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I disagree that LLVM bitcode makes reverse-engineering any easier on iOS. You can do everything in bitcode that you can do in native ARM code. The limitations a

I disagree that LLVM bitcode makes reverse-engineering any easier on iOS. You can do everything in bitcode that you can do in native ARM code. The limitations are already there in terms of code signing, sandboxing, and page protection and bitcode distribution doesn't change any of these OS attributes. Are you concerned about Apple reversing your code? You can still use arbitrary types (e.g. int64 instead of a pointer) and give them no additional data about the structure of your code. As you pointed out elsewhere, tools like OLLVM perform obfuscation at the bitcode level already. I'm guessing Apple made this change to make it easier to do program analysis of iOS apps being published. You can certainly find bugs easier in bitcode that has proper type information and clear differentiation between safe and unsafe branches. One of the first steps in program analysis tools is to lift native code to an IR in order to determine its correctness. With bitcode distribution, Apple has potentially made it easier on themselves to skip the lifting and type recovery steps for conforming apps in order to look for bugs. But unless they start requiring bitcode conform to certain additional standards, you can always transform bitcode to obfuscated bitcode, destroy type information via aliasing, etc.

nekitamo 11y ago
I concede that LLVM bitcode can be obfuscated just like machine code ( with enough effort, and LLVM bugs aside ) . However, you're still losing the ability to self-checksum your code, which in turn means you can't protect against things like piracy or unwanted modification of your binary. I agree also that Apple probably made this change to better analyze programs being submitted to the app store. That and to recompile programs to use intrinsics more efficiently, as new intrinsics become available. And finally, while I would not be concerned about Apple reversing my code, certain companies are and have to undertake steps to make that as difficult as possible.
- NateLawson 11y ago
  Yes, self-checksumming becomes impossible [1]. If you are concerned about anyone reversing your code (Apple or otherwise), you must obfuscate it. Either you work at the ARM level or the bitcode level (watchOS for now), but the basic techniques are the same. Or, you can avoid changing tools by doing source-level transformation. One of the companies probably impacted by this change is Arxan or other obfuscators. They have to change both front and backend to be LLVM -> LLVM. [1] Except for the case I mentioned before, where you predict the generated code for known architectures and use that to generate your constants. This still breaks if Apple generates code for a new architecture without your help.

nekitamo 11y ago

I concede that LLVM bitcode can be obfuscated just like machine code ( with enough effort, and LLVM bugs aside ) . However, you're still losing the ability to self-checksum your code, which in turn means you can't protect against things like piracy or unwanted modification of your binary. I agree also that Apple probably made this change to better analyze programs being submitted to the app store. That and to recompile programs to use intrinsics more efficiently, as new intrinsics become available. And finally, while I would not be concerned about Apple reversing my code, certain companies are and have to undertake steps to make that as difficult as possible.

NateLawson 11y ago
Yes, self-checksumming becomes impossible [1]. If you are concerned about anyone reversing your code (Apple or otherwise), you must obfuscate it. Either you work at the ARM level or the bitcode level (watchOS for now), but the basic techniques are the same. Or, you can avoid changing tools by doing source-level transformation. One of the companies probably impacted by this change is Arxan or other obfuscators. They have to change both front and backend to be LLVM -> LLVM. [1] Except for the case I mentioned before, where you predict the generated code for known architectures and use that to generate your constants. This still breaks if Apple generates code for a new architecture without your help.