Android internalsEgor ElizarovSPbSU 2012

Egor Elizarov SPbSU 20122

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Egor Elizarov SPbSU 20123

Lecture 4

“Androidized” kernel,

Bionic C library,

Logging system

yegor.yelizarov(at)gmail.com

http://vk.com/android_internalsRev: 1.1Last update: 05/30/2012

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Previous time

Makefile structure

Android.mk structure

Build system stages

Steps to add new module

Emulator; Qemu; KVM; ADB

Target file system structure

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Kernel types

Monolithic kernels

Microkernels

Hybrid kernels

Nanokernels

Exokernels

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Linux kernel

First release 5 October 1991

Vanilla – repository of Linus Torvalds

Monolithic kernel with loadable modules support

Languages: C, ASM

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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

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“Androidized” kernel

Based on Vanilla

Not in mainline until 3.x kernels

Usually board specific

Have android features

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System calls

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SysFS & DevFS

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Android / kernel interaction

System calls (Ex: Bionic routines)

SysFS (Ex: Power management)

DevFS (Ex: Volume deamon)

ProcFS (Ex: Debug system)

Netlink (Ex: Event hub)

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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)

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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

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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

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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

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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

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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

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More android features

Paranoid networking

Ram console

Physical memory (pmem)

Timed output / timed gpio

etc.

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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

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Bionic

Small LibC implementation

Partially POSIX compatible

BSD license

Support for x86, ARM, ARM thumb

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Bionic features

Not compatible with GNU libc

Fast pthread implementation (based on futexes)

Access to property values

User/group management

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Not in Bionic

Wide chars

C++ exceptions (limited implementation since NDKr5)

C++ STL

Full pthread

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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

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Logging system

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Logging in code

Kernel: printk()

Java: Log.*, SLog.*, EventLog.*

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Logging in code (2)

Native code: LOG* (LOGE)

Define for native code: LOG_TAG, LOG_NDEBUG

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Obtaining logs

dmesg

logcat

adb logcat

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Next time

Dalvik VM

Android application live cycle

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

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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

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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)