Android live-lock daemon (llkd)

Android 10 includes the Android live-lock daemon (llkd), which is designed to catch and mitigate kernel deadlocks. The llkd component provides a default standalone implementation, but you can alternatively integrate the llkd code into another service, either as part of the main loop or as a separate thread.

Detection scenarios

The llkd has two detection scenarios: Persistent D or Z state, and persistent stack signature.

Persistent D or Z state

If a thread is in D (uninterruptible sleep) or Z (zombie) state with no forward progress for longer than ro.llk.timeout_ms or ro.llk.[D|Z].timeout_ms, the llkd kills the process (or parent process). If a subsequent scan shows the same process continues to exist, the llkd confirms a live-lock condition and panics the kernel in a manner that provides the most detailed bug report for the condition.

The llkd includes a self watchdog that alarms if llkd locks up; watchdog is double the expected time to flow through the mainloop and sampling is every ro.llk_sample_ms.

Persistent stack signature

For userdebug releases, the llkd can detect kernel live-locks using persistent stack signature checking. If a thread in any state except Z has a persistent listed ro.llk.stack kernel symbol that is reported for longer than ro.llk.timeout_ms or ro.llk.stack.timeout_ms, the llkd kills the process (even if there is forward scheduling progress). If a subsequent scan shows the same process continues to exist, the llkd confirms a live-lock condition and panics the kernel in a manner that provides the most detailed bug report for the condition.

The lldk check persists continuously when the live lock condition exists and looks for the composed strings symbol+0x or symbol.cfi+0x in the /proc/pid/stack file on Linux. The list of symbols is in ro.llk.stack and defaults to the comma-separated list of cma_alloc,__get_user_pages,bit_wait_io,wait_on_page_bit_killable.

Symbols should be rare and short-lived enough that on a typical system the function is seen only once in a sample over the timeout period of ro.llk.stack.timeout_ms (samples occur every ro.llk.check_ms). Due to lack of ABA protection, this is the only way to prevent a false trigger. The symbol function must appear below the function calling the lock that could contend. If the lock is below or in the symbol function, the symbol appears in all affected processes, not just the one that caused the lockup.

Coverage

The default implementation of llkd does not monitor init, [kthreadd], or [kthreadd] spawns. For the llkd to cover [kthreadd]-spawned threads:

  • Drivers must not remain in a persistent D state,

OR

  • Drivers must have mechanisms to recover the thread should it be killed externally. For example, use wait_event_interruptible() instead of wait_event().

If one of the above conditions is met, the llkd denylist can be adjusted to cover kernel components. Stack symbol checking involves an additional process denylist to prevent sepolicy violations on services that block ptrace operations.

Android properties

The llkd responds to several Android properties (listed below).

  • Properties named prop_ms are in milliseconds.
  • Properties that use comma (,) separator for lists use a leading separator to preserve the default entry, then 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 entries resort to the specified default value.

ro.config.low_ram

Device is configured with limited memory.

ro.debuggable

Device is configured for userdebug or eng build.

ro.llk.sysrq_t

If property is eng, the default is not ro.config.low_ram or ro.debuggable. If true, dump all threads (sysrq t).

ro.llk.enable

Allow live-lock daemon to be enabled. Default is false.

llk.enable

Evaluated for eng builds. Default is ro.llk.enable.

ro.khungtask.enable

Allow [khungtask] daemon to be enabled. Default is false.

khungtask.enable

Evaluated for eng builds. Default is ro.khungtask.enable.

ro.llk.mlockall

Enable call to mlockall(). Default is false.

ro.khungtask.timeout

[khungtask] maximum time limit. Default is 12 minutes.

ro.llk.timeout_ms

D or Z maximum time limit. Default is 10 minutes. Double this value to set the alarm watchdog for llkd.

ro.llk.D.timeout_ms

D maximum time limit. Default is ro.llk.timeout_ms.

ro.llk.Z.timeout_ms

Z maximum time limit. Default is ro.llk.timeout_ms.

ro.llk.stack.timeout_ms

Checks for persistent stack symbols maximum time limit. Default is ro.llk.timeout_ms. Active only on userdebug or eng builds.

ro.llk.check_ms

Samples of threads for D or Z. Default is two minutes.

ro.llk.stack

Checks for kernel stack symbols that if persistently present can indicate a subsystem is locked up. Default is cma_alloc,__get_user_pages,bit_wait_io,wait_on_page_bit_killable comma-separated list of kernel symbols. The check doesn't do forward scheduling ABA except by polling every ro.llk_check_ms over the period ro.llk.stack.timeout_ms, so stack symbols should be exceptionally rare and fleeting (it is highly unlikely for a symbol to show up persistently in all samples of the stack). Checks for a match for symbol+0x or symbol.cfi+0x in stack expansion. Available only on userdebug or eng builds; security concerns on user builds result in limited privileges that prevent this check.

ro.llk.blacklist.process

The llkd does not watch the specified processes. Default is 0,1,2 (kernel, init, and [kthreadd]) plus process names init,[kthreadd],[khungtaskd],lmkd,llkd,watchdogd, [watchdogd],[watchdogd/0],...,[watchdogd/get_nprocs-1]. A process can be a comm, cmdline, or pid reference. An automated default can be larger than the current maximum property size of 92.

ro.llk.blacklist.parent

The llkd does not watch processes that have the specified parents. Default is 0,2,adbd&[setsid] (kernel, [kthreadd], and adbd only for zombie setsid). An ampersand (&) separator specifies 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 the ampersand to be escaped or quoted, although the init rc file where this is normally specified does not have this issue. A parent or target process can be a comm, cmdline, or pid reference.

ro.llk.blacklist.uid

The llkd does not watch processes that match the specified UIDs. Comma-separated list of UIS numbers or names. Default is empty or false.

ro.llk.blacklist.process.stack

The llkd does not monitor the specified subset of processes for live lock stack signatures. Default is process names init,lmkd.llkd,llkd,keystore,ueventd,apexd,logd. Prevents the sepolicy violation associated with processes that block ptrace (as these can't be checked). Active only on userdebug and eng builds. For details on build types, refer to Building Android.

Architectural concerns

  • Properties are limited to 92 characters (however, this is ignored for defaults defined in the include/llkd.h file in the sources).
  • The built-in [khungtask] daemon is too generic and trips on driver code that sits around in D state too much. Switching to S would make tasks killable (and resurrectable by drivers if needed).

Library interface (optional)

You can optionally incorporate the llkd into another privileged daemon using the following C interface from the libllkd component:

#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 automatically spawns, otherwise the caller must call llkCheckMilliseconds in its main loop. The function returns the period of time before the next expected call to this handler.