Android IPC Mechanism

Android IPC Mechanism

Jim Huang ( 黃敬群 )
Developer, 0xlab
jserv@0xlab.org

March 19, 2012 / 南台科大

Rights to copy
© Copyright 2012 0xlab
http://0xlab.org/
contact@0xlab.org
Attribution – ShareAlike 3.0
You are free Corrections, suggestions, contributions and
translations are welcome!
to copy, distribute, display, and perform the work
to make derivative works Latest update: Mar 21, 2012
to make commercial use of the work
Under the following conditions
Attribution. You must give the original author credit.
Share Alike. If you alter, transform, or build upon this work, you may distribute the
resulting work only under a license identical to this one.
For any reuse or distribution, you must make clear to others the license terms of this
work.
Any of these conditions can be waived if you get permission from the copyright holder.
Your fair use and other rights are in no way affected by the above.
License text: http://creativecommons.org/licenses/by-sa/3.0/legalcode

Agenda (0) IPC: The heart of Android
(1) Design Patterns
(2) Binder IPC Internals
(3) Use case: Graphics

Binder IPC: The heart of Android

Android Tasks

Process A Process B

Task Activity Activity

Content
Activity
Provider

.apk package
Service

.apk package

5

• Different component types Component View
– Activity
– Service
– Content Provider
– Broadcast Receiver

IPC = Inter-Process Communication

Activity
Activity Window Alarm
Manager Manager Manager

Kernel

Why IPC?

• Each process has its own address space
• Provides data isolation
• Prevents harmful direct interaction between two
different processes
– Sometimes, communication between processes is
required for modularization

IPC Mechanisms

• In GNU/Linux
– Signal
– Pipe
– Socket
– Semaphore
– Message queue
– Shared memory
• In Android
– Binder: lightweight RPC (Remote Procedure
Communication) mechanism

10

Binder History
• Developed under the name OpenBinder by Palm Inc.
under the leadership of Dianne Hackborn
• Android Binder is the customized re-implementation
of OpenBinder, which provides bindings to functions
and data from one execution environment to another

Background Problems

• Applications and Services may run in separate
processes but must communicate and share data
• IPC can introduce significant processing overhead
andsecurity holes

12

Binder: Android's Solution

• Driver to facilitate inter-process communication
• High performance through shared memory
• Per-process thread pool for processing requests
• Reference counting, and mapping of object
references across processes
• Synchronous calls between processes
“In the Android platform, the binder is used for
nearly everything that happens across processes
in the core platform. " – Dianne Hackborn
https://lkml.org/lkml/2009/6/25/3

IPC Abstraction

More abstract
Intent • Intent
– The highest level abstraction
AIDL • Inter process method invocation
– AIDL: Android Interface
Binder Definition Language
• binder: kernel driver
• ashmem: shared memory

Method invocation

caller

callee

In the same process

Inter-process method invocation

caller
interface

caller

interface
How?
callee

interface

callee

Inter-process method invocation

caller
interface

Proxy
caller

interface Binder in kernel

Binder Thread
callee
Stub
interface

callee

Pattern
• Abstracts and names a recurring AbstractService
design structure service

• Comprises class and/or object Client

– Dependencies
Proxy Service
– Structures service
1 1
service
The Proxy Pattern
– Interactions
– Conventions
• Specifies the design structure
explicitly
• is distilled from actual design
experience
• Android itself follows object oriented
design

Design Patterns used in Binder
(incomplete)

• Proxy Pattern
• Mediator Pattern
• Bridge Pattern

Proxy Pattern

• The proxy could interface to anything: a network
connection, a large object in memory, a file, or some
other resource that is expensive or impossible to
duplicate.

21

Proxy Pattern in Android
• Binder decomposes the method call and all its corresponding data to
a level that Linux can understand, transmitting it from the local
process and address space to the remote process and address
space, and reassembling and reenacting the call there.

22

Mediator Pattern

• With the mediator pattern, communication between
objects is encapsulated with a mediator object.

23

Bridge Pattern

• decouple an abstraction from its implementation so
that the two can vary independently

24

Bridge and Mediator Pattern in Android

Bridge patterns in linking
Java and C++

Mediator pattern

25

UML Representation

<<interface>>

implements

Proxy Stub

UML Representation

caller

<<interface>> calls

implements

Proxy Stub

extends

callee

AIDL

Auto generated from .aidl file caller

<<interface>>

Proxy Stub

callee

Use Case:
Who calls onPause() in Activity?
2
Send message
by Handler Activity

3
OnPause() is
queue Looper called in main thread

Activity
Manager Main
Binder Thread
Thread #1

Kernel 1

Call ”schedulePauseActivity”
across process

IPC Interaction in Android
(Application View)

3 parts:
• BnXXX: native
2 call interface 1 getService • BpXXX: proxy
• Client
Invoke BpXXX

Framework

Binder in Action

Process A Process B

Binder Terminology

• Binder
• Binder Object
– an instance of a class that implements the Binder interface.
– One Binder object can implement multiple Binders

• Binder Protocol
• IBinder Interface
– is a well-defined set of methods, properties and events that
a Binder can implement.

• Binder Token
– A numeric value that uniquely identifies a Binder

Facilities

• Simple inter process messaging system
• Managing
• Identifying
• Calls
• Notification
• Binder as a security access token

• Binder framework provides more than a simple
interprocess messaging system.
• Methods on remote objects can be called as if they
where local object methods.

Communication protocol

If one process sends data to another process, it is called transaction.
The data is called transaction data.

Service Manager (SM)
• Special Binder node with known Binder address
• Client does not know the address of remote Binder
– only Binder interface knows its own address
• Binder submits a name and its Binder token to SM
– Client retrieves Binder address with service name from
SM

Get Service list from SM
$ adb shell service list
Found 71 services:
0 stub_isms: [com.android.internal.telephony.ISms]
1 stub_phone: [com.android.internal.telephony.ITelephony]
2 stub_iphonesubinfo:
[com.android.internal.telephony.IPhoneSubInfo]
..
5 stub_telephony.registry:
[com.android.internal.telephony.ITelephonyRegistry]
...
7 stub_activity: [android.app.IActivityManager]
...
9 phone: [com.android.internal.telephony.ITelephony]
…
56 activity: [android.app.IActivityManager]
...
64 SurfaceFlinger: [android.ui.ISurfaceComposer]
...

Call remote method in ActivityManager
$ adb shell service list
...
56 activity: [android.app.IActivityManager]
...
$ adb service call activity 1598968902
Result: Parcel(
0x00000000: 0000001c 006e0061 00720064 0069006f '....a.n.d.r.o.i.'
0x00000010: 002e0064 00700061 002e0070 00410049 'd...a.p.p...I.A.'
0x00000020: 00740063 00760069 00740069 004d0079 'c.t.i.v.i.t.y.M.'
0x00000030: 006e0061 00670061 00720065 00000000 'a.n.a.g.e.r.....')

public abstract interface IBinder {
...
field public static final int INTERFACE_TRANSACTION
= 1598968902; // 0x5f4e5446
…
} Source: frameworks/base/api/current.txt

Interact with Android Service
$ adb service call phone 1 s16 "123"
Result: Parcel(00000000 '....')

interface ITelephony {
/* Dial a number. This doesn't place the call. It displays
* the Dialer screen. */ Source: frameworks/base/
void dial(String number); telephony/java/com/android/internal/telephony/ITelephony.aidl
service call SERVICE CODE [i32 INT | s16 STR] …

Options:

i32: Write the integer INT into the send parcel.

s16: Write the UTF-16 string STR into the send parcel.

$ adb shell service list Phone Application appears in foreground.
Found 71 services: parameter “1” → dial()
s16 "123" → String("123")
...
9 phone: [com.android.internal.telephony.ITelephony]

Implementation Layers of Binder

Implemented in Java

Implemented in C++

Implemented in C

API Layer
• AIDL (Android Interface Definition
Language)
– Ease the implementation of
Android remote services
– Defines an interface with method
of remote services
– AIDL parser generates Java class
• Proxy class for Client
• Stub class for Service
• Java API Wrapper
Introduce facilities to the binder
– Wraps the middleware layer

AIDL

• Data Types
– Java Primitives
– Containers
• String, List, Map, CharSequence
• List<>
• Multidimensional Array
– Parcelable
– Interface Reference
• Direction - in, out, inout
• oneway
– android.os.IBinder.FLAG_ONEWAY
43

AIDL Compiler
• Full-fledged Java(-only) Support
• Stub and Proxy Generator
// Interface
interface IRemoteService {
void ping();
}

IRemoteService mService = Client
IRemoteService.Stub.asInterface(service);

public class RemoteService extends Service {
public IBinder onBind(Intent intent) { return mBinder; }
private final IRemoteService.Stub mBinder =
new IRemoteService.Stub() {
public void ping() { // Nothing }
};
} Server
44

Parcels and Marshalling

• Simple inter process messaging system
• In an object oriented view, the transaction data is
called parcel.
• The procedure of building a parcel is called
marshalling an object.
• The procedure of rebuilding a object from a parcel is
called unmarshalling an object.

Parcel

• Marshalling – The transferring of data across
process boundaries
– Represented in native binary encoding
• Mostly handled by AIDL-generated code
• Extensible – Parcelable

47

android.os.Parcel

Delivering arguments of method

”flatten” ”unflatten”

transmit

Parcel Definition
• Container for a message (data and object references) that
can be sent through an IBinder.
• A Parcel can contain both
flattened data that will be
unflattened on the other side of
the IPC (using the various
methods here for writing specific
types, or the general Parcelable
interface), and references to live
IBinder objects that will result in
the other side receiving a proxy
IBinder connected with the
original IBinder in the Parcel.

Representation of Parcel

• Parcel is not for general-purpose serialization
– This class (and the corresponding Parcelable API
for placing arbitrary objects into a Parcel) is
designed as a high-performance IPC transport.
– Not appropriate to place any Parcel data into
persistent storage
• Functions for writing/reading primitive data types:
– writeByte(byte) / readByte()
– writeDouble(double) / readDouble()
– writeFloat(float) / readFloat()
– writeInt(int) / readInt()
– writeLong(long) / readLong()
– writeString(String) / readString()

Parcelable

• The Parcelable protocol provides an extremely
efficient (but low-level) protocol for objects to write
and read themselves from Parcels.
• Use the direct methods to write/read
– writeParcelable(Parcelable, int)
readParcelable(ClassLoader)
– writeParcelableArray(T[],int)
readParcelableArray(ClassLoader)

• These methods write both the class type and its data
to the Parcel, allowing that class to be reconstructed
from the appropriate class loader when later reading.

Bundles

• A special type-safe container, called Bundle, is
available for key/value maps of heterogeneous
values.
• This has many optimizations for improved
performance when reading and writing data, and its
type-safe API avoids difficult to debug type errors
when finally marshalling the data contents into a
Parcel.

52

Middleware Layer

• Implements the user space facilities
of the Binder framework in C++
• Implements structures and methods
to spawn and manage new threads
• Marshalling and unmarshalling of
specific data
• Provides interaction with the Binder
kernel driver

• frameworks/base/include/binder/IServiceManager.h
sp<IServiceManager> defaultServiceManager()
• frameworks/base/include/binder/IInterface.h
template BpInterface

Kernel Driver Layer
• Binder Driver supports the file
operations open, mmap, release, poll
and the system call ioctl
• ioctl arguments
– Binder driver command code
– Data buffer

• Command codes
– BINDER_WRITE_READ
– BINDER_SET_MAX_THREADS
– BINDER_SET_CONTEXT_MGR
– BINDER_THREAD_EXIT
– BINDER_VERSION
55

Binder Driver

• Multi-thread aware
– Have internal status per thead
– Compare to UNIX socket: sockets have internal
status per file descriptor (FD)

Binder Driver

• A pool of threads is associated to each service application to process
incoming IPC
• Binder performs mapping of object between two processes.
• Binder uses an object reference as an address in a process’s
memory space.
• Synchronous call, reference counting

Binder is different from UNIX socket

socket binder
internal status associated to FD associated to PID
(FD can be shared among
threads in the same
process)

read & write stream I/O done at once by
operation ioctl

network Yes No
transparency expected local only

$ adb cat /sys/devices/virtual/misc/binder/uevent
MAJOR=10 Binder
MINOR=47
DEVNAME=binder

from SM to Binder Driver

Client Service Manager
service list 1
Server
IXXX
Name:Handle onTransact(…)
2 Name:Handle thread pool
Handle=0 Name:Handle

transact(…)

3 4 5 User Space

Kernel Space

Binder Driver: /dev/binder

memory mapping

60

Transaction
BR → BinderDriverReturnProtocol
BC → BinderDriverCommandProtocol

binder_write_read
write_size write buffer
write_consumed
write_buffer
read_size
read_consumed read buffer
read_buffer

if (ioctl(fd, BINDER_WRITE_READ, &bwt ) >= 0)
err = NO_ERROR;
else
err = -errno;

Transaction of Binder
Process A and B have different memory space.
They can not see each other.
Kernel Process B
Process A
Binder

Copy memory by copy_from _user
Then, wake up process B

Kernel
Process B
Binder

Process A
Copy memory by copy_to_user

Internally, Android uses Binder for graphics data transaction across processes.
It is fairly efficient.

Limitation of Binder IPC

• Binders are used to to communicate over process
boundaries since different processes don't share a
common VM context
– no more direct access to each others Objects
(memory).
• Binders are not ideal for transferring large data
streams (like audio/video) since every object has to
be converted to (and back from) a Parcel.

63

Binder Performance

• Good
– Compact method index
– Native binary marshalling
– Support of ashmem shortcut
– No GUID
• Bad
– Dalvik Parcel overhead
– ioctl() path is not optimal
– Interface name overhead
– Global lock
64

Binder Security

• Binder’s Security Features
– Securely Determined Client Identity
• Binder.getCallingUid(), Binder.getCallingPid()
• Similar to Unix Domain Socket
getsockopt(..., SO_PEERCRED, ...)
– Interface Reference Security
• Client cannot guess Interface Reference
• Service Manager
– Directory Service for System Services
• Server should check client permission
Context.checkPermission(permission, pid, uid)

65

Binder sample program

• Build binder benchmark program
cd system/extras/tests/binder/benchmarks
mm
adb push
../../../../out/target/product/crespo/data/nativebenchmark/binderAddInts
/data/local/
• Execute
adb shell
su
/data/local/binderAddInts -d 5 -n 5 &
ps
...
root 17133 16754 4568 860 ffffffff 400e6284 S
/data/local/binderAddInts
root 17135 17133 2520 616 00000000 400e5cb0 R

Binder sample program

• Execute
/data/local/binderAddInts -d 5 -n 5 &
ps
...
root 17133 16754 4568 860 ffffffff 400e6284 S
root 17135 17133 2520 616 00000000 400e5cb0 R
cat /sys/kernel/debug/binder/transaction_log
transaction_log:3439847: call from 17133:17133 to 72:0 node
1 handle 0 size 124:4
transaction_log:3439850: reply from 72:72 to 17133:17133 node
0 handle 0 size 4:0
transaction_log:3439855: call from 17135:17135 to 17133:0
node 3439848 handle 1 size 8:0
...

Binder sysfs entries

• adb shell ls /sys/kernel/debug/binder
failed_transaction_log
proc
state
stats
transaction_log
transactions

• Target Method
– handle : Remote Interface Binder Transaction
– ptr & cookie : Local Interface
– code : Method ID
• Parcel - Input/Output Parameters
– data.ptr.buffer
– data_size
• Object Reference Management
– data.ptr.offsets
– offsets_size
• Security
– sender_pid
– sender_euid
• No Transaction GUID
– Transparent Recursion

Service Registration and Discovery
• System service is executed by IServiceManager::addService() calls.
– Parameter: handle to Binder Driver
• Look up the name of specific service in Binder Driver Map
– IServiceManager::getService() returns the handle of the found registered
services
• android.os.IBinder.INTERFACE_TRANSACTION: the actual name

Binder use case: Android Graphics

Real Case

Binder IPC is used for communicating between Graphics client and server.
Taken from http://www.cnblogs.com/xl19862005/archive/2011/11/17/2215363.html

Surface

Source: frameworks/base/core/java/android/view/Surface.java
• /* Handle on to a raw buffer that is being
managed by the screen compositor */
public class Surface implements Parcelable {
public Surface() {
mCanvas = new CompatibleCanvas();
}
private class CompatibleCanvas
extends Canvas { /* ... */ }
}
Surface instances can be written to and restored from a Parcel.
Surface instances can be written to and restored from a Parcel.

Delivering arguments of method

”flatten” ”unflatten”

transmit

 Properties
Android SurfaceFlinger
 Can combine 2D/3D surfaces and surfaces from multiple applications
 Surfaces passed as buffers via Binder IPC calls
 Can use OpenGL ES and 2D hardware accelerator for its compositions
 Double-buffering using page-flip

Everything is
Everything is
around Binder
around Binder

Camera + SurfaceFlinger + Binder

Reference

• Inter-process communication of Android, Tetsuyuki
Kobayashi
• 淺談 Android 系統進程間通信（ IPC ）機制 Binder 中
的 Server 和 Client 獲得 Service Manager 接口之路
http://blog.goggb.com/?post=1580
• Android Binder – Android Interprocess
Communication, Thorsten Schreiber
• Design Patterns in the Android Framework, Prof.
Sheng-De Wang

http://0xlab.org

Android IPC Mechanism

Recommended

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Android IPC Mechanism (20)

More from National Cheng Kung University (20)

Recently uploaded (20)

Android IPC Mechanism