Android Internals Part 1

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Android “OS” Internals
Prabhaker Mateti
A first glimpse of Android Internals
Android Version History
• 2008 1.0 Beta; 1.5 Cupcake; 1.6 Donut, Sep 2009
• 2009 2.0/2.1 (Éclair); revamped UI, introduced HTML5, W3C Geolocation
API and Exchange ActiveSync 2.5 support
• 2010 2.2 (Froyo), May 2010; speed improvements with JIT optimization
and the Chrome V8 JavaScript engine, added Wi-Fi tethering
• 2010 2.3 (Gingerbread), Dec 2010; refined the UI, improved keyboard and
copy/paste features, Near Field Communication
• 2011 3.0/3.1 (Honeycomb), Feb 2011; a tablet-oriented release
• 2011 4.0 (Ice Cream Sandwich)
• 2012 4.1 Jelly Bean (API level 16)
• 2012 4.2 Jelly Bean (API level 17)
• 2013 4.3 Jelly Bean (API level 18)
• 2013 4.4 KitKat (API level 19)
• http://en.wikipedia.org/wiki/Android_version_history
Mateti
Android Internals
2
Android System
• Open software platform for mobile devices
• A complete stack – OS, Middleware,
Applications
• An Open Handset Alliance (OHA) project
• Powered by Linux OS
• Application development mostly in Java
• Open source under the Apache 2 license
Mateti
Android Internals
3
Device Characteristics
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–
CPU: ARM 500-2600 Mhz; recently Intel Atom
RAM available to an App may only be a few MB
“Disk” (flash) access is slow cf to HDD
Lifecycle: Apps must pause/quit often, and restore to
give the illusion that they are always running
– UI design
•
•
•
•
screen may be HVGA (320x480) to 1920x1080 to …
may be in portrait (h > w) or landscape (w > h)
high DPI -- small text may not be readable
touch resolution is low (~25 pixel)
– Network access may be slow and intermittent
Mateti
Android Internals
4
Mateti
Android Internals
Std kernel parts not shown
5
Linux OS Inside
•
•
•
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Linux Kernel Works as a HAL
Linux/Android Device drivers
Linux Memory management
Linux Process management
Linux Networking
Kernel from the Linux FOSS project
Mateti
Android Internals
6
Android Runtime
Mateti
Android Internals
7
Android Java
• Java syntax is the same. But, not all libs are
included.
• Unused: Swing, AWT, SWT, lcdgui
• Android Java = Java SE – AWT/Swing
+ Android API
Mateti
Android Internals
8
Dalvik Virtual Machine
• Dalvik VM is a new JVM by Google
– Register-based versus stack-based JVM
– Different set of Java libraries than JDK
• Dalvik VM has been optimized for mobile
devices
– not so powerful CPU
– memory shortage
– Dalvik Executable .dex format is compact
– run multiple VMs efficiently.
Mateti
Android Internals
9
Dalvik Virtual Machine (Contd)
• Can have JIT enabled
• Relying on the Linux Kernel for:
– Threading
– Low-level memory management
• Projects for making JRuby, Groovy,
and Scala first class languages for Android.
Mateti
Android Internals
10
Art Virtual Machine
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•
•
•
Android Run Time (ART) libart.so
Replaces Dalvik libdvm.so (starting with 4.5?)
Faster And Battery improvements
https://source.android.com/devices/tech/dalv
ik/art.html
Mateti
Android Internals
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Libraries
• Surface Manager: A compositing window
manager similar to Compiz. Instead of drawing
directly to the screen buffer, drawing commands
go into off-screen bitmaps that are then
combined with other bitmaps to form the
display the user sees. Can create see-through
windows, fancy transitions, …
• 2D and 3D graphics: Use 3D hardware or a
software renderer. OpenGL.
• Media codecs: AAC, AVC (H.264), H.263, MP3,
MPEG-4, …
Mateti
Android Internals
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Libraries
• Browser engine:
– WebKit library for rendering web pages
– the same engine used in KDE, the Google
Chrome browser, Apple’s Safari browser, the
iPhone, and Symbian 60.
Mateti
Android Internals
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Mateti
Android Internals
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OpenGL ES
• OpenGL ES is a subset of OpenGL graphics standard.
• OpenGL ES is a … low-level interface between software
and graphics acceleration. OpenGL ES includes profiles
for floating-point and fixed-point systems and the
EGL™ specification ….
• OpenGL ES 1.X is for fixed function hardware and offers
acceleration, image quality and performance.
• OpenGL ES 2.X enables full programmable 3D graphics.
• http://www.khronos.org/opengles/
Mateti
Android Internals
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SQLite
• SQLite database engine
– Provides persistent storage.
– Also used in Firefox and the iPhone.
– android.database.sqlite
• Application would use to manage its own
private database.
• /system/xbin/sqlite3
Mateti
Android Internals
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Background: What is a program?
•
•
•
•
(Will add more details based on feedback.)
Precise def will be based on OS.
Do NOT use “program” and “process” interchangeably.
A program is a file
– Executable permissions
– Structure of content rigidly defined by an executable formats
•
•
•
•
•
Linux: ELF, a.out, coff
Windows: com, exe
Java: .class files
Android: .dex
Program v Object code files
– generated by a linker
– On Linux, /usr/bin/ld (historically misnamed)
– The compiler/IDE tool chain invokes the linker
•
APK file includes
– the .dex file
– along with other files describing resources.
•
“App” is an alternate term for a program
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Android Internals
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Background: What is a process?
• Process is a run-time volatile entity created by
an OS system call exec
• Processes have a virtual memory foot print.
– Code (machine instructions)
– Run time stack content
– Run time heap content
– Run time global variables
• Subject to paging and swapping
• Android details are more complex cf. Linux
Mateti
Android Internals
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Selected root Processes
• The following examples are typical
• % ps | wc –l was 220
• root
root
root
root
root
root
root
1 /init
1835 /system/bin/vold
1838 /system/bin/netd
1839 /system/bin/debuggerd
1840 /system/bin/sh
1848 zygote
2479 kcryptd
• URL ps-full-list.txt
Mateti
Android Internals
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Selected system+ Processes
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system 1834 /system/bin/servicemanager
system 1847 /system/bin/surfaceflinger
gps
1855 /system/bin/gpsd
media_rw 1880 /system/bin/sdcard
system 2775 com.sec.android.inputmethod
system 2824 com.sec.android.app.snotebook
wifi
3420 /system/bin/wpa_supplicant
dhcp
3533 /system/bin/dhcpcd
radio
2798 com.android.phone
Mateti
Android Internals
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Selected user Processes
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u0_a126
2656 com.android.systemui
u0_a16
2909 com.google.process.gapps
u0_a6
3110 android.process.acore
u0_a16
3162 com.google.process.location
u0_a6
3857 com.android.contacts
u0_a101
3906 com.sec.phone
u0_a77
4979 com.android.vending
u0_a203
5535 org.mozilla.firefox
u0_a236
5723 com.twitter.android
u0_a162
7604 com.kk.launcher
u0_a189
8461 com.devexpert.weather
u0_a112
12143 com.sec.android.app.music
u0_a58
12199 com.samsung.music
u0_a226
12230 com.android.chrome
u0_a25
29235 android.process.media
First column is user names
Mateti
Android Internals
21
File System
• ext3, ext4 of Linux
• Mount points
– One for system, one for the
apps, and one for
whatever.
• Each app has its own
sandbox accessible to it.
No one else can access its
data.
• /sdard
• /mnt/extSdCard
Mateti
Android Internals
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Partitions
• Example: Samsung T679
– ARMv7 (v7l)
– fdisk -l /dev/ block/ mmcblk0
– lists 37 partitions.
/dev/block/mmcblk0p15
/system
/dev/block/mmcblk0p16
/cache
/dev/block/mmcblk0p
/data
/dev/block/vold/179:33
/storage/sdcard1
/dev/block/vold/179:28
/storage/sdcard0
Mateti
Android Internals
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[email protected]:/ # df
Filesystem
/dev
/sys/fs/cgroup
/mnt/asec
/mnt/obb
/mnt/fuse
/system
/cache
/data
/persist
/firmware
/mnt/shell/emulated
Size
916.3M
916.3M
916.3M
916.3M
916.3M
826.8M
551.2M
5.7G
15.7M
64.0M
5.7G
Used
128.0K
12.0K
0.0K
0.0K
0.0K
713.1M
10.1M
3.9G
4.1M
44.4M
3.9G
Free
916.2M
916.3M
916.3M
916.3M
916.3M
113.7M
541.1M
1.8G
11.6M
19.5M
1.8G
Blksize
4096
4096
4096
4096
4096
4096
4096
4096
4096
16384
4096
(My rooted Nexus 4, May 2014)
Mateti
Android Internals
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Commands
• /system/bin
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Mateti
• /system/xbin
mount, swap, top, adb
blkid, bootanimation
backuptool.sh
bugreport
chmod, chown
du, e2fsck, fsck.exfat
gdbserver, grep, gzip
iptables, kill
ssh*, top, ps
Android Internals
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busybox
crond
dd, df, fdisk, tune2fs
nanddump
nslookup
nice
pidof, pkill, pwd
strace, su, sync, sha1sum
zip
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ls –l /proc/1 (trimmed)
dr-xr-xr-x
-r--r--r-lrwxrwxrwx
-r-------lrwxrwxrwx
dr-x-----dr-x------r--------r--r--r--rw-r--r--r--r--r--rw-------r--r--r--r--r--r--r-------dr-xr-xr-x
dr-x--x--x
-r--r--r--r--r--r-lrwxrwxrwx
-r--r--r--r--r--r--r--r--r--r--r--r--r--r--r--r--r--r-dr-xr-xr-x
-r--r--r--
Mateti
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Android Internals
attr
cmdline
cwd -> /
environ
exe -> /init
fd
fdinfo
io
limits
loginuid
maps
mem
mountinfo
mounts
mountstats
net
ns
pagemap
personality
root -> /
sessionid
smaps
stack
stat
statm
status
task
wchan
26
Android Hardware Abstraction (HAL)
• https://source.android.
com/devices/reference/
files.html
• Linux originated
–
–
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–
Mateti
• Mfr specific details
abstracted out
– All cameras, GPS, …
• Example /dev entries
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–
/dev/zero, /dev/null
/dev/random
/dev/input/*
/dev/tty
/dev/kmem
/sys/dev/block
Android Internals
Video
msm_camera
msm_dsps
msm_rotator
msm_vidc_dec
wcnss_wlan
27
IPC Mechanism in Android
• In GNU/Linux
– Pipes
– Shared Memory
– Message Queue
• In Android
– Binder
Mateti
Android Internals
28
Binder
• A kernel driver to facilitate inter-process
communication
• Lightweight RPC (Remote Procedure
Communication) mechanism
• Per-process thread pool for processing
requests
• Synchronous communication between
processes
Mateti
Android Internals
29
App Runtime Service
Mateti
Android Internals
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Binder IPC
Mateti
Android Internals
31
IPC over Binder
• Binder Driver supports the file operations open,
mmap, release, poll and ioctl
• an application opens the Binder kernel module
– root 10, 14 2014-05-11 22:43 /dev/binder
• This associates a file descriptor with that thread
• The kernel module uses the descriptor to identify
the initiators and recipients of Binder IPCs.
Mateti
Android Internals
32
Why Binder over conventional IPC
• Binder has additional features that sockets
don't have.
– E.g binder allows passing file descriptors across
processes.
• Pipes cannot perform RPC.
• Object reference counting, Object mapping.
• Binder has elaborate data referencing policies,
it is not a simplistic kernel driver.
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Android Internals
33
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Android Internals
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Mateti
Android Internals
35
Application Security
• Each Android
application
–
–
–
–
–
own Linux process.
own userid.
own sandbox file system
own set of preferences
own database.
• Other applications
cannot access any of its
data, unless it is
explicitly shared.
Mateti
• finer-grained security
features through a
"permission"
mechanism
• per-URI permissions for
granting ad-hoc access
• More later
Android Internals
36
/system/etc/permissions/…
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android.hardware.camera.front.xml
android.hardware.sensor.gyroscope.xml
android.hardware.telephony.gsm.xml
android.hardware.usb.host.xml
android.hardware.wifi.xml
com.cyanogenmod.android.xml
features.xml
platform.xml
Mateti
Android Internals
37
How to Explore Android Internals
• Install a “terminal” app. If your device is
rooted, you can change things. (We will
discuss “root” later.)
• adb shell
• Install an ssh server on the Android device,
and from Linux ssh into it.
– Highly useful.
– E.g., filezilla sftp client invoked on Linux
– Some devices already have /system/bin/sshd
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Android Internals
38
References
• Karim Yaghmour, Embedded Android book
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Android Internals
39
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