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 Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390 and tile.
11 12
12 Usage 13 Usage
13 ----- 14 -----
14 15
15 CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has 16 CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
16 thread scans the memory every 10 minutes (by d 17 thread scans the memory every 10 minutes (by default) and prints the
17 number of new unreferenced objects found. If t 18 number of new unreferenced objects found. If the ``debugfs`` isn't already
18 mounted, mount with:: 19 mounted, mount with::
19 20
20 # mount -t debugfs nodev /sys/kernel/debug/ 21 # mount -t debugfs nodev /sys/kernel/debug/
21 22
22 To display the details of all the possible sca 23 To display the details of all the possible scanned memory leaks::
23 24
24 # cat /sys/kernel/debug/kmemleak 25 # cat /sys/kernel/debug/kmemleak
25 26
26 To trigger an intermediate memory scan:: 27 To trigger an intermediate memory scan::
27 28
28 # echo scan > /sys/kernel/debug/kmemleak 29 # echo scan > /sys/kernel/debug/kmemleak
29 30
30 To clear the list of all current possible memo 31 To clear the list of all current possible memory leaks::
31 32
32 # echo clear > /sys/kernel/debug/kmemleak 33 # echo clear > /sys/kernel/debug/kmemleak
33 34
34 New leaks will then come up upon reading ``/sy 35 New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak``
35 again. 36 again.
36 37
37 Note that the orphan objects are listed in the 38 Note that the orphan objects are listed in the order they were allocated
38 and one object at the beginning of the list ma 39 and one object at the beginning of the list may cause other subsequent
39 objects to be reported as orphan. 40 objects to be reported as orphan.
40 41
41 Memory scanning parameters can be modified at 42 Memory scanning parameters can be modified at run-time by writing to the
42 ``/sys/kernel/debug/kmemleak`` file. The follo 43 ``/sys/kernel/debug/kmemleak`` file. The following parameters are supported:
43 44
44 - off 45 - off
45 disable kmemleak (irreversible) 46 disable kmemleak (irreversible)
46 - stack=on 47 - stack=on
47 enable the task stacks scanning (default) 48 enable the task stacks scanning (default)
48 - stack=off 49 - stack=off
49 disable the tasks stacks scanning 50 disable the tasks stacks scanning
50 - scan=on 51 - scan=on
51 start the automatic memory scanning thread 52 start the automatic memory scanning thread (default)
52 - scan=off 53 - scan=off
53 stop the automatic memory scanning thread 54 stop the automatic memory scanning thread
54 - scan=<secs> 55 - scan=<secs>
55 set the automatic memory scanning period i 56 set the automatic memory scanning period in seconds
56 (default 600, 0 to stop the automatic scan 57 (default 600, 0 to stop the automatic scanning)
57 - scan 58 - scan
58 trigger a memory scan 59 trigger a memory scan
59 - clear 60 - clear
60 clear list of current memory leak suspects 61 clear list of current memory leak suspects, done by
61 marking all current reported unreferenced 62 marking all current reported unreferenced objects grey,
62 or free all kmemleak objects if kmemleak h 63 or free all kmemleak objects if kmemleak has been disabled.
63 - dump=<addr> 64 - dump=<addr>
64 dump information about the object found at 65 dump information about the object found at <addr>
65 66
66 Kmemleak can also be disabled at boot-time by 67 Kmemleak can also be disabled at boot-time by passing ``kmemleak=off`` on
67 the kernel command line. 68 the kernel command line.
68 69
69 Memory may be allocated or freed before kmemle 70 Memory may be allocated or freed before kmemleak is initialised and
70 these actions are stored in an early log buffe 71 these actions are stored in an early log buffer. The size of this buffer
71 is configured via the CONFIG_DEBUG_KMEMLEAK_ME !! 72 is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
72 73
73 If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabl 74 If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
74 disabled by default. Passing ``kmemleak=on`` o 75 disabled by default. Passing ``kmemleak=on`` on the kernel command
75 line enables the function. 76 line enables the function.
76 77
77 If you are getting errors like "Error while wr 78 If you are getting errors like "Error while writing to stdout" or "write_loop:
78 Invalid argument", make sure kmemleak is prope 79 Invalid argument", make sure kmemleak is properly enabled.
79 80
80 Basic Algorithm 81 Basic Algorithm
81 --------------- 82 ---------------
82 83
83 The memory allocations via :c:func:`kmalloc`, 84 The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`,
84 :c:func:`kmem_cache_alloc` and 85 :c:func:`kmem_cache_alloc` and
85 friends are traced and the pointers, together 86 friends are traced and the pointers, together with additional
86 information like size and stack trace, are sto 87 information like size and stack trace, are stored in a rbtree.
87 The corresponding freeing function calls are t 88 The corresponding freeing function calls are tracked and the pointers
88 removed from the kmemleak data structures. 89 removed from the kmemleak data structures.
89 90
90 An allocated block of memory is considered orp 91 An allocated block of memory is considered orphan if no pointer to its
91 start address or to any location inside the bl 92 start address or to any location inside the block can be found by
92 scanning the memory (including saved registers 93 scanning the memory (including saved registers). This means that there
93 might be no way for the kernel to pass the add 94 might be no way for the kernel to pass the address of the allocated
94 block to a freeing function and therefore the 95 block to a freeing function and therefore the block is considered a
95 memory leak. 96 memory leak.
96 97
97 The scanning algorithm steps: 98 The scanning algorithm steps:
98 99
99 1. mark all objects as white (remaining whit 100 1. mark all objects as white (remaining white objects will later be
100 considered orphan) 101 considered orphan)
101 2. scan the memory starting with the data se 102 2. scan the memory starting with the data section and stacks, checking
102 the values against the addresses stored i 103 the values against the addresses stored in the rbtree. If
103 a pointer to a white object is found, the 104 a pointer to a white object is found, the object is added to the
104 gray list 105 gray list
105 3. scan the gray objects for matching addres 106 3. scan the gray objects for matching addresses (some white objects
106 can become gray and added at the end of t 107 can become gray and added at the end of the gray list) until the
107 gray set is finished 108 gray set is finished
108 4. the remaining white objects are considere 109 4. the remaining white objects are considered orphan and reported via
109 /sys/kernel/debug/kmemleak 110 /sys/kernel/debug/kmemleak
110 111
111 Some allocated memory blocks have pointers sto 112 Some allocated memory blocks have pointers stored in the kernel's
112 internal data structures and they cannot be de 113 internal data structures and they cannot be detected as orphans. To
113 avoid this, kmemleak can also store the number 114 avoid this, kmemleak can also store the number of values pointing to an
114 address inside the block address range that ne 115 address inside the block address range that need to be found so that the
115 block is not considered a leak. One example is 116 block is not considered a leak. One example is __vmalloc().
116 117
117 Testing specific sections with kmemleak 118 Testing specific sections with kmemleak
118 --------------------------------------- 119 ---------------------------------------
119 120
120 Upon initial bootup your /sys/kernel/debug/kme 121 Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
121 quite extensive. This can also be the case if 122 quite extensive. This can also be the case if you have very buggy code
122 when doing development. To work around these s 123 when doing development. To work around these situations you can use the
123 'clear' command to clear all reported unrefere 124 'clear' command to clear all reported unreferenced objects from the
124 /sys/kernel/debug/kmemleak output. By issuing 125 /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear'
125 you can find new unreferenced objects; this sh 126 you can find new unreferenced objects; this should help with testing
126 specific sections of code. 127 specific sections of code.
127 128
128 To test a critical section on demand with a cl 129 To test a critical section on demand with a clean kmemleak do::
129 130
130 # echo clear > /sys/kernel/debug/kmemleak 131 # echo clear > /sys/kernel/debug/kmemleak
131 ... test your kernel or modules ... 132 ... test your kernel or modules ...
132 # echo scan > /sys/kernel/debug/kmemleak 133 # echo scan > /sys/kernel/debug/kmemleak
133 134
134 Then as usual to get your report with:: 135 Then as usual to get your report with::
135 136
136 # cat /sys/kernel/debug/kmemleak 137 # cat /sys/kernel/debug/kmemleak
137 138
138 Freeing kmemleak internal objects 139 Freeing kmemleak internal objects
139 --------------------------------- 140 ---------------------------------
140 141
141 To allow access to previously found memory lea 142 To allow access to previously found memory leaks after kmemleak has been
142 disabled by the user or due to an fatal error, 143 disabled by the user or due to an fatal error, internal kmemleak objects
143 won't be freed when kmemleak is disabled, and 144 won't be freed when kmemleak is disabled, and those objects may occupy
144 a large part of physical memory. 145 a large part of physical memory.
145 146
146 In this situation, you may reclaim memory with 147 In this situation, you may reclaim memory with::
147 148
148 # echo clear > /sys/kernel/debug/kmemleak 149 # echo clear > /sys/kernel/debug/kmemleak
149 150
150 Kmemleak API 151 Kmemleak API
151 ------------ 152 ------------
152 153
153 See the include/linux/kmemleak.h header for th 154 See the include/linux/kmemleak.h header for the functions prototype.
154 155
155 - ``kmemleak_init`` - initialize 156 - ``kmemleak_init`` - initialize kmemleak
156 - ``kmemleak_alloc`` - notify of a 157 - ``kmemleak_alloc`` - notify of a memory block allocation
157 - ``kmemleak_alloc_percpu`` - notify of a 158 - ``kmemleak_alloc_percpu`` - notify of a percpu memory block allocation
158 - ``kmemleak_vmalloc`` - notify of a 159 - ``kmemleak_vmalloc`` - notify of a vmalloc() memory allocation
159 - ``kmemleak_free`` - notify of a 160 - ``kmemleak_free`` - notify of a memory block freeing
160 - ``kmemleak_free_part`` - notify of a 161 - ``kmemleak_free_part`` - notify of a partial memory block freeing
161 - ``kmemleak_free_percpu`` - notify of a 162 - ``kmemleak_free_percpu`` - notify of a percpu memory block freeing
162 - ``kmemleak_update_trace`` - update obje 163 - ``kmemleak_update_trace`` - update object allocation stack trace
163 - ``kmemleak_not_leak`` - mark an object as n 164 - ``kmemleak_not_leak`` - mark an object as not a leak
164 - ``kmemleak_ignore`` - do not scan 165 - ``kmemleak_ignore`` - do not scan or report an object as leak
165 - ``kmemleak_scan_area`` - add scan ar 166 - ``kmemleak_scan_area`` - add scan areas inside a memory block
166 - ``kmemleak_no_scan`` - do not scan a memor 167 - ``kmemleak_no_scan`` - do not scan a memory block
167 - ``kmemleak_erase`` - erase an ol 168 - ``kmemleak_erase`` - erase an old value in a pointer variable
168 - ``kmemleak_alloc_recursive`` - as kmemleak_a 169 - ``kmemleak_alloc_recursive`` - as kmemleak_alloc but checks the recursiveness
169 - ``kmemleak_free_recursive`` - as kmemleak 170 - ``kmemleak_free_recursive`` - as kmemleak_free but checks the recursiveness
170 171
171 The following functions take a physical addres 172 The following functions take a physical address as the object pointer
172 and only perform the corresponding action if t 173 and only perform the corresponding action if the address has a lowmem
173 mapping: 174 mapping:
174 175
175 - ``kmemleak_alloc_phys`` 176 - ``kmemleak_alloc_phys``
176 - ``kmemleak_free_part_phys`` 177 - ``kmemleak_free_part_phys``
>> 178 - ``kmemleak_not_leak_phys``
177 - ``kmemleak_ignore_phys`` 179 - ``kmemleak_ignore_phys``
178 180
179 Dealing with false positives/negatives 181 Dealing with false positives/negatives
180 -------------------------------------- 182 --------------------------------------
181 183
182 The false negatives are real memory leaks (orp 184 The false negatives are real memory leaks (orphan objects) but not
183 reported by kmemleak because values found duri 185 reported by kmemleak because values found during the memory scanning
184 point to such objects. To reduce the number of 186 point to such objects. To reduce the number of false negatives, kmemleak
185 provides the kmemleak_ignore, kmemleak_scan_ar 187 provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
186 kmemleak_erase functions (see above). The task 188 kmemleak_erase functions (see above). The task stacks also increase the
187 amount of false negatives and their scanning i 189 amount of false negatives and their scanning is not enabled by default.
188 190
189 The false positives are objects wrongly report 191 The false positives are objects wrongly reported as being memory leaks
190 (orphan). For objects known not to be leaks, k 192 (orphan). For objects known not to be leaks, kmemleak provides the
191 kmemleak_not_leak function. The kmemleak_ignor 193 kmemleak_not_leak function. The kmemleak_ignore could also be used if
192 the memory block is known not to contain other 194 the memory block is known not to contain other pointers and it will no
193 longer be scanned. 195 longer be scanned.
194 196
195 Some of the reported leaks are only transient, 197 Some of the reported leaks are only transient, especially on SMP
196 systems, because of pointers temporarily store 198 systems, because of pointers temporarily stored in CPU registers or
197 stacks. Kmemleak defines MSECS_MIN_AGE (defaul 199 stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
198 the minimum age of an object to be reported as 200 the minimum age of an object to be reported as a memory leak.
199 201
200 Limitations and Drawbacks 202 Limitations and Drawbacks
201 ------------------------- 203 -------------------------
202 204
203 The main drawback is the reduced performance o 205 The main drawback is the reduced performance of memory allocation and
204 freeing. To avoid other penalties, the memory 206 freeing. To avoid other penalties, the memory scanning is only performed
205 when the /sys/kernel/debug/kmemleak file is re 207 when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
206 intended for debugging purposes where the perf 208 intended for debugging purposes where the performance might not be the
207 most important requirement. 209 most important requirement.
208 210
209 To keep the algorithm simple, kmemleak scans f 211 To keep the algorithm simple, kmemleak scans for values pointing to any
210 address inside a block's address range. This m 212 address inside a block's address range. This may lead to an increased
211 number of false negatives. However, it is like 213 number of false negatives. However, it is likely that a real memory leak
212 will eventually become visible. 214 will eventually become visible.
213 215
214 Another source of false negatives is the data 216 Another source of false negatives is the data stored in non-pointer
215 values. In a future version, kmemleak could on 217 values. In a future version, kmemleak could only scan the pointer
216 members in the allocated structures. This feat 218 members in the allocated structures. This feature would solve many of
217 the false negative cases described above. 219 the false negative cases described above.
218 220
219 The tool can report false positives. These are 221 The tool can report false positives. These are cases where an allocated
220 block doesn't need to be freed (some cases in 222 block doesn't need to be freed (some cases in the init_call functions),
221 the pointer is calculated by other methods tha 223 the pointer is calculated by other methods than the usual container_of
222 macro or the pointer is stored in a location n 224 macro or the pointer is stored in a location not scanned by kmemleak.
223 225
224 Page allocations and ioremap are not tracked. 226 Page allocations and ioremap are not tracked.
225 227
226 Testing with kmemleak-test 228 Testing with kmemleak-test
227 -------------------------- 229 --------------------------
228 230
229 To check if you have all set up to use kmemlea 231 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 !! 232 module, a module that deliberately leaks memory. Set CONFIG_DEBUG_KMEMLEAK_TEST
231 as module (it can't be used as built-in) and b !! 233 as module (it can't be used as bult-in) and boot the kernel with kmemleak
232 enabled. Load the module and perform a scan wi 234 enabled. Load the module and perform a scan with::
233 235
234 # modprobe kmemleak-test 236 # modprobe kmemleak-test
235 # echo scan > /sys/kernel/debug/kmemle 237 # echo scan > /sys/kernel/debug/kmemleak
236 238
237 Note that the you may not get results instantl 239 Note that the you may not get results instantly or on the first scanning. When
238 kmemleak gets results, it'll log ``kmemleak: < 240 kmemleak gets results, it'll log ``kmemleak: <count of leaks> new suspected
239 memory leaks``. Then read the file to see then 241 memory leaks``. Then read the file to see then::
240 242
241 # cat /sys/kernel/debug/kmemleak 243 # cat /sys/kernel/debug/kmemleak
242 unreferenced object 0xffff89862ca702e8 244 unreferenced object 0xffff89862ca702e8 (size 32):
243 comm "modprobe", pid 2088, jiffies 4 245 comm "modprobe", pid 2088, jiffies 4294680594 (age 375.486s)
244 hex dump (first 32 bytes): 246 hex dump (first 32 bytes):
245 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6 247 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 6 248 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk.
247 backtrace: 249 backtrace:
248 [<00000000e0a73ec7>] 0xffffffffc01 250 [<00000000e0a73ec7>] 0xffffffffc01d2036
249 [<000000000c5d2a46>] do_one_initca 251 [<000000000c5d2a46>] do_one_initcall+0x41/0x1df
250 [<0000000046db7e0a>] do_init_modul 252 [<0000000046db7e0a>] do_init_module+0x55/0x200
251 [<00000000542b9814>] load_module+0 253 [<00000000542b9814>] load_module+0x203c/0x2480
252 [<00000000c2850256>] __do_sys_fini 254 [<00000000c2850256>] __do_sys_finit_module+0xba/0xe0
253 [<000000006564e7ef>] do_syscall_64 255 [<000000006564e7ef>] do_syscall_64+0x43/0x110
254 [<000000007c873fa6>] entry_SYSCALL 256 [<000000007c873fa6>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
255 ... 257 ...
256 258
257 Removing the module with ``rmmod kmemleak_test 259 Removing the module with ``rmmod kmemleak_test`` should also trigger some
258 kmemleak results. 260 kmemleak results.
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