Android Live-LocK Daemon (llkd) is used to catch kernel deadlocks and mitigate.
Code is structured to allow integration into another service as either as part of the main loop, or spun off as a thread should that be necessary. A default standalone implementation is provided by llkd component.
The 'C' interface from libllkd component is thus:
#include "llkd.h" bool llkInit(const char* threadname) /* return true if enabled */ unsigned llkCheckMillseconds(void) /* ms to sleep for next check */
If a threadname is provided, a thread will be automatically spawned, otherwise caller must call llkCheckMilliseconds in its main loop. Function will return the period of time before the next expected call to this handler.
There are two detection scenarios. Persistent D or Z state, and persistent stack signature.
If a thread is in D or Z state with no forward progress for longer than ro.llk.timeout_ms, or ro.llk.[D|Z].timeout_ms, kill the process or parent process respectively. If another scan shows the same process continues to exist, then have a confirmed live-lock condition and need to panic. Panic the kernel in a manner to provide the greatest bugreporting details as to the condition. Add a alarm self watchdog should llkd ever get locked up that is double the expected time to flow through the mainloop. Sampling is every ro.llk_sample_ms.
For usedebug releases only, persistent stack signature checking is enabled. If a thread in any state but Z, has a persistent listed ro.llk.stack kernel symbol always being reported, even if there is forward scheduling progress, for longer than ro.llk.timeout_ms, or ro.llk.stack.timeout_ms, then issue a kill to the process. If another scan shows the same process continues to exist, then have a confirmed live-lock condition and need to panic. There is no ABA detection since forward scheduling progress is allowed, thus the condition for the symbols are:
Default will not monitor init, or [kthreadd] and all that [kthreadd] spawns. This reduces the effectiveness of llkd by limiting its coverage. If there is value in covering [kthreadd] spawned threads, the requirement will be that the drivers not remain in a persistent 'D' state, or that they have mechanisms to recover the thread should it be killed externally (this is good driver coding hygiene, a common request to add such to publicly reviewed kernel.org maintained drivers). For instance use wait_event_interruptible() instead of wait_event(). The blacklists can be adjusted accordingly if these conditions are met to cover kernel components. For the stack symbol checking, there is an additional process blacklist so that we do not incide sepolicy violations on services that block ptrace operations.
An accompanying gTest set have been added, and will setup a persistent D or Z process, with and without forward progress, but not in a live-lock state because that would require a buggy kernel, or a module or kernel modification to stimulate. The test will check that llkd will mitigate first by killing the appropriate process. D state is setup by vfork() waiting for exec() in child process. Z state is setup by fork() and an un-waited for child process. Should be noted that both of these conditions should never happen on Android on purpose, and llkd effectively sweeps up processes that create these conditions. If the test can, it will reconfigure llkd to expedite the test duration by adjusting the ro.llk.* Android properties. Tests run the D state with some scheduling progress to ensure that ABA checking prevents false triggers. If 100% reliable ABA on platform, then ro.llk.killtest can be set to false; however this will result in some of the unit tests to panic kernel instead of deal with more graceful kill operation.
The following are the Android Properties llkd respond to. prop_ms named properties are in milliseconds. Properties that use comma (,) separator for lists, use a leading separator to preserve default and add or subtract entries with (optional) plus (+) and minus (-) prefixes respectively. For these lists, the string "false" is synonymous with an empty list, and blank or missing resorts to the specified default value.
device is configured with limited memory.
device is configured for userdebug or eng build.
default not ro.config.low_ram, or ro.debuggable if property is "eng". if true do sysrq t (dump all threads).
default false, allow live-lock daemon to be enabled.
default ro.llk.enable, and evaluated for eng.
default false, allow [khungtask] daemon to be enabled.
default ro.khungtask.enable and evaluated for eng.
default false, enable call to mlockall().
default value 12 minutes, [khungtask] maximum timelimit.
default 10 minutes, D or Z maximum timelimit, double this value and it sets the alarm watchdog for llkd.
default ro.llk.timeout_ms, D maximum timelimit.
default ro.llk.timeout_ms, Z maximum timelimit.
default ro.llk.timeout_ms, checking for persistent stack symbols maximum timelimit. Only active on userdebug or eng builds.
default 2 minutes samples of threads for D or Z.
default cma_alloc,__get_user_pages,bit_wait_io,wait_on_page_bit_killable comma separated list of kernel symbols. Look for kernel stack symbols that if ever persistently present can indicate a subsystem is locked up. Beware, check does not on purpose do forward scheduling ABA except by polling every ro.llk_check_ms over the period ro.llk.stack.timeout_ms, so stack symbol should be exceptionally rare and fleeting. One must be convinced that it is virtually impossible for symbol to show up persistently in all samples of the stack. Again, looks for a match for either " symbol+0x" or " symbol.cfi+0x" in stack expansion. Only available on userdebug or eng builds, limited privileges due to security concerns on user builds prevents this checking.
default 0,1,2 (kernel, init and [kthreadd]) plus process names init,[kthreadd],[khungtaskd],lmkd,llkd,watchdogd, [watchdogd],[watchdogd/0],...,[watchdogd/get_nprocs-1]. Do not watch these processes. A process can be comm, cmdline or pid reference. NB: automated default here can be larger than the current maximum property size of 92. NB: false is a very very very unlikely process to want to blacklist.
default 0,2,adbd&[setsid] (kernel, [kthreadd] and adbd only for zombie setsid). Do not watch processes that have this parent. An ampersand (&) separator is used to specify that the parent is ignored only in combination with the target child process. Ampersand was selected because it is never part of a process name, however a setprop in the shell requires it to be escaped or quoted; init rc file where this is normally specified does not have this issue. A parent or target processes can be specified as comm, cmdline or pid reference.
default empty or false, comma separated list of uid numbers or names. Do not watch processes that match this uid.
default process names init,lmkd.llkd,llkd,keystore,ueventd,apexd,logd. This subset of processes are not monitored for live lock stack signatures. Also prevents the sepolicy violation associated with processes that block ptrace, as these can not be checked anyways. Only active on userdebug and eng builds.