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dyld-shared-cache-extractor

★ 583 Open GitHub ↗

A CLI for extracting libraries from Apple's dyld shared cache file

// repository documentation

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# dyld-shared-cache-extractor

[As of macOS Big Sur][mjtsai], instead of shipping the system libraries
with macOS, Apple ships a generated cache of all built in dynamic
libraries and excludes the originals. This tool allows you to extract
these libraries from the cache for reverse engineering.

## Usage

Extract the default shared cache to `/tmp/libraries`:

```sh
dyld-shared-cache-extractor \
   /System/Volumes/Preboot/Cryptexes/OS/System/Library/dyld/dyld_shared_cache_arm64e \
   /tmp/libraries
```

If this fails it could be because the shared cache format has changed,
and the version you're trying to extract isn't supported by the version
of Xcode you have selected globally (which you can view with
`xcode-select -p` and `xcodebuild -version`). In this case you might
have to download a newer version of Xcode (potentially a beta version if
you're trying to extract the cache from a beta OS version) and override
the Xcode version when running `dyld-shared-cache-extractor`:

```sh
DEVELOPER_DIR=/Applications/Xcode-beta.app \
   dyld-shared-cache-extractor \
   /System/Volumes/Preboot/Cryptexes/OS/System/Library/dyld/dyld_shared_cache_arm64e \
   /tmp/libraries
```

If you want to prefer the system installation of `dsc_extractor.bundle`
instead of Xcode's version, you can pass it manually on the command
line:

```sh
dyld-shared-cache-extractor \
   /System/Volumes/Preboot/Cryptexes/OS/System/Library/dyld/dyld_shared_cache_arm64e \
   /tmp/libraries \
   /usr/lib/dsc_extractor.bundle
```

On macOS versions before Ventura the shared cache was in a different
location, you can extract on older macOS versions with:

```sh
dyld-shared-cache-extractor \
   /System/Library/dyld/dyld_shared_cache_arm64e \
   /tmp/libraries
```

## Installation

[Homebrew](https://brew.sh):

```sh
brew install keith/formulae/dyld-shared-cache-extractor
```

Manually:

```sh
cmake -B build
cmake --build build
cmake --install build
```

## More details

There are a few different ways you can interact with these shared
caches.

1. Depending on what you're doing inspecting them in [Hopper][hopper] is
   the easiest option
2. For a bit more functionality you can build the
   `dyld_shared_cache_util` target from the latest `dyld` [source
   dump][dump], but this requires some [modifications][modifications]

The problem with the 2 options above is that they can lag behind format
changes in the shared cache. This tool loads the private
`dsc_extractor.bundle` from Xcode, meaning it should always be able to
extract the shared cache files even from beta OS versions (potentially
using a beta Xcode version).

This logic is based on the function at the bottom of
`dyld3/shared-cache/dsc_extractor.cpp` from the `dyld` [source
dump][dump].

[dump]: https://opensource.apple.com
[hopper]: https://www.hopperapp.com
[mjtsai]: https://mjtsai.com/blog/2020/06/26/reverse-engineering-macos-11-0
[modifications]: https://lapcatsoftware.com/articles/bigsur.html

dyld-shared-cache-extractor

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