1 Kernel Memory Leak Detector 2 =========================== 3 4 Kmemleak provides a way of detecting possible kernel memory leaks in a 5 way similar to a `tracing garbage collector 6 <https://en.wikipedia.org/wiki/Tracing_garbage_collection>`_, 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 9 Valgrind tool (``memcheck --leak-check``) to detect the memory leaks in 10 user-space applications. 11 12 Usage 13 ----- 14 15 CONFIG_DEBUG_KMEMLEAK in "Kernel hacking" has to be enabled. A kernel 16 thread scans the memory every 10 minutes (by default) and prints the 17 number of new unreferenced objects found. If the ``debugfs`` isn't already 18 mounted, mount with:: 19 20 # mount -t debugfs nodev /sys/kernel/debug/ 21 22 To display the details of all the possible scanned memory leaks:: 23 24 # cat /sys/kernel/debug/kmemleak 25 26 To trigger an intermediate memory scan:: 27 28 # echo scan > /sys/kernel/debug/kmemleak 29 30 To clear the list of all current possible memory leaks:: 31 32 # echo clear > /sys/kernel/debug/kmemleak 33 34 New leaks will then come up upon reading ``/sys/kernel/debug/kmemleak`` 35 again. 36 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 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 67 the kernel command line. 68 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 71 is configured via the CONFIG_DEBUG_KMEMLEAK_MEM_POOL_SIZE option. 72 73 If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is 74 disabled by default. Passing ``kmemleak=on`` on the kernel command 75 line enables the function. 76 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 80 Basic Algorithm 81 --------------- 82 83 The memory allocations via :c:func:`kmalloc`, :c:func:`vmalloc`, 84 :c:func:`kmem_cache_alloc` and 85 friends are traced and the pointers, together with additional 86 information like size and stack trace, are stored in a rbtree. 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 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 110 111 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(). 116 117 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. 127 128 To 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 133 134 Then as usual to get your report with:: 135 136 # cat /sys/kernel/debug/kmemleak 137 138 Freeing kmemleak internal objects 139 --------------------------------- 140 141 To allow access to previously found memory leaks after kmemleak has been 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. 145 146 In this situation, you may reclaim memory with:: 147 148 # echo clear > /sys/kernel/debug/kmemleak 149 150 Kmemleak API 151 ------------ 152 153 See 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 170 171 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`` 177 - ``kmemleak_ignore_phys`` 178 179 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. 223 224 Page allocations and ioremap are not tracked. 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 230 module, a module that deliberately leaks memory. Set CONFIG_SAMPLE_KMEMLEAK 231 as module (it can't be used as built-in) and boot the kernel with kmemleak 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.
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