Dev:Dyld shared cache

Since iPhone OS 3.1, all system (private and public) libraries have been combined into a big cache file to improve performance. The original files are redundant and thus eliminated from the system.

If you're looking for binaries or libraries inside of /System/Library/Frameworks or /System/Library/PrivateFrameworks (or other directories) and can't, this is why.

As of iOS 13.5 application code is now in frameworks in the shared cache. The binaries you see in /Applications or /private/var/staged_system_apps are now just shims, so if you attempt to class-dump them it will error out as no ObjC section will be found.

OS X, along with any other *OS released by apple also uses a shared cache. Unlike iOS, OS X ships with the source binaries still on-disk, particularly so it can be updated with update_dyld_shared_cache. The cache is only vaguely documented in dyld man pages.

Extracting Frameworks and Libraries.
Since iOS 8, the SDK no longer includes extracted frameworks. The only way to obtain the libraries running on your device is via extraction.

Extracting the shared cache is useful in a few situations:


 * Linking against a framework or library not available in the public SDK
 * Using class-dump or similar tools to analyze Private Frameworks.

Until a tool capable of fixing extracted binaries is released, or unless you're able to obtain a .development cache, extracted framework may not be worth attempting to use in Reverse Engineering.

Obtaining a shared cache
The easiest method for getting a dyld_shared_cache is as such:


 * 1) Download or locate an ipsw for the target version and device
 * 2) Rename it to a .zip, extract, and locate the largest .dmg
 * 3) Mount the dmg and navigate to /System/Library/Caches/com.apple.dyld/
 * 4) Copy the dyld_shared_cache to your machine.

Other methods for cache retrieval
Since ASLR was implemented in iOS, trivial ways to pull the cache off the device have provided a "broken" cache, which can't be processed correctly by the aforementioned tools. This is because when read by processes in which ASLR is enabled, some offsetting is applied to the cache too. In order to circumvent this issue and pull a "valid" shared cache off the device, there are different options:


 * Copy the cache off the device using a program on which ASLR has been explicitly disabled, using the -mdynamic-no-pic compile flag.
 * Read the cache explicitly from the filesystem by setting the F_NOCACHE flag on the cache's file descriptor.
 * Copy the cache through AFC (filesystem browsers which use an AFC connection are fine) - on iOS 7 and 8, you'll want to install the package Apple File Conduit "2", hosted/maintained by saurik.
 * Use the copy that is probably laying around on your computer in ~/Library/Developer/Xcode/iOS\ DeviceSupport/ if you have Xcode.

Alternatively, dt.fetchsymbols can be used to extract the cache from an iOS device. This tool doesn't require file system access (jailbreak) or app installation.

Shared cache static analysis
Analyzing Private Frameworks can provide powerful insight into how iOS works, and can assist with writing better code as an extension developer.

If you're not sure what you need, and/or you don't have time to waste, you likely want Simulator Runtime frameworks.

Simulator Runtime Frameworks
The simulator runtime generated by XCode includes fully symbolicated, perfect x64 binaries for Private and Public frameworks. For most shared_cache analysis, unless you own IDA 7.5, you likely will be fine with using these. They're located in Add the folder here

XCode's Automatic Extraction
Before you go through the painful extraction process, macOS users should check `~/Library/Developer/Xcode/iOS DeviceSupport`. When you build and deploy a project via XCode, it will automatically extract the entire cache to this directory. This eliminates the need to deal with other tools.

Disassemblers capable of extraction
Owning IDA 7.5 is one of the few reasons to bother using a real cache for Reverse Engineering private frameworks. Other tools are not capable of doing so easily if at all.

IDA Pro
Common illegally distributed copies of IDA don't include these utilities, and you won't be able to use features documented here or elsewhere on a stolen copy.

IDA 7.5 includes a plethora of tools that make working with the dsc much more tolerable. It entirely eliminates the need for extraction or third-party scripts to fix output.

Details and instructions on using the integrated tools can be found on the IDA Pro page. If you own a copy, check the IDA page on this wiki for a guide on using it.

Hopper
Hopper is capable of loading singular modules from the shared cache. It does not fix references and as such produces mostly useless output.

It also allows loading the entire shared cache, but tends to crash when loading it

Ghidra
Ghidra is capable of loading the shared cache

More information is needed here

Standalone tools
Working on current iOS versions:


 * dsc_extractor (source code). More info here. It produces the best results among all tools, but without branch islands workaround. Do note this is the exact same output provided by XCode automatically.

Tools for previous iOS versions:


 * dyld_decache by KennyTM~ to extract these dylibs.
 * DySlim by comex to mount the whole cache file on Mac OS X.
 * decache by phoenixdev to nearly perfectly extract dylibs from iOS <= 6 cache file.
 * jtool is another option starting from iOS 8.
 * yasce by comex is/was the best option for iOS 8 (and above), but you will need a nightly build version of rust; something like "rustc 1.9.0-nightly (339a409bf 2016-03-01)".
 * dyld_cache_extract by macmade that works on macOS and provides a complete GUI. Clone repo and do 'git submodule update --remote' before buidling. It was reported to be not working on iOS 10.2's dyld_shared_cache_armv7s; gave a 561.1MB executable file.

Cache location
The cache is located in /System/Library/Caches/com.apple.dyld/dyld_shared_cache_armX, where X can be:

"Development" vs "Release" caches
In internal, private builds, apple often includes a second copy of the dyld_shared_cache suffixed with .development. It is located in the same folder as the regular cache and shares the same file format, but is noticeably larger in size.

This copy of the cache does not forgo the stub optimization that release builds of the cache endure. Extracting frameworks with any tool from a .developer cache can sometimes eliminate the need to rely on any tools you don't already have.

One place .development caches are present is filesystem dumps of "InternalUI" builds, one of which recently was leaked to the general public.

https://www.theiphonewiki.com/wiki/InternalUI_Builds includes an explanation of these builds and a disclaimer you should take note of before moving on.

Creation/background
The source code for the tool used at apple is here:

https://opensource.apple.com/source/dyld/dyld-733.6/dyld3/shared-cache/CacheBuilder.cpp.auto.html

Specifically linked is the file responsible for determining if a development build is being created, and if so to avoid removing stubs, presumably for speed.

Using .development caches with Dissassemblers
For a majority of programs without complex shared cache tools, the .development cache provides a breath of fresh air. Extracted frameworks produce functional assembly and readable pseudocode. If you've been struggling to find a working "dsc fixer" utility, this may be what you need.

IDA and .development caches
IDA Offers tools to deal with the lack of stubs in release caches. It then begs the question: is it worth using a .dev cache, still?

Previous versions of IDA failed to properly analyze the .dev cache, so if you are on pre 7.3, it is not possible.

Current versions have shortcomings regarding recognizing certain segments and omitting information, but DSCU can be used to load in that information. As such it's more advisable to use the .development cache if one is available.

It's clear .development caches are not the end-all solution to the optimization issue, but they certainly can be a bit of help when you're in a rush.

Example usage for dsc_extractor
Given that this is a bit of a source-code dump of an apple tool, it probably should be moved to its own page

Sorry this is still work in progress as of iOS 13.4.1 - the binary output is not usable in either Hopper or class-dump.

This tool is different in that it dumps every binary in the cache compared to other tools that allow extraction of individual binaries. We need to create a c++ file and use Apple's test program code to build an executable:

Copy and paste in the following Apple test program taken from the dyld package (look after the code for instructions on how to download it yourself).

This test program was taken from dydl 733.6. Get latest by browsing the versions here and download by swapping in a version number like:

Scroll to the bottom to find the test program code in the big comment.

Back to what we were doing, to build the file we created and install:

To use (Change the iOS version to what is in your folder, Xcode will copy the cache file from a connected device):

After a brief pause you'll see a lot of output as it dumps each binary, e.g.

And now, you can find all of the dumped binaries in the Binaries folder, the problem is the binaries aren't usable in class-dump and errors with "Cannot find offset", e.g.:

Example usage for decache
This will extract the binary of the private framework SpringBoardServices

If you get a message about an unsupported load command, ignore it. decache does not support some newer mach-o load commands, so the binary won't be able to run probably in the most cases. But for linking or reverse-engineering purposes it is still usable.

Example usage for jtool
To extract a specific binary from the cache ("UIKit" can be replaced with a different framework or library):

An example of one way to dump all the binaries at once (be careful with this, it creates huge files):

Problems with jtool
Please be aware that decache produces currently (16.04.15) better and more usable results then jtool, as jtool fails to resolve and fix the "uniqued" objectiv c selectors correctly.

Apple "uniques" objectiv c selectors, such as "alloc" (alloc is used almost everywhere), which are used in more then one place, into a single one. When extracting an image from the cache, the address of such a shared selector will most likely not be in the extracted image anymore, so this needs to fixed, which jtool apparently fails to do. (For more information: http://opensource.apple.com/source/dyld/dyld-132.13/launch-cache/update_dyld_shared_cache.cpp, look at the class ObjCSelectorUniquer)

Not working since iOS 11
jtool2 is the newer version this user reports it not working on the iOS 11 shared cache - shows a warning "File is likely truncated (or header corrupt?)" and then doesn't get past "LC_DYLD_INFO..." http://newosxbook.com/forum/viewtopic.php?f=3&t=19577&start=50#p24183 Still same error exists on the cache from iOS 13.

Class dumping
See this section of Reverse Engineering Tools.