dyld_shared_cache

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Revision as of 05:56, 5 December 2014 by PoomSmart (talk | contribs) (Cache extraction: one more.)
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Since iPhoneOS 3.1, all default (private and public) libraries have been combined into a big cache file to improve performance in /System/Library/Caches/com.apple.dyld/dyld_shared_cache_armX, where X can be:

X Device ARM Architecture
v6 ARMv6
v7 ARMv7
v7s
64 ARMv8

The original files are no longer useful for non-on-device-developers, so they are eliminated from the system.

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

Cache extraction

Developers who do not use the SDK cannot link programs on iOS directly due to the missing dylibs. One first needs to extract the appropriate dylibs from the dyld_shared_cache. Starting with iOS 8, the framework binaries shipped with the iOS SDK only contain the symbols, not the compiled code anymore. Binaries extracted from the dyld_shared_cache contain all compiled code and are therefore useful for reverse-engineering purposes.

Options:

  • You could use dyld_decache by KennyTM~ to extract these dylibs.
  • Alternatively, you could use DySlim by comex to mount the whole cache file on Mac OS X.
  • decache by phoenixdev also works quite well.
  • dsc_extractor (source code). More info here.
  • jtool is another option if other tools fail (which seems to be common starting with iOS 8).

Example usage for jtool

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

jtool -extract UIKit path/to/dyld_shared_cache

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

jtool -lv cache_armv7 | cut -c 24- | tail +5 | while read line ; do jtool -extract $line cache_armv7 ; done

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.
  • Pull the cache off a decrypted root filesystem DMG which you can find inside the IPSW.

Class dumping

See this section of Reverse Engineering Tools.

References

  • Cache or Check? — an analysis of the dyld_shared_cache system by D. Howett.