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
