Analysis of the Facebook.app for iOS

Analysis of the Facebook.app for iOS

Posted Oct 18, 2016

Did you ever wonder why the Facebook.app for iOS is such a big download? This post tries to give some answers. The version 66.0 (released on 7 October 2016) was analyzed on an iPad Air 2 (64-bit).

Here is what you see when downloading Facebook on an iPad Air 2:

App content

A scan of the content of the Facebook app using GrandPerspective gives already a good overview:





As you can see more than 100 MB are taken by the binary itself. The images (Assets.car), the localizations and other resources are only responsible for a small part of the size.

One note: The Facebook app seems to use the open source JSC framework. Unless there is a technical reason to use it, Facebook could remove this framework and use instead the JavaScriptCore framework built in iOS. JavaScriptCore is available on iOS 7 and later and Facebook targets iOS 8 and later.

Slicing

One important point to note is that the Facebook binary I downloaded only contains 64-bit code. There is no 32-bit code. Facebook uses the Slicing mechanism which is described by Apple here:

Entropy

Let’s look at the Facebook binary. The entropy graph generated by Hopper gives a good idea of the content:



The red part (with biggest entropy) is actual code while the green and orange parts are other sections (strings, constants, …). Just by looking at the entropy graph, we know that around 50% of the binary is code.

Sections

By looking at the sections of the binary we indeed see that the

(__TEXT, __text)

section is more than 50 MB. Here is the list of sections sorted by size:

(__TEXT, __text): 56.60 MB
(__DATA, __objc_const): 16.57 MB
(__RODATA, __cstring): 5.02 MB
(__RODATA, __objc_methname): 4.55 MB
(__DATA, __const): 4.46 MB
(__RODATA, __gcc_except_tab): 3.87 MB
(__RODATA, __objc_methtype): 2.66 MB
(__DATA, __objc_data): 2.41 MB
(__DATA, __cfstring): 2.20 MB
(__DATA, __data): 1.66 MB
(__TEXT, __unwind_info): 1.58 MB
(__RODATA, __objc_classname): 1.42 MB
(__RODATA, __const): 1.40 MB
(__DATA, __bss): 0.89 MB
(__DATA, __objc_selrefs): 0.80 MB
(__DATA, __common): 0.54 MB
(__DATA, __objc_ivar): 0.38 MB
(__DATA, __objc_classlist): 0.24 MB
(__DATA, __objc_classrefs): 0.18 MB
(__DATA, __objc_superrefs): 0.13 MB
(__TEXT, __const): 0.07 MB
(__DATA, __objc_protolist): 0.06 MB
(__TEXT, __stubs): 0.02 MB
(__TEXT, __stub_helper): 0.02 MB
(__DATA, __la_symbol_ptr): 0.01 MB
(__DATA, __got): 0.01 MB
(__TEXT, __ustring): 0.01 MB
(__DATA, fbsessiongks): 0.01 MB
(__TEXT, __eh_frame): 0.01 MB
(__DATA, __mod_init_func): 0.00 MB
(__DATA, __objc_protorefs): 0.00 MB
(__DATA, __objc_catlist): 0.00 MB
(__DATA, __objc_nlclslist): 0.00 MB
(__DATA, FBInjectable): 0.00 MB
(__DATA, __mod_term_func): 0.00 MB
(__DATA, __objc_imageinfo): 0.00 MB

__RODATA segment

If you read carefully the list of sections, you did notice that the Facebook.app has a

__RODATA

segment containing sections generally found inside the

__TEXT

segment. The reason for this oddity is to work around an App Review limitation. As you can read on the App Review page:


Facebook targets iOS 8.0 and later and as such its

__TEXT

segment should not exceed 60 MB. But if you sum up the

__TEXT

and

__RODATA

segments, you end up with more than 77 MB:

Total size of the __TEXT sections: 58.3 MB
Total size of the __RODATA sections: 18.9 MB

Facebook avoids this limitation by moving some if the

__TEXT

sections into the read only

__RODATA

segment. Implementing this trick is really simple: you just need to add a linker flag to rename the chosen sections. And it appears you need absolutely nothing at runtime: the renamed sections will be found automatically. This linker flag is described in the ld man page:


You could use it to rename the

(__TEXT, __cstring)

section to

(__RODATA, __cstring)

by simply adding this line into the Other Linker Flags (OTHER_LDFLAGS):

-Wl,-rename_section,__TEXT,__cstring,__RODATA,__cstring

You can download a sample macOS Xcode project here. If you load the original binary (without the section rename) using MachOView, you will see the

(__TEXT, __cstring)

section:





After adding the linker flag and recompiling, a

(__RODATA, __cstring)

section will be visible and the binary will run as expected:





Note that this workaround is only needed as long as Facebook supports iOS 8. When targeting iOS 9 or later, the limitation is 400 MB which should be enough without splitting the

__TEXT

segment.

Third party libraries

So what does the code contain? Sadly I could not find a license/readme file containing the list of third party libraries. Also the Info view in Settings.app > Facebook > Settings > Infos is desperately empty - this seems like a Facebook.app bug.

By looking at the class names, I could build a list of third party libraries used. This list is most likely incomplete but hopefully correct:

AsyncDisplayKit: http://asyncdisplaykit.org

keeps even the most complex user interfaces smooth and responsive

Bolts-ObjC: https://github.com/BoltsFramework/Bolts-ObjC

Bolts is a collection of low-level libraries designed to make developing mobile apps easier

CocoaAsyncSocket: https://github.com/robbiehanson/CocoaAsyncSocket

Asynchronous socket networking library for Mac and iOS

CocoaLumberjack: https://github.com/CocoaLumberjack/CocoaLumberjack

A fast & simple, yet powerful & flexible logging framework for Mac and iOS

ComponentKit: http://componentkit.org

ComponentKit takes a functional, declarative approach to building UI

EGODatabase: https://github.com/enormego/egodatabase

EGODatabase is a thread-safe Objective-C SQLite wrapper with full support for asynchronous SQLite calls as well as built in NSOperationQueue support

FLAnimatedImage: https://github.com/Flipboard/FLAnimatedImage

Performant animated GIF engine for iOS

FXBlurView: https://github.com/nicklockwood/FXBlurView

UIView subclass that replicates the iOS 7 realtime background blur effect, but works on iOS 5 and above

Google Toolbox for Mac: https://github.com/google/google-toolbox-for-mac

Google Toolbox for Mac

JSONKit: https://github.com/johnezang/JSONKit

Objective-C JSON

libphonenumber: https://github.com/iziz/libPhoneNumber-iOS

iOS port from libphonenumber

OAuth2Client: https://github.com/nxtbgthng/OAuth2Client

Client library for OAuth2

OCMock: http://ocmock.org

Mock objects for Objective-C

pop: https://github.com/facebook/pop

An extensible iOS and OS X animation library, useful for physics-based interactions.

nighthawk-webrtc: https://github.com/ceaglest/nighthawk-webrtc

Custom builds of WebRTC for iOS

React Native: https://facebook.github.io/react-native/

Build Native Mobile Apps using JavaScript and React

ZipArchive: https://github.com/ZipArchive/ZipArchive

ZipArchive is a simple utility class for zipping and unzipping files on iOS and Mac

ZipZap: https://github.com/pixelglow/ZipZap

zip file I/O library for Mac OS X and iOS

Objective-C awards

The awards for the best Objective-C classes is attributed to:

NTNativeTemplateHackyWorkaroundBecauseNSMapTableIsBuggyContainer

The award for the longest Objective-C method is attributed to:

- (void)[FBComposerCompositionStateMutation matchBootstrap:
activityAttachmentUpdatedMutation:
activitySharingComposition:
albumDescription:
albumTitle:
appProvidedHashtag:
audienceMutation:
audienceTouchedByUser:
author:
clearContent:
containerPublishTargetForMediaCollection:
composerCancelAnalytics:
defaultAudienceMutation:
destinationMutation:
feedOnlyToggleState:
hashtags:
inspirationMutation:
isImplicitLocationExplicitlyBlockedByUser:
lifeEventDateMutation:
lifeEventRelationshipUpdated:
lifeEventTitleOrIconUpdated:
linkShareMutation:
shareOriginalPostMutation:
removeMediaAttachmentWithAssetID:
replaceAllExistingMediaAttachments:
addSingleMediaAttachment:
replaceSingleExistingMediaAttachmentIfExisting:
mediaAttachmentCaptionMutation:
mediaAttachmentMentionsMutation:
mediaReorderingMutation:
mentions:
moviesComposition:
moviesLoadedTaggedUsers:
photoAutoTaggingState:
photoTaggingAssetMutation:
placeMutation:
placeSuggestions:
placeTaggingState:
platformLoadedActionMutation:
platformLoadedAppAttribution:
platformLoadedImageAttachments:
platformLoadedOGMediaAttachmentsMutation:
platformLoadedPlaceMutation:
platformLoadedRef:
platformLoadedRobotext:
platformLoadedTaggedUsers:
platformLoadedVideoAttachment:
postContentTypeMutation:
postPromptPayload:
publishAsQuestionAndAnswerSession:
togglePublishAsQuestionAndAnswerSession:
preferredMarketplaceMutation:
productItemCategoryGraphQLID:
productItemPickupDeliveryInfo:
productItemPriceMutation:
productItemShouldPostToMarketplace:
productItemSuggestedPlace:
productItemSuggestedZipCode:
productItemTitle:
publishTarget:
remoteImageAttachments:
removeSponsor:
removeStorylineAttachment:
removeStickerAttachment:
replaceStorylineAttachmentWithMediaAttachments:
selectedStarRating:
shouldCreateProfileBadge:
statusTextMutation:
stickers:
taggableActivityComposition:
targeting:
withTagsMutation:
poll:
sponsor:
videoAttachmentMutation:
videoTaggingAssetMutation:
videoTaggingAnalytics:
boostedComponentData:
albumTracker:]

Conclusion

The Facebook app is a complex app and not just a simple web view. It contains a lot of third party libraries and even splits its

__TEXT

segment in order to not reach the 60 MB limit imposed by Apple.

Update 10.04.2017:

There is a newer blog post about the version 87.0: Analysis of the Facebook.app for iOS [v. 87.0]https://blog.timac.org/2017/0410-analysis-of-the-facebook-app-for-ios-v-87-0
https://blog.timac.org/2016/1018-analysis-of-the-facebook-app-for-ios/