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. 11Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390 and tile. 12 13Usage 14----- 15 16CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel 17thread scans the memory every 10 minutes (by default) and prints the 18number of new unreferenced objects found. If the ``debugfs`` isn't already 19mounted, mount with:: 20 21 # mount -t debugfs nodev /sys/kernel/debug/ 22 23To display the details of all the possible scanned memory leaks:: 24 25 # cat /sys/kernel/debug/kmemleak 26 27To trigger an intermediate memory scan:: 28 29 # echo scan > /sys/kernel/debug/kmemleak 30 31To clear the list of all current possible memory leaks:: 32 33 # echo clear > /sys/kernel/debug/kmemleak 34 35New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak`` 36again. 37 38Note that the orphan objects are listed in the order they were allocated 39and one object at the beginning of the list may cause other subsequent 40objects to be reported as orphan. 41 42Memory scanning parameters can be modified at run-time by writing to the 43``/sys/kernel/debug/kmemleak`` file. The following parameters are supported: 44 45- off 46 disable kmemleak (irreversible) 47- stack=on 48 enable the task stacks scanning (default) 49- stack=off 50 disable the tasks stacks scanning 51- scan=on 52 start the automatic memory scanning thread (default) 53- scan=off 54 stop the automatic memory scanning thread 55- scan=<secs> 56 set the automatic memory scanning period in seconds 57 (default 600, 0 to stop the automatic scanning) 58- scan 59 trigger a memory scan 60- clear 61 clear list of current memory leak suspects, done by 62 marking all current reported unreferenced objects grey, 63 or free all kmemleak objects if kmemleak has been disabled. 64- dump=<addr> 65 dump information about the object found at <addr> 66 67Kmemleak can also be disabled at boot-time by passing ``kmemleak=off`` on 68the kernel command line. 69 70Memory may be allocated or freed before kmemleak is initialised and 71these actions are stored in an early log buffer. The size of this buffer 72is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option. 73 74If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is 75disabled by default. Passing ``kmemleak=on`` on the kernel command 76line enables the function. 77 78If you are getting errors like "Error while writing to stdout" or "write_loop: 79Invalid argument", make sure kmemleak is properly enabled. 80 81Basic Algorithm 82--------------- 83 84The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`, 85:c:func:`kmem_cache_alloc` and 86friends are traced and the pointers, together with additional 87information like size and stack trace, are stored in a rbtree. 88The corresponding freeing function calls are tracked and the pointers 89removed from the kmemleak data structures. 90 91An allocated block of memory is considered orphan if no pointer to its 92start address or to any location inside the block can be found by 93scanning the memory (including saved registers). This means that there 94might be no way for the kernel to pass the address of the allocated 95block to a freeing function and therefore the block is considered a 96memory leak. 97 98The scanning algorithm steps: 99 100 1. mark all objects as white (remaining white objects will later be 101 considered orphan) 102 2. scan the memory starting with the data section and stacks, checking 103 the values against the addresses stored in the rbtree. If 104 a pointer to a white object is found, the object is added to the 105 gray list 106 3. scan the gray objects for matching addresses (some white objects 107 can become gray and added at the end of the gray list) until the 108 gray set is finished 109 4. the remaining white objects are considered orphan and reported via 110 /sys/kernel/debug/kmemleak 111 112Some allocated memory blocks have pointers stored in the kernel's 113internal data structures and they cannot be detected as orphans. To 114avoid this, kmemleak can also store the number of values pointing to an 115address inside the block address range that need to be found so that the 116block is not considered a leak. One example is __vmalloc(). 117 118Testing specific sections with kmemleak 119--------------------------------------- 120 121Upon initial bootup your /sys/kernel/debug/kmemleak output page may be 122quite extensive. This can also be the case if you have very buggy code 123when doing development. To work around these situations you can use the 124'clear' command to clear all reported unreferenced objects from the 125/sys/kernel/debug/kmemleak output. By issuing a 'scan' after a 'clear' 126you can find new unreferenced objects; this should help with testing 127specific sections of code. 128 129To test a critical section on demand with a clean kmemleak do:: 130 131 # echo clear > /sys/kernel/debug/kmemleak 132 ... test your kernel or modules ... 133 # echo scan > /sys/kernel/debug/kmemleak 134 135Then as usual to get your report with:: 136 137 # cat /sys/kernel/debug/kmemleak 138 139Freeing kmemleak internal objects 140--------------------------------- 141 142To allow access to previously found memory leaks after kmemleak has been 143disabled by the user or due to an fatal error, internal kmemleak objects 144won't be freed when kmemleak is disabled, and those objects may occupy 145a large part of physical memory. 146 147In this situation, you may reclaim memory with:: 148 149 # echo clear > /sys/kernel/debug/kmemleak 150 151Kmemleak API 152------------ 153 154See the include/linux/kmemleak.h header for the functions prototype. 155 156- ``kmemleak_init`` - initialize kmemleak 157- ``kmemleak_alloc`` - notify of a memory block allocation 158- ``kmemleak_alloc_percpu`` - notify of a percpu memory block allocation 159- ``kmemleak_vmalloc`` - notify of a vmalloc() memory allocation 160- ``kmemleak_free`` - notify of a memory block freeing 161- ``kmemleak_free_part`` - notify of a partial memory block freeing 162- ``kmemleak_free_percpu`` - notify of a percpu memory block freeing 163- ``kmemleak_update_trace`` - update object allocation stack trace 164- ``kmemleak_not_leak`` - mark an object as not a leak 165- ``kmemleak_ignore`` - do not scan or report an object as leak 166- ``kmemleak_scan_area`` - add scan areas inside a memory block 167- ``kmemleak_no_scan`` - do not scan a memory block 168- ``kmemleak_erase`` - erase an old value in a pointer variable 169- ``kmemleak_alloc_recursive`` - as kmemleak_alloc but checks the recursiveness 170- ``kmemleak_free_recursive`` - as kmemleak_free but checks the recursiveness 171 172The following functions take a physical address as the object pointer 173and only perform the corresponding action if the address has a lowmem 174mapping: 175 176- ``kmemleak_alloc_phys`` 177- ``kmemleak_free_part_phys`` 178- ``kmemleak_not_leak_phys`` 179- ``kmemleak_ignore_phys`` 180 181Dealing with false positives/negatives 182-------------------------------------- 183 184The false negatives are real memory leaks (orphan objects) but not 185reported by kmemleak because values found during the memory scanning 186point to such objects. To reduce the number of false negatives, kmemleak 187provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and 188kmemleak_erase functions (see above). The task stacks also increase the 189amount of false negatives and their scanning is not enabled by default. 190 191The false positives are objects wrongly reported as being memory leaks 192(orphan). For objects known not to be leaks, kmemleak provides the 193kmemleak_not_leak function. The kmemleak_ignore could also be used if 194the memory block is known not to contain other pointers and it will no 195longer be scanned. 196 197Some of the reported leaks are only transient, especially on SMP 198systems, because of pointers temporarily stored in CPU registers or 199stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing 200the minimum age of an object to be reported as a memory leak. 201 202Limitations and Drawbacks 203------------------------- 204 205The main drawback is the reduced performance of memory allocation and 206freeing. To avoid other penalties, the memory scanning is only performed 207when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is 208intended for debugging purposes where the performance might not be the 209most important requirement. 210 211To keep the algorithm simple, kmemleak scans for values pointing to any 212address inside a block's address range. This may lead to an increased 213number of false negatives. However, it is likely that a real memory leak 214will eventually become visible. 215 216Another source of false negatives is the data stored in non-pointer 217values. In a future version, kmemleak could only scan the pointer 218members in the allocated structures. This feature would solve many of 219the false negative cases described above. 220 221The tool can report false positives. These are cases where an allocated 222block doesn't need to be freed (some cases in the init_call functions), 223the pointer is calculated by other methods than the usual container_of 224macro or the pointer is stored in a location not scanned by kmemleak. 225 226Page allocations and ioremap are not tracked. 227 228Testing with kmemleak-test 229-------------------------- 230 231To check if you have all set up to use kmemleak, you can use the kmemleak-test 232module, a module that deliberately leaks memory. Set CONFIG_DEBUG_KMEMLEAK_TEST 233as module (it can't be used as bult-in) and boot the kernel with kmemleak 234enabled. Load the module and perform a scan with:: 235 236 # modprobe kmemleak-test 237 # echo scan > /sys/kernel/debug/kmemleak 238 239Note that the you may not get results instantly or on the first scanning. When 240kmemleak gets results, it'll log ``kmemleak: <count of leaks> new suspected 241memory leaks``. Then read the file to see then:: 242 243 # cat /sys/kernel/debug/kmemleak 244 unreferenced object 0xffff89862ca702e8 (size 32): 245 comm "modprobe", pid 2088, jiffies 4294680594 (age 375.486s) 246 hex dump (first 32 bytes): 247 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk 248 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. 249 backtrace: 250 [<00000000e0a73ec7>] 0xffffffffc01d2036 251 [<000000000c5d2a46>] do_one_initcall+0x41/0x1df 252 [<0000000046db7e0a>] do_init_module+0x55/0x200 253 [<00000000542b9814>] load_module+0x203c/0x2480 254 [<00000000c2850256>] __do_sys_finit_module+0xba/0xe0 255 [<000000006564e7ef>] do_syscall_64+0x43/0x110 256 [<000000007c873fa6>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 257 ... 258 259Removing the module with ``rmmod kmemleak_test`` should also trigger some 260kmemleak results. 261