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1 | Kernel Memory Leak Detector |
2 | =========================== | |
3 | ||
04f70336 | 4 | Kmemleak provides a way of detecting possible kernel memory leaks in a |
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5 | way similar to a `tracing garbage collector |
6 | <https://en.wikipedia.org/wiki/Tracing_garbage_collection>`_, | |
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7 | with the difference that the orphan objects are not freed but only |
8 | reported via /sys/kernel/debug/kmemleak. A similar method is used by the | |
ca90a7a3 | 9 | Valgrind tool (``memcheck --leak-check``) to detect the memory leaks in |
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10 | user-space applications. |
11 | ||
12 | Usage | |
13 | ----- | |
14 | ||
15 | CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel | |
bab4a34a | 16 | thread scans the memory every 10 minutes (by default) and prints the |
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17 | number of new unreferenced objects found. If the ``debugfs`` isn't already |
18 | mounted, mount with:: | |
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19 | |
20 | # mount -t debugfs nodev /sys/kernel/debug/ | |
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21 | |
22 | To display the details of all the possible scanned memory leaks:: | |
23 | ||
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24 | # cat /sys/kernel/debug/kmemleak |
25 | ||
ca90a7a3 | 26 | To trigger an intermediate memory scan:: |
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27 | |
28 | # echo scan > /sys/kernel/debug/kmemleak | |
29 | ||
ca90a7a3 | 30 | To clear the list of all current possible memory leaks:: |
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31 | |
32 | # echo clear > /sys/kernel/debug/kmemleak | |
33 | ||
ca90a7a3 | 34 | New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak`` |
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35 | again. |
36 | ||
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37 | Note that the orphan objects are listed in the order they were allocated |
38 | and one object at the beginning of the list may cause other subsequent | |
39 | objects to be reported as orphan. | |
40 | ||
41 | Memory scanning parameters can be modified at run-time by writing to the | |
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42 | ``/sys/kernel/debug/kmemleak`` file. The following parameters are supported: |
43 | ||
44 | - off | |
45 | disable kmemleak (irreversible) | |
46 | - stack=on | |
47 | enable the task stacks scanning (default) | |
48 | - stack=off | |
49 | disable the tasks stacks scanning | |
50 | - scan=on | |
51 | start the automatic memory scanning thread (default) | |
52 | - scan=off | |
53 | stop the automatic memory scanning thread | |
54 | - scan=<secs> | |
55 | set the automatic memory scanning period in seconds | |
56 | (default 600, 0 to stop the automatic scanning) | |
57 | - scan | |
58 | trigger a memory scan | |
59 | - clear | |
60 | clear list of current memory leak suspects, done by | |
61 | marking all current reported unreferenced objects grey, | |
62 | or free all kmemleak objects if kmemleak has been disabled. | |
63 | - dump=<addr> | |
64 | dump information about the object found at <addr> | |
65 | ||
66 | Kmemleak can also be disabled at boot-time by passing ``kmemleak=off`` on | |
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67 | the kernel command line. |
68 | ||
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69 | Memory may be allocated or freed before kmemleak is initialised and |
70 | these actions are stored in an early log buffer. The size of this buffer | |
2c861bf5 | 71 | is configured via the CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE option. |
a9d9058a | 72 | |
6808a40d | 73 | If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is |
ca90a7a3 | 74 | disabled by default. Passing ``kmemleak=on`` on the kernel command |
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75 | line enables the function. |
76 | ||
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77 | If you are getting errors like "Error while writing to stdout" or "write_loop: |
78 | Invalid argument", make sure kmemleak is properly enabled. | |
79 | ||
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80 | Basic Algorithm |
81 | --------------- | |
82 | ||
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83 | The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`, |
84 | :c:func:`kmem_cache_alloc` and | |
04f70336 | 85 | friends are traced and the pointers, together with additional |
4762c984 | 86 | information like size and stack trace, are stored in a rbtree. |
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87 | The corresponding freeing function calls are tracked and the pointers |
88 | removed from the kmemleak data structures. | |
89 | ||
90 | An allocated block of memory is considered orphan if no pointer to its | |
91 | start address or to any location inside the block can be found by | |
92 | scanning the memory (including saved registers). This means that there | |
93 | might be no way for the kernel to pass the address of the allocated | |
94 | block to a freeing function and therefore the block is considered a | |
95 | memory leak. | |
96 | ||
97 | The scanning algorithm steps: | |
98 | ||
99 | 1. mark all objects as white (remaining white objects will later be | |
100 | considered orphan) | |
101 | 2. scan the memory starting with the data section and stacks, checking | |
4762c984 | 102 | the values against the addresses stored in the rbtree. If |
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103 | a pointer to a white object is found, the object is added to the |
104 | gray list | |
105 | 3. scan the gray objects for matching addresses (some white objects | |
106 | can become gray and added at the end of the gray list) until the | |
107 | gray set is finished | |
108 | 4. the remaining white objects are considered orphan and reported via | |
109 | /sys/kernel/debug/kmemleak | |
110 | ||
111 | Some allocated memory blocks have pointers stored in the kernel's | |
112 | internal data structures and they cannot be detected as orphans. To | |
113 | avoid this, kmemleak can also store the number of values pointing to an | |
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 __vmalloc(). | |
116 | ||
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117 | Testing specific sections with kmemleak |
118 | --------------------------------------- | |
119 | ||
120 | Upon initial bootup your /sys/kernel/debug/kmemleak output page may be | |
121 | quite extensive. This can also be the case if you have very buggy code | |
122 | when doing development. To work around these situations you can use the | |
123 | 'clear' command to clear all reported unreferenced objects from the | |
124 | /sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' | |
125 | you can find new unreferenced objects; this should help with testing | |
126 | specific sections of code. | |
127 | ||
ca90a7a3 | 128 | To test a critical section on demand with a clean kmemleak do:: |
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129 | |
130 | # echo clear > /sys/kernel/debug/kmemleak | |
131 | ... test your kernel or modules ... | |
132 | # echo scan > /sys/kernel/debug/kmemleak | |
133 | ||
ca90a7a3 | 134 | Then as usual to get your report with:: |
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135 | |
136 | # cat /sys/kernel/debug/kmemleak | |
137 | ||
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138 | Freeing kmemleak internal objects |
139 | --------------------------------- | |
140 | ||
abb3b1f8 | 141 | To allow access to previously found memory leaks after kmemleak has been |
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142 | disabled by the user or due to an fatal error, internal kmemleak objects |
143 | won't be freed when kmemleak is disabled, and those objects may occupy | |
144 | a large part of physical memory. | |
145 | ||
ca90a7a3 | 146 | In this situation, you may reclaim memory with:: |
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147 | |
148 | # echo clear > /sys/kernel/debug/kmemleak | |
149 | ||
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150 | Kmemleak API |
151 | ------------ | |
152 | ||
153 | See the include/linux/kmemleak.h header for the functions prototype. | |
154 | ||
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155 | - ``kmemleak_init`` - initialize kmemleak |
156 | - ``kmemleak_alloc`` - notify of a memory block allocation | |
157 | - ``kmemleak_alloc_percpu`` - notify of a percpu memory block allocation | |
94f4a161 | 158 | - ``kmemleak_vmalloc`` - notify of a vmalloc() memory allocation |
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159 | - ``kmemleak_free`` - notify of a memory block freeing |
160 | - ``kmemleak_free_part`` - notify of a partial memory block freeing | |
161 | - ``kmemleak_free_percpu`` - notify of a percpu memory block freeing | |
162 | - ``kmemleak_update_trace`` - update object allocation stack trace | |
163 | - ``kmemleak_not_leak`` - mark an object as not a leak | |
164 | - ``kmemleak_ignore`` - do not scan or report an object as leak | |
165 | - ``kmemleak_scan_area`` - add scan areas inside a memory block | |
166 | - ``kmemleak_no_scan`` - do not scan a memory block | |
167 | - ``kmemleak_erase`` - erase an old value in a pointer variable | |
168 | - ``kmemleak_alloc_recursive`` - as kmemleak_alloc but checks the recursiveness | |
169 | - ``kmemleak_free_recursive`` - as kmemleak_free but checks the recursiveness | |
04f70336 | 170 | |
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171 | The following functions take a physical address as the object pointer |
172 | and only perform the corresponding action if the address has a lowmem | |
173 | mapping: | |
174 | ||
175 | - ``kmemleak_alloc_phys`` | |
176 | - ``kmemleak_free_part_phys`` | |
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177 | - ``kmemleak_ignore_phys`` |
178 | ||
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179 | Dealing with false positives/negatives |
180 | -------------------------------------- | |
181 | ||
182 | The false negatives are real memory leaks (orphan objects) but not | |
183 | reported by kmemleak because values found during the memory scanning | |
184 | point to such objects. To reduce the number of false negatives, kmemleak | |
185 | provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and | |
186 | kmemleak_erase functions (see above). The task stacks also increase the | |
187 | amount of false negatives and their scanning is not enabled by default. | |
188 | ||
189 | The false positives are objects wrongly reported as being memory leaks | |
190 | (orphan). For objects known not to be leaks, kmemleak provides the | |
191 | kmemleak_not_leak function. The kmemleak_ignore could also be used if | |
192 | the memory block is known not to contain other pointers and it will no | |
193 | longer be scanned. | |
194 | ||
195 | Some of the reported leaks are only transient, especially on SMP | |
196 | systems, because of pointers temporarily stored in CPU registers or | |
197 | stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing | |
198 | the minimum age of an object to be reported as a memory leak. | |
199 | ||
200 | Limitations and Drawbacks | |
201 | ------------------------- | |
202 | ||
203 | The main drawback is the reduced performance of memory allocation and | |
204 | freeing. To avoid other penalties, the memory scanning is only performed | |
205 | when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is | |
206 | intended for debugging purposes where the performance might not be the | |
207 | most important requirement. | |
208 | ||
209 | To keep the algorithm simple, kmemleak scans for values pointing to any | |
210 | address inside a block's address range. This may lead to an increased | |
211 | number of false negatives. However, it is likely that a real memory leak | |
212 | will eventually become visible. | |
213 | ||
214 | Another source of false negatives is the data stored in non-pointer | |
215 | values. In a future version, kmemleak could only scan the pointer | |
216 | members in the allocated structures. This feature would solve many of | |
217 | the false negative cases described above. | |
218 | ||
219 | The tool can report false positives. These are cases where an allocated | |
220 | block doesn't need to be freed (some cases in the init_call functions), | |
221 | the pointer is calculated by other methods than the usual container_of | |
222 | macro or the pointer is stored in a location not scanned by kmemleak. | |
223 | ||
21b86bd5 | 224 | Page allocations and ioremap are not tracked. |
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225 | |
226 | Testing with kmemleak-test | |
227 | -------------------------- | |
228 | ||
229 | To check if you have all set up to use kmemleak, you can use the kmemleak-test | |
a110f8eb | 230 | module, a module that deliberately leaks memory. Set CONFIG_SAMPLE_KMEMLEAK |
1abbef4f | 231 | as module (it can't be used as built-in) and boot the kernel with kmemleak |
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232 | enabled. Load the module and perform a scan with:: |
233 | ||
234 | # modprobe kmemleak-test | |
235 | # echo scan > /sys/kernel/debug/kmemleak | |
236 | ||
237 | Note that the you may not get results instantly or on the first scanning. When | |
238 | kmemleak gets results, it'll log ``kmemleak: <count of leaks> new suspected | |
239 | memory leaks``. Then read the file to see then:: | |
240 | ||
241 | # cat /sys/kernel/debug/kmemleak | |
242 | unreferenced object 0xffff89862ca702e8 (size 32): | |
243 | comm "modprobe", pid 2088, jiffies 4294680594 (age 375.486s) | |
244 | hex dump (first 32 bytes): | |
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 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. | |
247 | backtrace: | |
248 | [<00000000e0a73ec7>] 0xffffffffc01d2036 | |
249 | [<000000000c5d2a46>] do_one_initcall+0x41/0x1df | |
250 | [<0000000046db7e0a>] do_init_module+0x55/0x200 | |
251 | [<00000000542b9814>] load_module+0x203c/0x2480 | |
252 | [<00000000c2850256>] __do_sys_finit_module+0xba/0xe0 | |
253 | [<000000006564e7ef>] do_syscall_64+0x43/0x110 | |
254 | [<000000007c873fa6>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 | |
255 | ... | |
256 | ||
257 | Removing the module with ``rmmod kmemleak_test`` should also trigger some | |
258 | kmemleak results. |