1 Kernel Memory Leak Detector 1 Kernel Memory Leak Detector
2 =========================== 2 ===========================
3 3
4 Kmemleak provides a way of detecting possible 4 Kmemleak provides a way of detecting possible kernel memory leaks in a
5 way similar to a `tracing garbage collector 5 way similar to a `tracing garbage collector
6 <https://en.wikipedia.org/wiki/Tracing_garbage 6 <https://en.wikipedia.org/wiki/Tracing_garbage_collection>`_,
7 with the difference that the orphan objects ar 7 with the difference that the orphan objects are not freed but only
8 reported via /sys/kernel/debug/kmemleak. A sim 8 reported via /sys/kernel/debug/kmemleak. A similar method is used by the
9 Valgrind tool (``memcheck --leak-check``) to d 9 Valgrind tool (``memcheck --leak-check``) to detect the memory leaks in
10 user-space applications. 10 user-space applications.
11 11
12 Usage 12 Usage
13 ----- 13 -----
14 14
15 CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has 15 CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
16 thread scans the memory every 10 minutes (by d 16 thread scans the memory every 10 minutes (by default) and prints the
17 number of new unreferenced objects found. If t 17 number of new unreferenced objects found. If the ``debugfs`` isn't already
18 mounted, mount with:: 18 mounted, mount with::
19 19
20 # mount -t debugfs nodev /sys/kernel/debug/ 20 # mount -t debugfs nodev /sys/kernel/debug/
21 21
22 To display the details of all the possible sca 22 To display the details of all the possible scanned memory leaks::
23 23
24 # cat /sys/kernel/debug/kmemleak 24 # cat /sys/kernel/debug/kmemleak
25 25
26 To trigger an intermediate memory scan:: 26 To trigger an intermediate memory scan::
27 27
28 # echo scan > /sys/kernel/debug/kmemleak 28 # echo scan > /sys/kernel/debug/kmemleak
29 29
30 To clear the list of all current possible memo 30 To clear the list of all current possible memory leaks::
31 31
32 # echo clear > /sys/kernel/debug/kmemleak 32 # echo clear > /sys/kernel/debug/kmemleak
33 33
34 New leaks will then come up upon reading ``/sy 34 New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak``
35 again. 35 again.
36 36
37 Note that the orphan objects are listed in the 37 Note that the orphan objects are listed in the order they were allocated
38 and one object at the beginning of the list ma 38 and one object at the beginning of the list may cause other subsequent
39 objects to be reported as orphan. 39 objects to be reported as orphan.
40 40
41 Memory scanning parameters can be modified at 41 Memory scanning parameters can be modified at run-time by writing to the
42 ``/sys/kernel/debug/kmemleak`` file. The follo 42 ``/sys/kernel/debug/kmemleak`` file. The following parameters are supported:
43 43
44 - off 44 - off
45 disable kmemleak (irreversible) 45 disable kmemleak (irreversible)
46 - stack=on 46 - stack=on
47 enable the task stacks scanning (default) 47 enable the task stacks scanning (default)
48 - stack=off 48 - stack=off
49 disable the tasks stacks scanning 49 disable the tasks stacks scanning
50 - scan=on 50 - scan=on
51 start the automatic memory scanning thread 51 start the automatic memory scanning thread (default)
52 - scan=off 52 - scan=off
53 stop the automatic memory scanning thread 53 stop the automatic memory scanning thread
54 - scan=<secs> 54 - scan=<secs>
55 set the automatic memory scanning period i 55 set the automatic memory scanning period in seconds
56 (default 600, 0 to stop the automatic scan 56 (default 600, 0 to stop the automatic scanning)
57 - scan 57 - scan
58 trigger a memory scan 58 trigger a memory scan
59 - clear 59 - clear
60 clear list of current memory leak suspects 60 clear list of current memory leak suspects, done by
61 marking all current reported unreferenced 61 marking all current reported unreferenced objects grey,
62 or free all kmemleak objects if kmemleak h 62 or free all kmemleak objects if kmemleak has been disabled.
63 - dump=<addr> 63 - dump=<addr>
64 dump information about the object found at 64 dump information about the object found at <addr>
65 65
66 Kmemleak can also be disabled at boot-time by 66 Kmemleak can also be disabled at boot-time by passing ``kmemleak=off`` on
67 the kernel command line. 67 the kernel command line.
68 68
69 Memory may be allocated or freed before kmemle 69 Memory may be allocated or freed before kmemleak is initialised and
70 these actions are stored in an early log buffe 70 these actions are stored in an early log buffer. The size of this buffer
71 is configured via the CONFIG_DEBUG_KMEMLEAK_ME 71 is configured via the CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE option.
72 72
73 If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabl 73 If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
74 disabled by default. Passing ``kmemleak=on`` o 74 disabled by default. Passing ``kmemleak=on`` on the kernel command
75 line enables the function. 75 line enables the function.
76 76
77 If you are getting errors like "Error while wr 77 If you are getting errors like "Error while writing to stdout" or "write_loop:
78 Invalid argument", make sure kmemleak is prope 78 Invalid argument", make sure kmemleak is properly enabled.
79 79
80 Basic Algorithm 80 Basic Algorithm
81 --------------- 81 ---------------
82 82
83 The memory allocations via :c:func:`kmalloc`, 83 The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`,
84 :c:func:`kmem_cache_alloc` and 84 :c:func:`kmem_cache_alloc` and
85 friends are traced and the pointers, together 85 friends are traced and the pointers, together with additional
86 information like size and stack trace, are sto 86 information like size and stack trace, are stored in a rbtree.
87 The corresponding freeing function calls are t 87 The corresponding freeing function calls are tracked and the pointers
88 removed from the kmemleak data structures. 88 removed from the kmemleak data structures.
89 89
90 An allocated block of memory is considered orp 90 An allocated block of memory is considered orphan if no pointer to its
91 start address or to any location inside the bl 91 start address or to any location inside the block can be found by
92 scanning the memory (including saved registers 92 scanning the memory (including saved registers). This means that there
93 might be no way for the kernel to pass the add 93 might be no way for the kernel to pass the address of the allocated
94 block to a freeing function and therefore the 94 block to a freeing function and therefore the block is considered a
95 memory leak. 95 memory leak.
96 96
97 The scanning algorithm steps: 97 The scanning algorithm steps:
98 98
99 1. mark all objects as white (remaining whit 99 1. mark all objects as white (remaining white objects will later be
100 considered orphan) 100 considered orphan)
101 2. scan the memory starting with the data se 101 2. scan the memory starting with the data section and stacks, checking
102 the values against the addresses stored i 102 the values against the addresses stored in the rbtree. If
103 a pointer to a white object is found, the 103 a pointer to a white object is found, the object is added to the
104 gray list 104 gray list
105 3. scan the gray objects for matching addres 105 3. scan the gray objects for matching addresses (some white objects
106 can become gray and added at the end of t 106 can become gray and added at the end of the gray list) until the
107 gray set is finished 107 gray set is finished
108 4. the remaining white objects are considere 108 4. the remaining white objects are considered orphan and reported via
109 /sys/kernel/debug/kmemleak 109 /sys/kernel/debug/kmemleak
110 110
111 Some allocated memory blocks have pointers sto 111 Some allocated memory blocks have pointers stored in the kernel's
112 internal data structures and they cannot be de 112 internal data structures and they cannot be detected as orphans. To
113 avoid this, kmemleak can also store the number 113 avoid this, kmemleak can also store the number of values pointing to an
114 address inside the block address range that ne 114 address inside the block address range that need to be found so that the
115 block is not considered a leak. One example is 115 block is not considered a leak. One example is __vmalloc().
116 116
117 Testing specific sections with kmemleak 117 Testing specific sections with kmemleak
118 --------------------------------------- 118 ---------------------------------------
119 119
120 Upon initial bootup your /sys/kernel/debug/kme 120 Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
121 quite extensive. This can also be the case if 121 quite extensive. This can also be the case if you have very buggy code
122 when doing development. To work around these s 122 when doing development. To work around these situations you can use the
123 'clear' command to clear all reported unrefere 123 'clear' command to clear all reported unreferenced objects from the
124 /sys/kernel/debug/kmemleak output. By issuing 124 /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
125 you can find new unreferenced objects; this sh 125 you can find new unreferenced objects; this should help with testing
126 specific sections of code. 126 specific sections of code.
127 127
128 To test a critical section on demand with a cl 128 To test a critical section on demand with a clean kmemleak do::
129 129
130 # echo clear > /sys/kernel/debug/kmemleak 130 # echo clear > /sys/kernel/debug/kmemleak
131 ... test your kernel or modules ... 131 ... test your kernel or modules ...
132 # echo scan > /sys/kernel/debug/kmemleak 132 # echo scan > /sys/kernel/debug/kmemleak
133 133
134 Then as usual to get your report with:: 134 Then as usual to get your report with::
135 135
136 # cat /sys/kernel/debug/kmemleak 136 # cat /sys/kernel/debug/kmemleak
137 137
138 Freeing kmemleak internal objects 138 Freeing kmemleak internal objects
139 --------------------------------- 139 ---------------------------------
140 140
141 To allow access to previously found memory lea 141 To allow access to previously found memory leaks after kmemleak has been
142 disabled by the user or due to an fatal error, 142 disabled by the user or due to an fatal error, internal kmemleak objects
143 won't be freed when kmemleak is disabled, and 143 won't be freed when kmemleak is disabled, and those objects may occupy
144 a large part of physical memory. 144 a large part of physical memory.
145 145
146 In this situation, you may reclaim memory with 146 In this situation, you may reclaim memory with::
147 147
148 # echo clear > /sys/kernel/debug/kmemleak 148 # echo clear > /sys/kernel/debug/kmemleak
149 149
150 Kmemleak API 150 Kmemleak API
151 ------------ 151 ------------
152 152
153 See the include/linux/kmemleak.h header for th 153 See the include/linux/kmemleak.h header for the functions prototype.
154 154
155 - ``kmemleak_init`` - initialize 155 - ``kmemleak_init`` - initialize kmemleak
156 - ``kmemleak_alloc`` - notify of a 156 - ``kmemleak_alloc`` - notify of a memory block allocation
157 - ``kmemleak_alloc_percpu`` - notify of a 157 - ``kmemleak_alloc_percpu`` - notify of a percpu memory block allocation
158 - ``kmemleak_vmalloc`` - notify of a 158 - ``kmemleak_vmalloc`` - notify of a vmalloc() memory allocation
159 - ``kmemleak_free`` - notify of a 159 - ``kmemleak_free`` - notify of a memory block freeing
160 - ``kmemleak_free_part`` - notify of a 160 - ``kmemleak_free_part`` - notify of a partial memory block freeing
161 - ``kmemleak_free_percpu`` - notify of a 161 - ``kmemleak_free_percpu`` - notify of a percpu memory block freeing
162 - ``kmemleak_update_trace`` - update obje 162 - ``kmemleak_update_trace`` - update object allocation stack trace
163 - ``kmemleak_not_leak`` - mark an object as n 163 - ``kmemleak_not_leak`` - mark an object as not a leak
164 - ``kmemleak_ignore`` - do not scan 164 - ``kmemleak_ignore`` - do not scan or report an object as leak
165 - ``kmemleak_scan_area`` - add scan ar 165 - ``kmemleak_scan_area`` - add scan areas inside a memory block
166 - ``kmemleak_no_scan`` - do not scan a memor 166 - ``kmemleak_no_scan`` - do not scan a memory block
167 - ``kmemleak_erase`` - erase an ol 167 - ``kmemleak_erase`` - erase an old value in a pointer variable
168 - ``kmemleak_alloc_recursive`` - as kmemleak_a 168 - ``kmemleak_alloc_recursive`` - as kmemleak_alloc but checks the recursiveness
169 - ``kmemleak_free_recursive`` - as kmemleak 169 - ``kmemleak_free_recursive`` - as kmemleak_free but checks the recursiveness
170 170
171 The following functions take a physical addres 171 The following functions take a physical address as the object pointer
172 and only perform the corresponding action if t 172 and only perform the corresponding action if the address has a lowmem
173 mapping: 173 mapping:
174 174
175 - ``kmemleak_alloc_phys`` 175 - ``kmemleak_alloc_phys``
176 - ``kmemleak_free_part_phys`` 176 - ``kmemleak_free_part_phys``
177 - ``kmemleak_ignore_phys`` 177 - ``kmemleak_ignore_phys``
178 178
179 Dealing with false positives/negatives 179 Dealing with false positives/negatives
180 -------------------------------------- 180 --------------------------------------
181 181
182 The false negatives are real memory leaks (orp 182 The false negatives are real memory leaks (orphan objects) but not
183 reported by kmemleak because values found duri 183 reported by kmemleak because values found during the memory scanning
184 point to such objects. To reduce the number of 184 point to such objects. To reduce the number of false negatives, kmemleak
185 provides the kmemleak_ignore, kmemleak_scan_ar 185 provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
186 kmemleak_erase functions (see above). The task 186 kmemleak_erase functions (see above). The task stacks also increase the
187 amount of false negatives and their scanning i 187 amount of false negatives and their scanning is not enabled by default.
188 188
189 The false positives are objects wrongly report 189 The false positives are objects wrongly reported as being memory leaks
190 (orphan). For objects known not to be leaks, k 190 (orphan). For objects known not to be leaks, kmemleak provides the
191 kmemleak_not_leak function. The kmemleak_ignor 191 kmemleak_not_leak function. The kmemleak_ignore could also be used if
192 the memory block is known not to contain other 192 the memory block is known not to contain other pointers and it will no
193 longer be scanned. 193 longer be scanned.
194 194
195 Some of the reported leaks are only transient, 195 Some of the reported leaks are only transient, especially on SMP
196 systems, because of pointers temporarily store 196 systems, because of pointers temporarily stored in CPU registers or
197 stacks. Kmemleak defines MSECS_MIN_AGE (defaul 197 stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
198 the minimum age of an object to be reported as 198 the minimum age of an object to be reported as a memory leak.
199 199
200 Limitations and Drawbacks 200 Limitations and Drawbacks
201 ------------------------- 201 -------------------------
202 202
203 The main drawback is the reduced performance o 203 The main drawback is the reduced performance of memory allocation and
204 freeing. To avoid other penalties, the memory 204 freeing. To avoid other penalties, the memory scanning is only performed
205 when the /sys/kernel/debug/kmemleak file is re 205 when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
206 intended for debugging purposes where the perf 206 intended for debugging purposes where the performance might not be the
207 most important requirement. 207 most important requirement.
208 208
209 To keep the algorithm simple, kmemleak scans f 209 To keep the algorithm simple, kmemleak scans for values pointing to any
210 address inside a block's address range. This m 210 address inside a block's address range. This may lead to an increased
211 number of false negatives. However, it is like 211 number of false negatives. However, it is likely that a real memory leak
212 will eventually become visible. 212 will eventually become visible.
213 213
214 Another source of false negatives is the data 214 Another source of false negatives is the data stored in non-pointer
215 values. In a future version, kmemleak could on 215 values. In a future version, kmemleak could only scan the pointer
216 members in the allocated structures. This feat 216 members in the allocated structures. This feature would solve many of
217 the false negative cases described above. 217 the false negative cases described above.
218 218
219 The tool can report false positives. These are 219 The tool can report false positives. These are cases where an allocated
220 block doesn't need to be freed (some cases in 220 block doesn't need to be freed (some cases in the init_call functions),
221 the pointer is calculated by other methods tha 221 the pointer is calculated by other methods than the usual container_of
222 macro or the pointer is stored in a location n 222 macro or the pointer is stored in a location not scanned by kmemleak.
223 223
224 Page allocations and ioremap are not tracked. 224 Page allocations and ioremap are not tracked.
225 225
226 Testing with kmemleak-test 226 Testing with kmemleak-test
227 -------------------------- 227 --------------------------
228 228
229 To check if you have all set up to use kmemlea 229 To check if you have all set up to use kmemleak, you can use the kmemleak-test
230 module, a module that deliberately leaks memor 230 module, a module that deliberately leaks memory. Set CONFIG_SAMPLE_KMEMLEAK
231 as module (it can't be used as built-in) and b 231 as module (it can't be used as built-in) and boot the kernel with kmemleak
232 enabled. Load the module and perform a scan wi 232 enabled. Load the module and perform a scan with::
233 233
234 # modprobe kmemleak-test 234 # modprobe kmemleak-test
235 # echo scan > /sys/kernel/debug/kmemle 235 # echo scan > /sys/kernel/debug/kmemleak
236 236
237 Note that the you may not get results instantl 237 Note that the you may not get results instantly or on the first scanning. When
238 kmemleak gets results, it'll log ``kmemleak: < 238 kmemleak gets results, it'll log ``kmemleak: <count of leaks> new suspected
239 memory leaks``. Then read the file to see then 239 memory leaks``. Then read the file to see then::
240 240
241 # cat /sys/kernel/debug/kmemleak 241 # cat /sys/kernel/debug/kmemleak
242 unreferenced object 0xffff89862ca702e8 242 unreferenced object 0xffff89862ca702e8 (size 32):
243 comm "modprobe", pid 2088, jiffies 4 243 comm "modprobe", pid 2088, jiffies 4294680594 (age 375.486s)
244 hex dump (first 32 bytes): 244 hex dump (first 32 bytes):
245 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6 245 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
246 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6 246 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk.
247 backtrace: 247 backtrace:
248 [<00000000e0a73ec7>] 0xffffffffc01 248 [<00000000e0a73ec7>] 0xffffffffc01d2036
249 [<000000000c5d2a46>] do_one_initca 249 [<000000000c5d2a46>] do_one_initcall+0x41/0x1df
250 [<0000000046db7e0a>] do_init_modul 250 [<0000000046db7e0a>] do_init_module+0x55/0x200
251 [<00000000542b9814>] load_module+0 251 [<00000000542b9814>] load_module+0x203c/0x2480
252 [<00000000c2850256>] __do_sys_fini 252 [<00000000c2850256>] __do_sys_finit_module+0xba/0xe0
253 [<000000006564e7ef>] do_syscall_64 253 [<000000006564e7ef>] do_syscall_64+0x43/0x110
254 [<000000007c873fa6>] entry_SYSCALL 254 [<000000007c873fa6>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
255 ... 255 ...
256 256
257 Removing the module with ``rmmod kmemleak_test 257 Removing the module with ``rmmod kmemleak_test`` should also trigger some
258 kmemleak results. 258 kmemleak results.
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