1Kernel Memory Leak Detector
2===========================
3
4Kmemleak provides a way of detecting possible kernel memory leaks in a
5way similar to a `tracing garbage collector
6<https://en.wikipedia.org/wiki/Tracing_garbage_collection>`_,
7with the difference that the orphan objects are not freed but only
8reported via /sys/kernel/debug/kmemleak. A similar method is used by the
9Valgrind tool (``memcheck --leak-check``) to detect the memory leaks in
10user-space applications.
11
12Usage
13-----
14
15CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel
16thread scans the memory every 10 minutes (by default) and prints the
17number of new unreferenced objects found. If the ``debugfs`` isn't already
18mounted, mount with::
19
20  # mount -t debugfs nodev /sys/kernel/debug/
21
22To display the details of all the possible scanned memory leaks::
23
24  # cat /sys/kernel/debug/kmemleak
25
26To trigger an intermediate memory scan::
27
28  # echo scan > /sys/kernel/debug/kmemleak
29
30To clear the list of all current possible memory leaks::
31
32  # echo clear > /sys/kernel/debug/kmemleak
33
34New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak``
35again.
36
37Note that the orphan objects are listed in the order they were allocated
38and one object at the beginning of the list may cause other subsequent
39objects to be reported as orphan.
40
41Memory scanning parameters can be modified at run-time by writing to the
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
66Kmemleak can also be disabled at boot-time by passing ``kmemleak=off`` on
67the kernel command line.
68
69Memory may be allocated or freed before kmemleak is initialised and
70these actions are stored in an early log buffer. The size of this buffer
71is configured via the CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE option.
72
73If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
74disabled by default. Passing ``kmemleak=on`` on the kernel command
75line enables the function. 
76
77If you are getting errors like "Error while writing to stdout" or "write_loop:
78Invalid argument", make sure kmemleak is properly enabled.
79
80Basic Algorithm
81---------------
82
83The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`,
84:c:func:`kmem_cache_alloc` and
85friends are traced and the pointers, together with additional
86information like size and stack trace, are stored in a rbtree.
87The corresponding freeing function calls are tracked and the pointers
88removed from the kmemleak data structures.
89
90An allocated block of memory is considered orphan if no pointer to its
91start address or to any location inside the block can be found by
92scanning the memory (including saved registers). This means that there
93might be no way for the kernel to pass the address of the allocated
94block to a freeing function and therefore the block is considered a
95memory leak.
96
97The 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
102     the values against the addresses stored in the rbtree. If
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
111Some allocated memory blocks have pointers stored in the kernel's
112internal data structures and they cannot be detected as orphans. To
113avoid this, kmemleak can also store the number of values pointing to an
114address inside the block address range that need to be found so that the
115block is not considered a leak. One example is __vmalloc().
116
117Testing specific sections with kmemleak
118---------------------------------------
119
120Upon initial bootup your /sys/kernel/debug/kmemleak output page may be
121quite extensive. This can also be the case if you have very buggy code
122when 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'
125you can find new unreferenced objects; this should help with testing
126specific sections of code.
127
128To test a critical section on demand with a clean kmemleak do::
129
130  # echo clear > /sys/kernel/debug/kmemleak
131  ... test your kernel or modules ...
132  # echo scan > /sys/kernel/debug/kmemleak
133
134Then as usual to get your report with::
135
136  # cat /sys/kernel/debug/kmemleak
137
138Freeing kmemleak internal objects
139---------------------------------
140
141To allow access to previously found memory leaks after kmemleak has been
142disabled by the user or due to an fatal error, internal kmemleak objects
143won't be freed when kmemleak is disabled, and those objects may occupy
144a large part of physical memory.
145
146In this situation, you may reclaim memory with::
147
148  # echo clear > /sys/kernel/debug/kmemleak
149
150Kmemleak API
151------------
152
153See the include/linux/kmemleak.h header for the functions prototype.
154
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
158- ``kmemleak_vmalloc``		 - notify of a vmalloc() memory allocation
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
170
171The following functions take a physical address as the object pointer
172and only perform the corresponding action if the address has a lowmem
173mapping:
174
175- ``kmemleak_alloc_phys``
176- ``kmemleak_free_part_phys``
177- ``kmemleak_ignore_phys``
178
179Dealing with false positives/negatives
180--------------------------------------
181
182The false negatives are real memory leaks (orphan objects) but not
183reported by kmemleak because values found during the memory scanning
184point to such objects. To reduce the number of false negatives, kmemleak
185provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and
186kmemleak_erase functions (see above). The task stacks also increase the
187amount of false negatives and their scanning is not enabled by default.
188
189The false positives are objects wrongly reported as being memory leaks
190(orphan). For objects known not to be leaks, kmemleak provides the
191kmemleak_not_leak function. The kmemleak_ignore could also be used if
192the memory block is known not to contain other pointers and it will no
193longer be scanned.
194
195Some of the reported leaks are only transient, especially on SMP
196systems, because of pointers temporarily stored in CPU registers or
197stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing
198the minimum age of an object to be reported as a memory leak.
199
200Limitations and Drawbacks
201-------------------------
202
203The main drawback is the reduced performance of memory allocation and
204freeing. To avoid other penalties, the memory scanning is only performed
205when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is
206intended for debugging purposes where the performance might not be the
207most important requirement.
208
209To keep the algorithm simple, kmemleak scans for values pointing to any
210address inside a block's address range. This may lead to an increased
211number of false negatives. However, it is likely that a real memory leak
212will eventually become visible.
213
214Another source of false negatives is the data stored in non-pointer
215values. In a future version, kmemleak could only scan the pointer
216members in the allocated structures. This feature would solve many of
217the false negative cases described above.
218
219The tool can report false positives. These are cases where an allocated
220block doesn't need to be freed (some cases in the init_call functions),
221the pointer is calculated by other methods than the usual container_of
222macro or the pointer is stored in a location not scanned by kmemleak.
223
224Page allocations and ioremap are not tracked.
225
226Testing with kmemleak-test
227--------------------------
228
229To check if you have all set up to use kmemleak, you can use the kmemleak-test
230module, a module that deliberately leaks memory. Set CONFIG_SAMPLE_KMEMLEAK
231as module (it can't be used as built-in) and boot the kernel with kmemleak
232enabled. Load the module and perform a scan with::
233
234        # modprobe kmemleak-test
235        # echo scan > /sys/kernel/debug/kmemleak
236
237Note that the you may not get results instantly or on the first scanning. When
238kmemleak gets results, it'll log ``kmemleak: <count of leaks> new suspected
239memory 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
257Removing the module with ``rmmod kmemleak_test`` should also trigger some
258kmemleak results.
259