to almost any function within the
27 kernel comes with risks. A callback can be c 27 kernel comes with risks. A callback can be called from any context
28 (normal, softirq, irq, and NMI). Callbacks c 28 (normal, softirq, irq, and NMI). Callbacks can also be called just before
29 going to idle, during CPU bring up and taked 29 going to idle, during CPU bring up and takedown, or going to user space.
30 This requires extra care to what can be done 30 This requires extra care to what can be done inside a callback. A callback
31 can be called outside the protective scope o 31 can be called outside the protective scope of RCU.
32 32
33 There are helper functions to help against rec !! 33 The ftrace infrastructure has some protections against recursions and RCU
34 RCU is watching. These are explained below. !! 34 but one must still be very careful how they use the callbacks.
35 35
36 36
37 The ftrace_ops structure 37 The ftrace_ops structure
38 ======================== 38 ========================
39 39
40 To register a function callback, a ftrace_ops 40 To register a function callback, a ftrace_ops is required. This structure
41 is used to tell ftrace what function should be 41 is used to tell ftrace what function should be called as the callback
42 as well as what protections the callback will 42 as well as what protections the callback will perform and not require
43 ftrace to handle. 43 ftrace to handle.
44 44
45 There is only one field that is needed to be s 45 There is only one field that is needed to be set when registering
46 an ftrace_ops with ftrace: 46 an ftrace_ops with ftrace:
47 47
48 .. code-block:: c 48 .. code-block:: c
49 49
50 struct ftrace_ops ops = { 50 struct ftrace_ops ops = {
51 .func = my_callback_ 51 .func = my_callback_func,
52 .flags = MY_FTRACE_FL 52 .flags = MY_FTRACE_FLAGS
53 .private = any_private_ 53 .private = any_private_data_structure,
54 }; 54 };
55 55
56 Both .flags and .private are optional. Only .f 56 Both .flags and .private are optional. Only .func is required.
57 57
58 To enable tracing call:: !! 58 To enable tracing call:
59 59
60 register_ftrace_function(&ops); !! 60 .. c:function:: register_ftrace_function(&ops);
61 61
62 To disable tracing call:: !! 62 To disable tracing call:
63 63
64 unregister_ftrace_function(&ops); !! 64 .. c:function:: unregister_ftrace_function(&ops);
65 65
66 The above is defined by including the header:: !! 66 The above is defined by including the header:
67 67
68 #include <linux/ftrace.h> !! 68 .. c:function:: #include <linux/ftrace.h>
69 69
70 The registered callback will start being calle 70 The registered callback will start being called some time after the
71 register_ftrace_function() is called and befor 71 register_ftrace_function() is called and before it returns. The exact time
72 that callbacks start being called is dependent 72 that callbacks start being called is dependent upon architecture and scheduling
73 of services. The callback itself will have to 73 of services. The callback itself will have to handle any synchronization if it
74 must begin at an exact moment. 74 must begin at an exact moment.
75 75
76 The unregister_ftrace_function() will guarante 76 The unregister_ftrace_function() will guarantee that the callback is
77 no longer being called by functions after the 77 no longer being called by functions after the unregister_ftrace_function()
78 returns. Note that to perform this guarantee, 78 returns. Note that to perform this guarantee, the unregister_ftrace_function()
79 may take some time to finish. 79 may take some time to finish.
80 80
81 81
82 The callback function 82 The callback function
83 ===================== 83 =====================
84 84
85 The prototype of the callback function is as f 85 The prototype of the callback function is as follows (as of v4.14):
86 86
87 .. code-block:: c 87 .. code-block:: c
88 88
89 void callback_func(unsigned long ip, unsign 89 void callback_func(unsigned long ip, unsigned long parent_ip,
90 struct ftrace_ops *op, s 90 struct ftrace_ops *op, struct pt_regs *regs);
91 91
92 @ip 92 @ip
93 This is the instruction pointer of th 93 This is the instruction pointer of the function that is being traced.
94 (where the fentry or mcount is within 94 (where the fentry or mcount is within the function)
95 95
96 @parent_ip 96 @parent_ip
97 This is the instruction pointer of the 97 This is the instruction pointer of the function that called the
98 the function being traced (where the c 98 the function being traced (where the call of the function occurred).
99 99
100 @op 100 @op
101 This is a pointer to ftrace_ops that w 101 This is a pointer to ftrace_ops that was used to register the callback.
102 This can be used to pass data to the c 102 This can be used to pass data to the callback via the private pointer.
103 103
104 @regs 104 @regs
105 If the FTRACE_OPS_FL_SAVE_REGS or FTRA 105 If the FTRACE_OPS_FL_SAVE_REGS or FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED
106 flags are set in the ftrace_ops struct 106 flags are set in the ftrace_ops structure, then this will be pointing
107 to the pt_regs structure like it would 107 to the pt_regs structure like it would be if an breakpoint was placed
108 at the start of the function where ftr 108 at the start of the function where ftrace was tracing. Otherwise it
109 either contains garbage, or NULL. 109 either contains garbage, or NULL.
110 110
111 Protect your callback <<
112 ===================== <<
113 <<
114 As functions can be called from anywhere, and <<
115 called by a callback may also be traced, and c <<
116 recursion protection must be used. There are t <<
117 can help in this regard. If you start your cod <<
118 <<
119 .. code-block:: c <<
120 <<
121 int bit; <<
122 <<
123 bit = ftrace_test_recursion_trylock(ip <<
124 if (bit < 0) <<
125 return; <<
126 <<
127 and end it with: <<
128 <<
129 .. code-block:: c <<
130 <<
131 ftrace_test_recursion_unlock(bit); <<
132 <<
133 The code in between will be safe to use, even <<
134 function that the callback is tracing. Note, o <<
135 ftrace_test_recursion_trylock() will disable p <<
136 ftrace_test_recursion_unlock() will enable it <<
137 enabled). The instruction pointer (ip) and its <<
138 ftrace_test_recursion_trylock() to record wher <<
139 (if CONFIG_FTRACE_RECORD_RECURSION is set). <<
140 <<
141 Alternatively, if the FTRACE_OPS_FL_RECURSION <<
142 (as explained below), then a helper trampoline <<
143 for recursion for the callback and no recursio <<
144 But this is at the expense of a slightly more <<
145 function call. <<
146 <<
147 If your callback accesses any data or critical <<
148 protection, it is best to make sure that RCU i <<
149 that data or critical section will not be prot <<
150 case add: <<
151 <<
152 .. code-block:: c <<
153 <<
154 if (!rcu_is_watching()) <<
155 return; <<
156 <<
157 Alternatively, if the FTRACE_OPS_FL_RCU flag i <<
158 (as explained below), then a helper trampoline <<
159 for rcu_is_watching for the callback and no ot <<
160 But this is at the expense of a slightly more <<
161 function call. <<
162 <<
163 111
164 The ftrace FLAGS 112 The ftrace FLAGS
165 ================ 113 ================
166 114
167 The ftrace_ops flags are all defined and docum 115 The ftrace_ops flags are all defined and documented in include/linux/ftrace.h.
168 Some of the flags are used for internal infras 116 Some of the flags are used for internal infrastructure of ftrace, but the
169 ones that users should be aware of are the fol 117 ones that users should be aware of are the following:
170 118
171 FTRACE_OPS_FL_SAVE_REGS 119 FTRACE_OPS_FL_SAVE_REGS
172 If the callback requires reading or mo 120 If the callback requires reading or modifying the pt_regs
173 passed to the callback, then it must s 121 passed to the callback, then it must set this flag. Registering
174 a ftrace_ops with this flag set on an 122 a ftrace_ops with this flag set on an architecture that does not
175 support passing of pt_regs to the call 123 support passing of pt_regs to the callback will fail.
176 124
177 FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED 125 FTRACE_OPS_FL_SAVE_REGS_IF_SUPPORTED
178 Similar to SAVE_REGS but the registeri 126 Similar to SAVE_REGS but the registering of a
179 ftrace_ops on an architecture that doe 127 ftrace_ops on an architecture that does not support passing of regs
180 will not fail with this flag set. But 128 will not fail with this flag set. But the callback must check if
181 regs is NULL or not to determine if th 129 regs is NULL or not to determine if the architecture supports it.
182 130
183 FTRACE_OPS_FL_RECURSION !! 131 FTRACE_OPS_FL_RECURSION_SAFE
184 By default, it is expected that the ca !! 132 By default, a wrapper is added around the callback to
185 But if the callback is not that worrie !! 133 make sure that recursion of the function does not occur. That is,
186 setting this bit will add the recursio !! 134 if a function that is called as a result of the callback's execution
187 callback by calling a helper function !! 135 is also traced, ftrace will prevent the callback from being called
188 protection and only call the callback !! 136 again. But this wrapper adds some overhead, and if the callback is
189 !! 137 safe from recursion, it can set this flag to disable the ftrace
190 Note, if this flag is not set, and rec !! 138 protection.
191 cause the system to crash, and possibl !! 139
192 !! 140 Note, if this flag is set, and recursion does occur, it could cause
193 Note, if this flag is set, then the ca !! 141 the system to crash, and possibly reboot via a triple fault.
194 with preemption disabled. If it is not !! 142
195 (but not guaranteed) that the callback !! 143 It is OK if another callback traces a function that is called by a
196 preemptable context. !! 144 callback that is marked recursion safe. Recursion safe callbacks
>> 145 must never trace any function that are called by the callback
>> 146 itself or any nested functions that those functions call.
>> 147
>> 148 If this flag is set, it is possible that the callback will also
>> 149 be called with preemption enabled (when CONFIG_PREEMPT is set),
>> 150 but this is not guaranteed.
197 151
198 FTRACE_OPS_FL_IPMODIFY 152 FTRACE_OPS_FL_IPMODIFY
199 Requires FTRACE_OPS_FL_SAVE_REGS set. 153 Requires FTRACE_OPS_FL_SAVE_REGS set. If the callback is to "hijack"
200 the traced function (have another func 154 the traced function (have another function called instead of the
201 traced function), it requires setting 155 traced function), it requires setting this flag. This is what live
202 kernel patches uses. Without this flag 156 kernel patches uses. Without this flag the pt_regs->ip can not be
203 modified. 157 modified.
204 158
205 Note, only one ftrace_ops with FTRACE_ 159 Note, only one ftrace_ops with FTRACE_OPS_FL_IPMODIFY set may be
206 registered to any given function at a 160 registered to any given function at a time.
207 161
208 FTRACE_OPS_FL_RCU 162 FTRACE_OPS_FL_RCU
209 If this is set, then the callback will 163 If this is set, then the callback will only be called by functions
210 where RCU is "watching". This is requi 164 where RCU is "watching". This is required if the callback function
211 performs any rcu_read_lock() operation 165 performs any rcu_read_lock() operation.
212 166
213 RCU stops watching when the system goe 167 RCU stops watching when the system goes idle, the time when a CPU
214 is taken down and comes back online, a 168 is taken down and comes back online, and when entering from kernel
215 to user space and back to kernel space 169 to user space and back to kernel space. During these transitions,
216 a callback may be executed and RCU syn 170 a callback may be executed and RCU synchronization will not protect
217 it. 171 it.
218 <<
219 FTRACE_OPS_FL_PERMANENT <<
220 If this is set on any ftrace ops, then <<
221 writing 0 to the proc sysctl ftrace_en <<
222 the flag set cannot be registered if f <<
223 <<
224 Livepatch uses it not to lose the func <<
225 stays protected. <<
226 172
227 173
228 Filtering which functions to trace 174 Filtering which functions to trace
229 ================================== 175 ==================================
230 176
231 If a callback is only to be called from specif 177 If a callback is only to be called from specific functions, a filter must be
232 set up. The filters are added by name, or ip i 178 set up. The filters are added by name, or ip if it is known.
233 179
234 .. code-block:: c 180 .. code-block:: c
235 181
236 int ftrace_set_filter(struct ftrace_ops *op 182 int ftrace_set_filter(struct ftrace_ops *ops, unsigned char *buf,
237 int len, int reset); 183 int len, int reset);
238 184
239 @ops 185 @ops
240 The ops to set the filter with 186 The ops to set the filter with
241 187
242 @buf 188 @buf
243 The string that holds the function fil 189 The string that holds the function filter text.
244 @len 190 @len
245 The length of the string. 191 The length of the string.
246 192
247 @reset 193 @reset
248 Non-zero to reset all filters before a 194 Non-zero to reset all filters before applying this filter.
249 195
250 Filters denote which functions should be enabl 196 Filters denote which functions should be enabled when tracing is enabled.
251 If @buf is NULL and reset is set, all function 197 If @buf is NULL and reset is set, all functions will be enabled for tracing.
252 198
253 The @buf can also be a glob expression to enab 199 The @buf can also be a glob expression to enable all functions that
254 match a specific pattern. 200 match a specific pattern.
255 201
256 See Filter Commands in :file:`Documentation/tr 202 See Filter Commands in :file:`Documentation/trace/ftrace.rst`.
257 203
258 To just trace the schedule function: 204 To just trace the schedule function:
259 205
260 .. code-block:: c 206 .. code-block:: c
261 207
262 ret = ftrace_set_filter(&ops, "schedule", s 208 ret = ftrace_set_filter(&ops, "schedule", strlen("schedule"), 0);
263 209
264 To add more functions, call the ftrace_set_fil 210 To add more functions, call the ftrace_set_filter() more than once with the
265 @reset parameter set to zero. To remove the cu 211 @reset parameter set to zero. To remove the current filter set and replace it
266 with new functions defined by @buf, have @rese 212 with new functions defined by @buf, have @reset be non-zero.
267 213
268 To remove all the filtered functions and trace 214 To remove all the filtered functions and trace all functions:
269 215
270 .. code-block:: c 216 .. code-block:: c
271 217
272 ret = ftrace_set_filter(&ops, NULL, 0, 1); 218 ret = ftrace_set_filter(&ops, NULL, 0, 1);
273 219
274 220
275 Sometimes more than one function has the same 221 Sometimes more than one function has the same name. To trace just a specific
276 function in this case, ftrace_set_filter_ip() 222 function in this case, ftrace_set_filter_ip() can be used.
277 223
278 .. code-block:: c 224 .. code-block:: c
279 225
280 ret = ftrace_set_filter_ip(&ops, ip, 0, 0); 226 ret = ftrace_set_filter_ip(&ops, ip, 0, 0);
281 227
282 Although the ip must be the address where the 228 Although the ip must be the address where the call to fentry or mcount is
283 located in the function. This function is used 229 located in the function. This function is used by perf and kprobes that
284 gets the ip address from the user (usually usi 230 gets the ip address from the user (usually using debug info from the kernel).
285 231
286 If a glob is used to set the filter, functions 232 If a glob is used to set the filter, functions can be added to a "notrace"
287 list that will prevent those functions from ca 233 list that will prevent those functions from calling the callback.
288 The "notrace" list takes precedence over the " 234 The "notrace" list takes precedence over the "filter" list. If the
289 two lists are non-empty and contain the same f 235 two lists are non-empty and contain the same functions, the callback will not
290 be called by any function. 236 be called by any function.
291 237
292 An empty "notrace" list means to allow all fun 238 An empty "notrace" list means to allow all functions defined by the filter
293 to be traced. 239 to be traced.
294 240
295 .. code-block:: c 241 .. code-block:: c
296 242
297 int ftrace_set_notrace(struct ftrace_ops *o 243 int ftrace_set_notrace(struct ftrace_ops *ops, unsigned char *buf,
298 int len, int reset); 244 int len, int reset);
299 245
300 This takes the same parameters as ftrace_set_f 246 This takes the same parameters as ftrace_set_filter() but will add the
301 functions it finds to not be traced. This is a 247 functions it finds to not be traced. This is a separate list from the
302 filter list, and this function does not modify 248 filter list, and this function does not modify the filter list.
303 249
304 A non-zero @reset will clear the "notrace" lis 250 A non-zero @reset will clear the "notrace" list before adding functions
305 that match @buf to it. 251 that match @buf to it.
306 252
307 Clearing the "notrace" list is the same as cle 253 Clearing the "notrace" list is the same as clearing the filter list
308 254
309 .. code-block:: c 255 .. code-block:: c
310 256
311 ret = ftrace_set_notrace(&ops, NULL, 0, 1); 257 ret = ftrace_set_notrace(&ops, NULL, 0, 1);
312 258
313 The filter and notrace lists may be changed at 259 The filter and notrace lists may be changed at any time. If only a set of
314 functions should call the callback, it is best 260 functions should call the callback, it is best to set the filters before
315 registering the callback. But the changes may 261 registering the callback. But the changes may also happen after the callback
316 has been registered. 262 has been registered.
317 263
318 If a filter is in place, and the @reset is non 264 If a filter is in place, and the @reset is non-zero, and @buf contains a
319 matching glob to functions, the switch will ha 265 matching glob to functions, the switch will happen during the time of
320 the ftrace_set_filter() call. At no time will 266 the ftrace_set_filter() call. At no time will all functions call the callback.
321 267
322 .. code-block:: c 268 .. code-block:: c
323 269
324 ftrace_set_filter(&ops, "schedule", strlen( 270 ftrace_set_filter(&ops, "schedule", strlen("schedule"), 1);
325 271
326 register_ftrace_function(&ops); 272 register_ftrace_function(&ops);
327 273
328 msleep(10); 274 msleep(10);
329 275
330 ftrace_set_filter(&ops, "try_to_wake_up", s 276 ftrace_set_filter(&ops, "try_to_wake_up", strlen("try_to_wake_up"), 1);
331 277
332 is not the same as: 278 is not the same as:
333 279
334 .. code-block:: c 280 .. code-block:: c
335 281
336 ftrace_set_filter(&ops, "schedule", strlen( 282 ftrace_set_filter(&ops, "schedule", strlen("schedule"), 1);
337 283
338 register_ftrace_function(&ops); 284 register_ftrace_function(&ops);
339 285
340 msleep(10); 286 msleep(10);
341 287
342 ftrace_set_filter(&ops, NULL, 0, 1); 288 ftrace_set_filter(&ops, NULL, 0, 1);
343 289
344 ftrace_set_filter(&ops, "try_to_wake_up", s 290 ftrace_set_filter(&ops, "try_to_wake_up", strlen("try_to_wake_up"), 0);
345 291
346 As the latter will have a short time where all 292 As the latter will have a short time where all functions will call
347 the callback, between the time of the reset, a 293 the callback, between the time of the reset, and the time of the
348 new setting of the filter. 294 new setting of the filter.
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