Android internals 04 - “Androdized” kernel, Bionic, Logging subsystem (rev_1.1)

Android internals
Egor Elizarov
SPbSU 2012

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2 Egor Elizarov SPbSU 2012

Lecture 4

“Androidized” kernel,
Bionic C library,
Logging system

yegor.yelizarov(at)gmail.com
Rev: 1.1
Last update: 05/30/2012 http://vk.com/android_internals


Previous time

Makefile structure

Android.mk structure

Build system stages

Steps to add new module

Emulator; Qemu; KVM; ADB

Target file system structure


Kernel types


Monolithic kernels

Microkernels

Hybrid kernels

Nanokernels

Exokernels


Linux kernel


First release 5 October 1991

Vanilla – repository of Linus Torvalds

Monolithic kernel with loadable modules
support

Languages: C, ASM


Linux drivers


Code in drivers/ directory

Can be build-in or modules (*.ko)

Devices usually have corresponding element
in /dev directory

Network devices doesn't have /dev/* elements


“Androidized” kernel


Based on Vanilla

Not in mainline until 3.x kernels

Usually board specific

Have android features


System calls


SysFS & DevFS


Android / kernel interaction


System calls (Ex: Bionic routines)

SysFS (Ex: Power management)

DevFS (Ex: Volume deamon)

ProcFS (Ex: Debug system)

Netlink (Ex: Event hub)


Wakelocks

Early suspend (Go to suspend as often as
possible)

/sys/power/wake_lock, /sys/power/wake_unlock

Could not suspend if wakelock occupied

Wake locks have labels (Ex:
PowerManagerService)


Low memory killer

 Behind Linux Out Of Memory logic

Allow to set different OOM thresholds for different kinds
of processes

Set up user-space policies during init
 Policies are adjusted in runtime by Activity Manager
 kernel/drivers/staging/android/lowmemorykiller.c


Binder


RPC/IPC akin to COM

Derived from OpenBinder project

Transmit “parcels” between processes

/dev/binder & /proc/binder

Code: kernel/drivers/staging/android/binder.c


Anonymous SHared MEMory

Derived from POSIX SHM part of SysV IPC

Named memory block that is shared between
processes

Have reference counting to destroy memory
regions

Shrink memory regions if system has lack of
memory


Alarm

Based on Linux High-Resolution Timer & Real-
Time Clock

Allow set system time and adjust alarms

Use RTC to exit suspend and HRT to get precise
alarm

/dev/alarm & /dev/rtc/ & /sys/class/rtc


Logger


Light weight logger

Use three ring buffer queues inside kernel

No file writes or context switching operations

/dev/log/*

Code: kernel/drivers/staging/android/logger.c


More android features


Paranoid networking

Ram console

Physical memory (pmem)

Timed output / timed gpio

etc.


Getting & building kernel
 git clone https://android.googlesource.com/kernel/goldfish.git
 cd goldfish
 git checkout -b android-goldfish-2.6.29 origin/android-goldfish-
2.6.29
 make goldfish_defconfig
 make menuconfig
 make ARCH=arm
CROSS_COMPILE=$ANDROID_ROOT/prebuilt/linux-
x86/toolchain/arm-eabi-4.4.3/bin/arm-eabi- uImage -j4


Bionic


Small LibC implementation

Partially POSIX compatible

BSD license

Support for x86, ARM, ARM thumb


Bionic features


Not compatible with GNU libc

Fast pthread implementation (based on futexes)

Access to property values

User/group management


Not in Bionic


Wide chars

C++ exceptions (limited implementation since
NDKr5)

C++ STL

Full pthread


Adding syscall


./bionic/libc/README.TXT

Find syscall number in kernel (unistd.h)

Add line to ./bionic/libc/SYSCALLS.TXT

Run bionic/libc/tools/gensyscalls.py

Add wrapper to bionic if needed


Logging system


Logging in code

Kernel: printk()


Java: Log.*, SLog.*, EventLog.*


Logging in code (2)


Native code: LOG* (LOGE)

Define for native code: LOG_TAG,
LOG_NDEBUG


Obtaining logs


dmesg

logcat

adb logcat


Next time


Dalvik VM

Android application live
cycle

Source: http://android-developers.blogspot.com/


Useful links
 http://vk.com/android_internals

 K. Yaghmour. Embedded Android. Early Release, O'Reilly, October 2011

 http://wiki.osdev.org/Kernels

 http://www.kernel.org/

 http://cs736-android.pbworks.com/w/page/5834465/ASHMEM

 http://elinux.org/Android_Kernel_Features

 R. Love. Linux kernel development. 3rd edition, Addison-Westley, June 2010

 http://source.android.com/source/building-kernels.html

 $ANDROID_ROOT/bionic/libc/README

 http://codingrelic.geekhold.com/2008/11/six-million-dollar-libc.html


Thanks to


Sergey Matyukevich for review and advices
(www.linkedin.com/pub/sergey-matyukevich/31/889/769)


Nikolay F. Fominykh for review and advices


Nikita Shulga for advices and notes
(http://www.linkedin.com/pub/nikita-shulga/8/582/287)


Grigory Tolstolytkin for advices and notes
(http://www.linkedin.com/pub/grigory-
tolstolytkin/2a/b41/74)


Android internals 04 - “Androdized” kernel, Bionic, Logging subsystem (rev_1.1)

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