Duke Systems
Android (Simplified)
Jeff Chase
Duke University
Part 1: Background
From Bloomberg
Krall and General Counsel Bruce Sewell
have amassed a team of lawyers from
inside Apple and some of the top U.S. law
firms to fight Samsung, HTC, and Google’s
Motorola Mobility unit over Google’s
Android mobile operating system and the
smartphones and tablets that run on it.
The fight is central to Apple’s identity…it
got 46.4 percent of its sales from the
iPhone last quarter and 26 percent from
the iPad. The iPhone generated $47 billion
in sales last fiscal year….
Apple and Samsung together make more
than half of the smartphones sold in the
world. Samsung is the biggest player in the
global market and Apple is dominant in the
U.S. The companies are vying for …a
market that …grew 62 percent to $219 B…
Google Chrome blamed for new MacBook Air crashes - The W...
http://www.washingtonpost.com/business/technology/google-c...
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Why new MacBook Airs crash when running
Google’s Chrome
By VentureBeat.com, Published: June 29
A spate of crashing new MacBook Air laptops running Google’s Chrome browser had the tech
conspiracists in a tizzy, what with the frenemies relationship between iPhone-maker Apple and
Android-creator Google. But the root cause of the problem turns out to be much more mundane: the
MacBook’s graphic accelerator.
“We have identified a leak of graphic resources in the Chrome browser related to the drawing of
plugins on Mac OS X,” Google told Gizmodo Thursday. The leaky resource is causing a kernel panic
in the new MacBook Air’s Intel HD 4000 graphics chip, which has the Mountain View, Calif. Internet
giant befuddled, terming the problem a potential “bug” by Apple.
While both Google and Apple investigate, the Chrome browser was updated to disable some graphics
acceleration on the MacBook Air. A more permanent fix will be pushed out “in the coming days,”
Google assures concerned MacBook Air owners.
The response to all the fuss seems to have driven Mac fans away from Chrome and more solidly
toward Apple’s Safari browser. “Chrome is great for my Dad’s 2003 vintage PC. But on my Mac it
offers no advantage…it’s just another piece of redundant software to maintain. Safari all the way,”
said a 9to5Mac user named “Tigerlilly.”
Others questioned Google placing blame on Apple for a “bug” in the new MacBook Air. Which gets
us back to the conspiracy theories. Presumably, Apple developers tested the new MacBook Air on a
wide range of software, looking for problems. The trouble with Chrome may have surfaced, but been
overlooked. After all, wouldn’t a problem with Chrome push Apple fans deeper into the arms of
Safari — as is happening now?
Copyright 2012, VentureBeat
© The Washington Post Company
“Conspiracy theory”?
Leading tech companies are filled with good
system builders with vision, strong principles,
and a commitment to technological purity.
Others in the company focus on market
strategy and tactics. By US law their
commitment to profits for the company’s
owners dominates other values.
Good system builders learn to understand the
social, legal, and market context for their
work. Technology choices are always
intertwined with market structure and strategic
concerns.
Unix, looking backward: UI+IPC
• Conceived around keystrokes and byte streams
– User-visible environment is centered on a text-based
command shell.
• Limited view of how programs interact
– files: byte streams in a shared name space
– pipes: byte streams between pairs of sibling processes
Unix, looking backward: upcalls
• Limited view of how programs interact with the OS.
– The kernel directs control flow into user process at a fixed entry
point: e.g., entry for exec() is _crt0 or “main”.
– Process may also register a signal handlers for events relating to
the process, (generally) signalled by the kernel.
– Process lives until it exits voluntarily or fails
• “receives an unhandled signal that is fatal by default”.
data
Protected
system calls
data
...and upcalls
(e.g., signals)
X Windows (1985)
Big change: GUI.
1. Windows
2. Window server
3. App events
4. Widget toolkit
Unix, looking backward: security
• Presumes multiple users sharing a machine.
• Each user has a userID.
– UserID owns all files created by all programs user runs.
– Any program can access any file owned by userID.
• Each user trusts all programs it chooses to run.
– We “deputize” every program.
– Some deputies get confused.
– Result: decades of confused deputy security problems.
• Contrary view: give programs the privileges they
need, and nothing more.
– Principle of Least Privilege
Confused deputy
Mal wants the power. Can
Mal trick Bob to get it?
Bob has the Power. Bob
wishes to hold the power
and use it properly.
Alice considers Bob her deputy in the use of this
Power. Alice trusts Bob to deny the power to Mal.
http://finntrack.co.uk/, erights.org
http://www.cap-lore.com/CapTheory/ConfusedDeputyM.html
Android protection
• Each application (“app”) runs with its own identity.
– Each app has a private space of files, processes, etc. that
defines a “sandbox”.
– It does not matter that they run on behalf of the same user:
the code matters more than the user. No deputies!
• The system mediates access to the sensors and UI
by applications.
– GPS, camera, microphone, touchpad, etc.
• Each app declares the named permissions it needs.
– subject to user approval
• Each app declares the permissions another app
needs to interact with it.
Part 2, Android technology
• The goal here is to present an overview of the
Android structure, and rationale for that structure.
• We won’t learn how to program Android apps.
Detailed docs are available.
– developer.android.com
• What we offer is the conceptual underpinnings and
connections to other OS concepts.
– We choose Android for its instructive value: open source.
• Some images are borrowed from material on the
web. E.g., Google material:
– Anatomy and Physiology of Android
Virtual
Machine
(JVM)
C/C++
[http://www.android.com]
Dalvik JVM (Interpreter)
Dalvik interprets
Java bytecode.
Bytecode is an abstract
machine instruction set.
Java source compiles
to bytecode.
Android apps
compile to bytecode.
Dalvik has various
optimizations for the
Android platform.
Android: components
• Apps declare typed components.
– metadata list of components (manifest)
Component
intents, RPC
• Components have upcall
interfaces visible to system.
• System instantiates and destroys
components driven by events in
the system and UI.
• System upcalls components to
notify them of lifecycle events.
• Apps may interact by typed
messages among components.
– events (intents)
– object invocation (binder RPC)
upcalls
System
App
Components run in contexts
context
(app process)
• Components are Java code.
– A component is a class in an app.
instances
– Its name is (appname, classname).
– Apps are named as packages.
• Components run in JVM contexts.
– Each component runs at most one
instance in exactly one context.
JVM+lib
component
launch
(activate)
– Context is a process with a JVM and a
trusted system library.
– Android library defines context object with
system API.
app
Apps are isolated
app “sandbox”
• Components in the same app
generally share a context.
app
files
• Components in different apps are
always in different contexts.
• Apps cannot reference each
other’s memory contexts:
– “soft” JVM protection
context
JVM+lib
– hard process boundaries
• Apps interact only via IPC.
– intents, events
– service RPC and content put/get
• Apps run with distinct user IDs.
– Principle of Least Privilege
app
Component launch
app context
• To launch a component:
– Select a JVM context to run it.
– Tell JVM to instantiate the class.
• System communicates with the
context via its system library.
• System obtains info about the
component from app manifest.
launch
System
– Class  component type: the
component class descends from a
system base class.
– List of event profiles (intent filters)
that trigger component launch.
read
manifest
If there is no JVM context active for the
component’s app, then the system must start one.
JVM+lib
load
class
manifest
app
App launch
Zygote is a preinitialized “warm”
JVM image for unborn children.
Zygote
Activity
start
Manager
JVM+lib
Service
etc.
fork
children
JVM+lib
setuid to
app uid
JVM+lib
Linux kernel
How do we launch the application’s code? Exec?
App launch
Zygote
Activity
Manager
Service
etc.
forked child
context
JVM+lib
launch
JVM+lib
open
read
App files
Linux kernel
No exec needed: all Android contexts run the same
Linux program: the JVM. Fork is just right!
Binder: object RPC channels
Activity
Manager
Service
etc.
Services
register to
advertise for
clients.
JVM+lib
Bindings are
reference-counted.
A client binds
to a service.
JVM+lib
Android binder
an add-on kernel driver
for binder RPC
Linux kernel
Android services and libraries communicate by sending
messages through shared-memory channels set up by binder.
Android environment server
Activity
Manager
Service
etc.
JVM+lib
JVM+lib
The Activity Manager maintains a binding to every app context.
Apps call system APIs and receive events via binder RPC calls
to/from Android Activity Manager etc.
Post-note
• Zygote also forks a system service manager on
system startup.
• SM is a process that forks a lot of the basic Java
android services.
• SM looks for installed apps with binder services in
the manifest, and starts those components.
- They include daemons that listen to usb, audio etc.
and send events
Deactivating components and apps
Activity
Manager
Service
etc.
JVM+lib
X
If a service has no
bound clients, the
system may
deactivate it.
JVM+lib
The Activity Manager decides when to deactivate
components and tear down app contexts.
Deactivating components and apps
Activity
Manager
Service
etc.
JVM+lib
If an app has no
active components,
the system may
deactivate it.
X
JVM+lib
The Activity Manager decides when to deactivate
components and tear down app contexts.
Deactivating components and apps
Activity
Manager
Service
etc.
JVM+lib
If an app has no
active components,
the system may
deactivate it.
X
JVM+lib
The Activity Manager decides when to deactivate
components and tear down app contexts.
Deactivating components and apps
Activity
Manager
Service
etc.
JVM+lib
The user navigates with
the screen and buttons,
activating components
and moving on.
The components set up interactions among
themselves as needed to serve the user.
The system monitors activity and memory pressure and
cleans up behind components as needed.
The four component types
1. Activity. Display a screen.
–
Push on a “back stack”.
–
May be launched by other apps.
2. Service. Serve an API.
–
Establish an external binder interface.
–
Public methods are externally visible.
3. Provider. Get/put content objects.
–
Serve a URI space with MIME types.
–
Backed by SQLite database tables.
4. Receiver. Respond to events.
–
E.g., low battery.
Intents for activities and receivers
• Intents are named events.
– Components signal intents with various attributes and data.
– They declare filters to specify which intents they may receive.
– Filter specifies named permissions the sender must have.
• A component may invoke an activity with an explicit
intent, which invokes a named target component.
• A component may broadcast an implicit intent for
delivery to any interested receiver component.
– Sender names permissions that each receiver must have.
– The event may be sent to its receivers in order or in parallel.
• See also: implicit vs. explicit invocation in Garlan/Shaw.
– Explicit intents and ordered broadcasts may receive a result.
Post-note
• We did not discuss the state diagrams on the
following slides in any detail.
• But understand that each executing
component is a finite state machine.
– States are defined by the component type.
– Transitions are driven by UI events and/or other
system or app events.
– These events generate system upcalls or intents
to the component, which change its state.
– Components in certain states are eligible to be
reclaimed by the system.
Activity
• System upcalls
component as its
state changes due to
user actions.
• If another activity is
started, the activity is
paused.
• If a paused activity is
not visible to the
user, it is stopped.
• A stopped activity
may be destroyed.
• And its app process
may be killed.
Saving/restoring activity state
Service
• Services advertise one
or more binder
endpoints.
• Clients choose to
bind/unbind (or unbind
when stopped).
• A service with no bound
clients may be shut
down.
Service
For later
• Threading models and concurrency
– Each app has a main thread (activity thread) that controls its
UI and invokes the upcalls of its components as they are
needed. Apps must never block the activity thread.
– Components can create other threads in various ways.
• Binder/RPC structure
– Service threading/queue models and request handling
– RPC data translation
• Permission structure
– An extensible namespace of permissions whose meaning is
defined by system or by apps. For any interaction, both
components define the permissions needed by the other.
A note on “subsystems”
• A subsystem is a server that provides system
functions to untrusting contexts, but runs with only
partial system privilege.
– E.g., this code cannot manipulate the hardware state except
by invoking the kernel.
• Android AMS manipulates contexts.
• With no special kernel support! It
uses same syscalls as anyone else.
• Unix provides no syscalls for
managing another context (just kill).
• AMS controls user contexts by
forking them with a trusted lib, and
issuing RPC commands to that lib.
Android
AMS
subsystem
Linux
kernel
JVM+lib
binder