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Objective-C disassembly on macOS Big Sur

June 25 2020 by Jeff Johnson
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This is a follow-up to my blog post yesterday Extract the system libraries on macOS Big Sur, in which I explained how to extract the system libraries from the dyld shared cache. Although you can successfully disassemble these extracted libraries, there's still a problem: the otool command-line tool fails to understand many Objective-C references in the disassembly. Let's take a look at an example from my favorite framework, AppKit. The following is from the implementation of the -[NSApplication init] method. In order to call [super init], the implementation has to get the NSApplication Objective-C class. For convenience I use my own command-line tool riptool, which is a wrapper around otool that resolves rip-relative addresses.

00007fff235f7284	movq	0x5f789a0d(%rip) [0x7fff82d80c98], %rax ## Objc class ref: bad class ref
00007fff235f728b	movq	%rax, 0x8(%rdi)

Oh no, "bad class ref", that's bad! Compare with AppKit from macOS 10.14.6 Mojave (still my primary system):

0000000000003eb0	movq	0x10d3b11(%rip) [0x10d79c8], %rax ## Objc class ref: NSApplication
0000000000003eb7	movq	%rax, 0x8(%rdi)

On Mojave, the address 0x10d79c8 is in the __objc_superrefs section, as otool -l /System/Library/Frameworks/AppKit.framework/AppKit shows.

  sectname __objc_superrefs
   segname __DATA
      addr 0x00000000010d77f0
      size 0x00000000000032a0

If we look at that section with otool -s __DATA __objc_superrefs /System/Library/Frameworks/AppKit.framework/AppKit we see this:

00000000010d79c0	68 d1 0e 01 00 00 00 00 98 be 0f 01 00 00 00 00

The address 0x10d79c8 has the little-endian value 0x10fbe98. And nm -n /System/Library/Frameworks/AppKit.framework/AppKit confirms that this is a pointer to the NSApplication Objective-C class:

00000000010fbe98 S _OBJC_CLASS_$_NSApplication

This class is in the __objc_data section:

  sectname __objc_data
   segname __DATA
      addr 0x00000000010eb980
      size 0x0000000000022d80

That's what we see on Mojave. What about on Big Sur? The address 0x7fff82d80c98, the "bad class ref", is also in the __objc_superrefs section, like on Mojave:

  sectname __objc_superrefs
   segname __DATA
      addr 0x00007fff82d80aa8
      size 0x0000000000003480

Here's what the section looks like on Big Sur:

00007fff82d80c98	f8 5b da 62 00 02 00 00 60 d6 d9 62 00 02 00 00

The address 0x7fff82d80c98 has little-endian value 0x62da5bf8. But that's not a direct pointer to the NSApplication Objective-C class:

00007fff82da5bf8 S _OBJC_CLASS_$_NSApplication

Still, the class is in the __objc_data section:

  sectname __objc_data
   segname __DATA
      addr 0x00007fff82d96310
      size 0x0000000000024040

Doing the math 00007fff82da5bf8 - 0x62da5bf8 = 0x7FFF20000000. That's an interesting address, because it's not even in AppKit! If you do dyld_shared_cache_util -list -vmaddr you find:

0x7FFF23450000 /System/Library/Frameworks/AppKit.framework/Versions/C/AppKit

My theory is that 0x7FFF20000000 is the address of the beginning of the dyld shared cache. Here's the first library in the list:

0x7FFF20040000 /System/Library/Accounts/Notification/INDAccountNotificationPlugin.bundle/Contents/MacOS/INDAccountNotificationPlugin

I don't know why it's offset by 0x40000 from the beginning, but close enough! It seems that prior to Big Sur, Objective-C references in a Mach-O file are offsets from the beginning on the file, whereas on Big Sur, Objective-C references in a Mach-O file are offsets from the beginning of the dyld shared cache. Roughly speaking.

I'm sharing this information with the hope that it helps to create better tools for disassembling system libraries on macOS Big Sur. It's all very preliminary, so anything I say is subject to revision or correction and cannot be used against me in a court of law or public opinion.

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