Kernel Memory Leak Detector¶
Kmemleak provides a way of detecting possible kernel memory leaks in a
way similar to a tracing garbage collector
with the difference that the orphan objects are not freed but only
reported via /sys/kernel/debug/kmemleak. A similar method is used by the
Valgrind tool (
memcheck --leak-check) to detect the memory leaks in
Kmemleak is supported on x86, arm, powerpc, sparc, sh, microblaze, ppc, mips, s390 and tile.
CONFIG_DEBUG_KMEMLEAK in “Kernel hacking” has to be enabled. A kernel thread scans the memory every 10 minutes (by default) and prints the number of new unreferenced objects found. To display the details of all the possible memory leaks:
# mount -t debugfs nodev /sys/kernel/debug/ # cat /sys/kernel/debug/kmemleak
To trigger an intermediate memory scan:
# echo scan > /sys/kernel/debug/kmemleak
To clear the list of all current possible memory leaks:
# echo clear > /sys/kernel/debug/kmemleak
New leaks will then come up upon reading
Note that the orphan objects are listed in the order they were allocated and one object at the beginning of the list may cause other subsequent objects to be reported as orphan.
Memory scanning parameters can be modified at run-time by writing to the
/sys/kernel/debug/kmemleak file. The following parameters are supported:
- disable kmemleak (irreversible)
- enable the task stacks scanning (default)
- disable the tasks stacks scanning
- start the automatic memory scanning thread (default)
- stop the automatic memory scanning thread
- set the automatic memory scanning period in seconds (default 600, 0 to stop the automatic scanning)
- trigger a memory scan
- clear list of current memory leak suspects, done by marking all current reported unreferenced objects grey, or free all kmemleak objects if kmemleak has been disabled.
- dump information about the object found at <addr>
Kmemleak can also be disabled at boot-time by passing
the kernel command line.
Memory may be allocated or freed before kmemleak is initialised and these actions are stored in an early log buffer. The size of this buffer is configured via the CONFIG_DEBUG_KMEMLEAK_EARLY_LOG_SIZE option.
If CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF are enabled, the kmemleak is
disabled by default. Passing
kmemleak=on on the kernel command
line enables the function.
The memory allocations via
friends are traced and the pointers, together with additional
information like size and stack trace, are stored in a rbtree.
The corresponding freeing function calls are tracked and the pointers
removed from the kmemleak data structures.
An allocated block of memory is considered orphan if no pointer to its start address or to any location inside the block can be found by scanning the memory (including saved registers). This means that there might be no way for the kernel to pass the address of the allocated block to a freeing function and therefore the block is considered a memory leak.
The scanning algorithm steps:
- mark all objects as white (remaining white objects will later be considered orphan)
- scan the memory starting with the data section and stacks, checking the values against the addresses stored in the rbtree. If a pointer to a white object is found, the object is added to the gray list
- scan the gray objects for matching addresses (some white objects can become gray and added at the end of the gray list) until the gray set is finished
- the remaining white objects are considered orphan and reported via /sys/kernel/debug/kmemleak
Some allocated memory blocks have pointers stored in the kernel’s internal data structures and they cannot be detected as orphans. To avoid this, kmemleak can also store the number of values pointing to an address inside the block address range that need to be found so that the block is not considered a leak. One example is __vmalloc().
Testing specific sections with kmemleak¶
Upon initial bootup your /sys/kernel/debug/kmemleak output page may be quite extensive. This can also be the case if you have very buggy code when doing development. To work around these situations you can use the ‘clear’ command to clear all reported unreferenced objects from the /sys/kernel/debug/kmemleak output. By issuing a ‘scan’ after a ‘clear’ you can find new unreferenced objects; this should help with testing specific sections of code.
To test a critical section on demand with a clean kmemleak do:
# echo clear > /sys/kernel/debug/kmemleak ... test your kernel or modules ... # echo scan > /sys/kernel/debug/kmemleak
Then as usual to get your report with:
# cat /sys/kernel/debug/kmemleak
Freeing kmemleak internal objects¶
To allow access to previously found memory leaks after kmemleak has been disabled by the user or due to an fatal error, internal kmemleak objects won’t be freed when kmemleak is disabled, and those objects may occupy a large part of physical memory.
In this situation, you may reclaim memory with:
# echo clear > /sys/kernel/debug/kmemleak
See the include/linux/kmemleak.h header for the functions prototype.
kmemleak_init- initialize kmemleak
kmemleak_alloc- notify of a memory block allocation
kmemleak_alloc_percpu- notify of a percpu memory block allocation
kmemleak_vmalloc- notify of a vmalloc() memory allocation
kmemleak_free- notify of a memory block freeing
kmemleak_free_part- notify of a partial memory block freeing
kmemleak_free_percpu- notify of a percpu memory block freeing
kmemleak_update_trace- update object allocation stack trace
kmemleak_not_leak- mark an object as not a leak
kmemleak_ignore- do not scan or report an object as leak
kmemleak_scan_area- add scan areas inside a memory block
kmemleak_no_scan- do not scan a memory block
kmemleak_erase- erase an old value in a pointer variable
kmemleak_alloc_recursive- as kmemleak_alloc but checks the recursiveness
kmemleak_free_recursive- as kmemleak_free but checks the recursiveness
The following functions take a physical address as the object pointer and only perform the corresponding action if the address has a lowmem mapping:
Dealing with false positives/negatives¶
The false negatives are real memory leaks (orphan objects) but not reported by kmemleak because values found during the memory scanning point to such objects. To reduce the number of false negatives, kmemleak provides the kmemleak_ignore, kmemleak_scan_area, kmemleak_no_scan and kmemleak_erase functions (see above). The task stacks also increase the amount of false negatives and their scanning is not enabled by default.
The false positives are objects wrongly reported as being memory leaks (orphan). For objects known not to be leaks, kmemleak provides the kmemleak_not_leak function. The kmemleak_ignore could also be used if the memory block is known not to contain other pointers and it will no longer be scanned.
Some of the reported leaks are only transient, especially on SMP systems, because of pointers temporarily stored in CPU registers or stacks. Kmemleak defines MSECS_MIN_AGE (defaulting to 1000) representing the minimum age of an object to be reported as a memory leak.
Limitations and Drawbacks¶
The main drawback is the reduced performance of memory allocation and freeing. To avoid other penalties, the memory scanning is only performed when the /sys/kernel/debug/kmemleak file is read. Anyway, this tool is intended for debugging purposes where the performance might not be the most important requirement.
To keep the algorithm simple, kmemleak scans for values pointing to any address inside a block’s address range. This may lead to an increased number of false negatives. However, it is likely that a real memory leak will eventually become visible.
Another source of false negatives is the data stored in non-pointer values. In a future version, kmemleak could only scan the pointer members in the allocated structures. This feature would solve many of the false negative cases described above.
The tool can report false positives. These are cases where an allocated block doesn’t need to be freed (some cases in the init_call functions), the pointer is calculated by other methods than the usual container_of macro or the pointer is stored in a location not scanned by kmemleak.
Page allocations and ioremap are not tracked.