Difference between revisions of "Dyld shared cache"

From iPhone Development Wiki
Jump to: navigation, search
(Not working since iOS 11)
(From a MacOS Device)
(28 intermediate revisions by 5 users not shown)
Line 3: Line 3:
 
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.
 
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 <tt>[http://theiphonewiki.com/wiki//System/Library/Frameworks /System/Library/Frameworks]</tt> or <tt>/System/Library/PrivateFrameworks</tt> (or other directories) and can't, this is why.
+
If you're looking for binaries or libraries inside of <tt>/System/Library/Frameworks</tt> or <tt>/System/Library/PrivateFrameworks</tt> (or other directories) and can't, this is why.
  
OS X also uses a shared cache. Unlike iOS, OS X ships with the source binaries still on-disk, particularly so it can be updated with [https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/update_dyld_shared_cache.1.html update_dyld_shared_cache]. The cache is only vaguely documented in dyld man pages.
+
As of iOS 13.5 application code is now in frameworks in the shared cache. The binaries you see in <tt>/Applications</tt> or <tt>/private/var/staged_system_apps</tt> are now just shims, so if you attempt to class-dump them it will error out as no ObjC section will be found.
  
= Cache location =
+
OS X, along with any other *OS released by apple also uses a shared cache. Unlike iOS, macOS before 11 Big Sur used to ship with the source binaries still on-disk, particularly so it can be updated with [https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/update_dyld_shared_cache.1.html update_dyld_shared_cache]. Starting with macOS 11, update_dyld_shared_cache is deprecated and, as in iOS, the only copy of the libraries is in the "cache". The cache is only vaguely documented in dyld man pages.
 +
 
 +
== Obtaining a shared cache ==
 +
 
 +
=== For an iOS Device ===
 +
 
 +
==== Using the ipsw tool ====
 +
 
 +
# Install [https://github.com/blacktop/ipsw ipsw].
 +
# Run <code>ipsw download --version [target iOS version] --device [target device model (e.g. iPhone10,3)</code>
 +
# Run <code>ipsw extract --dyld</code>
 +
 
 +
==== Manually ====
 +
 
 +
# Download or locate an .ipsw file for the target version and device 
 +
# Rename it to a .zip, extract, and locate the largest .dmg 
 +
# Mount the dmg and navigate to /System/Library/Caches/com.apple.dyld/ 
 +
# Copy the dyld_shared_cache to your machine.
 +
 
 +
=== From a MacOS Device ===
 +
 
 +
On newer machines, it is located at <code>/System/Library/dyld/</code>
 +
 
 +
Previously, it was located at <code>/usr/lib/dyld</code>
 +
 
 +
== 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.
 +
* Reverse engineering the code in private or public frameworks within iOS.
 +
 
 +
[https://github.com/arandomdev/DyldExtractor DyldExtractor] is currently (as of iOS 14) capable of extracting nearly perfect frameworks from the shared cache. However, these cannot yet be loaded by dyld, as more work needs done.
 +
 
 +
== Shared cache static analysis ==
 +
 
 +
=== Simulator Runtime Frameworks ===
 +
 
 +
''Can someone with an M1 mac comment on this?''
 +
 
 +
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 or the frameworks you are looking for are not available in simulators, you are fine with these. They're located under <code>/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot</code> (Xcode 11 and higher) or <code>/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot</code> (Xcode 10 and lower).
 +
 
 +
=== Disassemblers capable of extraction ===
 +
 
 +
Using [https://github.com/arandomdev/DyldExtractor DyldExtractor] is recommended for extraction. The only tool capable of extracting on its own is IDA Pro 7.5, and using an extractor can still be easier.
 +
 
 +
 
 +
==== BinaryNinja ====
 +
 
 +
BinaryNinja, utilizing the [https://github.com/cxnder/bn-dyldsharedcache bn-dyldsharecache] Plugin, is capable of loading images from the dyld_shared_cache. It's built on DyldExtractor, meaning the output is better than IDA's DSCU, and far easier to use.
 +
 
 +
==== IDA Pro ====
 +
 
 +
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. It's not worth your time.
 +
 
 +
==== Ghidra ====
 +
 
 +
Ghidra is capable of loading the shared cache
 +
 
 +
''More information is needed here''
 +
 
 +
There are [https://github.com/NationalSecurityAgency/ghidra/issues/682 known issues] with loading a shared cache.
 +
 
 +
=== Standalone tools ===
 +
 
 +
Working on current iOS versions:
 +
 
 +
* [https://github.com/arandomdev/DyldExtractor DyldExtractor]. Python project capable of extracting and heavily fixing up frameworks from the shared cache. This tool produces nearly perfect output on current iOS versions.
 +
 
 +
"Working":
 +
 
 +
* [http://opensource.apple.com/source/dyld/ dsc_extractor (source code)]. More info [https://gist.github.com/NSExceptional/85527151eeec4b0640187a0a165da1cd 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:
 +
 
 +
* [https://github.com/kennytm/Miscellaneous/downloads dyld_decache] by KennyTM~ to extract these dylibs.
 +
* [https://gist.github.com/455086/ DySlim] by comex to mount the whole cache file on Mac OS X.
 +
* [https://github.com/phoenix3200/decache decache] by phoenixdev to nearly perfectly extract dylibs from iOS <= 6 cache file.
 +
* [http://www.newosxbook.com/index.php?page=downloads jtool] is another option starting from iOS 8.
 +
* [https://github.com/comex/imaon2 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)".
 +
* [https://github.com/macmade/dyld_cache_extract 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 <tt>/System/Library/Caches/com.apple.dyld/dyld_shared_cache_armX</tt>, where X can be:
 
The cache is located in <tt>/System/Library/Caches/com.apple.dyld/dyld_shared_cache_armX</tt>, where X can be:
Line 33: Line 127:
 
|}
 
|}
  
= Cache extraction =
 
  
It used to be possible to compile and link from an iOS device without an SDK, but the introduction of dyld_shared_cache has made using an SDK a necessity. Alternatively, one may need to extract the appropriate dylibs from the dyld_shared_cache, if it's not available in the SDK.
+
=== Other Disassemblers ===
  
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.
+
''Information on how other disassemblers interact with these caches is needed.''
  
Options:
 
  
* [https://github.com/kennytm/Miscellaneous/downloads dyld_decache] by KennyTM~ to extract these dylibs.
+
=== Example usage for jtool ===
* [https://gist.github.com/455086/ DySlim] by comex to mount the whole cache file on Mac OS X.
 
* [https://github.com/phoenix3200/decache decache] by phoenixdev to nearly perfectly extract dylibs from iOS <= 6 cache file.
 
* [http://opensource.apple.com/source/dyld/ dsc_extractor (source code)]. More info [https://gist.github.com/NSExceptional/85527151eeec4b0640187a0a165da1cd here]. It produces the best results among all tools, but without branch islands workaround.
 
* [http://www.newosxbook.com/index.php?page=downloads jtool] is another option starting from iOS 8.
 
* [https://github.com/comex/imaon2 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)".
 
* [https://github.com/macmade/dyld_cache_extract 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.
 
 
 
== Example usage for decache ==
 
 
 
This will extract the binary of the private framework SpringBoardServices
 
 
 
<source lang=bash>
 
decache -c path/to/dyld_shared_cache -x /System/Library/PrivateFrameworks/SpringBoardServices.framework/SpringBoardServices -o SpringBoardServices
 
</source>
 
 
 
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):
 
To extract a specific binary from the cache ("UIKit" can be replaced with a different framework or library):
Line 72: Line 146:
 
</source>
 
</source>
  
=== Problems with jtool ===
+
==== 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.
 
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.
Line 78: Line 152:
 
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)
 
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 ===
+
==== 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.
 
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.
Line 100: Line 174:
 
</source>
 
</source>
  
= Cache retrieval =
+
== "Development" vs "Release" caches ==
 +
 
 +
=== 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
  
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:
+
Specifically linked is the file responsible for determining if a development build is being created, and if so to avoid removing stubs. This is likely done to make debugging issues in frameworks much less painful.
  
* Copy the cache off the device using a program on which ASLR has been explicitly disabled, using the <tt>-mdynamic-no-pic</tt> compile flag.
+
Evidence of these builds is present in dyld source (''link to a github mirror here please''), and they're likely used in internal iOS builds.
* Read the cache explicitly from the filesystem by setting the <tt>F_NOCACHE</tt> 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.
 
* Use the copy that is probably laying around on your computer in "~/Library/Developer/Xcode/iOS DeviceSupport/" if you have Xcode.
 
  
Alternatively, [https://github.com/npupyshev/dt.fetchsymbols 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.
+
== Class dumping ==
  
= Class dumping =
+
See [[Reverse_Engineering_Tools#Class/Metadata_Dumping_tools|this section of Reverse Engineering Tools]].
  
See [[Reverse_Engineering_Tools#class-dump.2C_class_dump_z.2C_classdump-dyld|this section of Reverse Engineering Tools]].
+
== External Links ==
 
= External Links =
 
  
 
* [http://blog.howett.net/?p=75 Cache or Check?] — an analysis of the dyld_shared_cache system by D. Howett.
 
* [http://blog.howett.net/?p=75 Cache or Check?] — an analysis of the dyld_shared_cache system by D. Howett.
 
* [https://github.com/deepinstinct/dsc_fix dsc_fix] — an IDA script that aids in reverse engineering dyld_shared_cache libraries
 
* [https://github.com/deepinstinct/dsc_fix dsc_fix] — an IDA script that aids in reverse engineering dyld_shared_cache libraries

Revision as of 08:33, 7 August 2022


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, macOS before 11 Big Sur used to ship with the source binaries still on-disk, particularly so it can be updated with update_dyld_shared_cache. Starting with macOS 11, update_dyld_shared_cache is deprecated and, as in iOS, the only copy of the libraries is in the "cache". The cache is only vaguely documented in dyld man pages.

Obtaining a shared cache

For an iOS Device

Using the ipsw tool

  1. Install ipsw.
  2. Run ipsw download --version [target iOS version] --device [target device model (e.g. iPhone10,3)
  3. Run ipsw extract --dyld

Manually

  1. Download or locate an .ipsw file 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.

From a MacOS Device

On newer machines, it is located at /System/Library/dyld/

Previously, it was located at /usr/lib/dyld

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.
  • Reverse engineering the code in private or public frameworks within iOS.

DyldExtractor is currently (as of iOS 14) capable of extracting nearly perfect frameworks from the shared cache. However, these cannot yet be loaded by dyld, as more work needs done.

Shared cache static analysis

Simulator Runtime Frameworks

Can someone with an M1 mac comment on this?

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 or the frameworks you are looking for are not available in simulators, you are fine with these. They're located under /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Library/Developer/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot (Xcode 11 and higher) or /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot (Xcode 10 and lower).

Disassemblers capable of extraction

Using DyldExtractor is recommended for extraction. The only tool capable of extracting on its own is IDA Pro 7.5, and using an extractor can still be easier.


BinaryNinja

BinaryNinja, utilizing the bn-dyldsharecache Plugin, is capable of loading images from the dyld_shared_cache. It's built on DyldExtractor, meaning the output is better than IDA's DSCU, and far easier to use.

IDA Pro

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. It's not worth your time.

Ghidra

Ghidra is capable of loading the shared cache

More information is needed here

There are known issues with loading a shared cache.

Standalone tools

Working on current iOS versions:

  • DyldExtractor. Python project capable of extracting and heavily fixing up frameworks from the shared cache. This tool produces nearly perfect output on current iOS versions.

"Working":

  • 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:

X Device ARM Architecture
v6 ARMv6
v7 ARMv7
v7s
v7k
64 ARMv8
64e ARMv8.3


Other Disassemblers

Information on how other disassemblers interact with these caches is needed.


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):

cache=dyld_shared_cache_arm64
mkdir -p extracted && jtool -lv $cache | cut -c 24- | tail +5 | while read line; do mkdir -p extracted/"$(dirname "$line")"; jtool -extract $line $cache; mv $cache."$(basename "$line")" extracted/$line; done

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.

$ ./jtool2 -e Stocks ./dyld_shared_cache_arm64 
Warning: File is likely truncated (or header corrupt?) Binding opcodes falls outside file
Warning: File is likely truncated (or header corrupt?) Binding opcodes falls outside file
Warning: File is likely truncated (or header corrupt?) LC_FUNCTION_STARTS falls outside file
0x176d9000-0x17731000 __TEXT   (360448 bytes)
0x2ccc2070-0x2ccd54a8 __DATA_CONST   (78904 bytes)
0x30571f60-0x30575f60 __DATA   (16384 bytes)
0x314c9a28-0x314ca000 __DATA_DIRTY   (1496 bytes)
0x316b3000-0x37064000 __LINKEDIT   (94048256 bytes)
   0x31e3c4d0-0x31e3eaf8 Exports   (9768 bytes)
   0x34689f18-0x346912f8 Symbol Table   (29664 bytes)
   0x35a46500-0x35a46b90 Function Starts   (1680 bytes)
   0x35bb1930-0x35bb1960 Data In Code   (48 bytes)
   0x35d66710-0x37029aba String Table   (19674026 bytes)
LC_DYLD_INFO...

"Development" vs "Release" caches

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. This is likely done to make debugging issues in frameworks much less painful.

Evidence of these builds is present in dyld source (link to a github mirror here please), and they're likely used in internal iOS builds.

Class dumping

See this section of Reverse Engineering Tools.

External Links

  • Cache or Check? — an analysis of the dyld_shared_cache system by D. Howett.
  • dsc_fix — an IDA script that aids in reverse engineering dyld_shared_cache libraries