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Are there any tools, tutorials, or resources out there for helping to make sense of NaCL binaries? They have some features that should make things easier – most crucially, fully decidable disassembly and code/data disambiguation. But looking at them in a disassembler, it's hard to make sense of what's going on because most of the interesting stuff is done by calls into the NaCL runtime, and I can't seem to find good documentation on what functions are available there or how to resolve the calls in a disassembly.

There's a lot of code like (sample from hotword-x86-64.nexe):

   21d70:       81 c4 b8 00 00 00       add    esp,0xb8
   21d76:       4c 01 fc                add    rsp,r15
   21d79:       41 5b                   pop    r11
   21d7b:       0f 1f 44 00 00          nop    DWORD PTR [rax+rax*1+0x0]
   21d80:       41 83 e3 e0             and    r11d,0xffffffe0
   21d84:       4d 01 fb                add    r11,r15
   21d87:       41 ff e3                jmp    r11

And this makes it pretty hard to follow control flow.

These binaries are of some interest because they're in some ways Google's answer to ActiveX, and they've been used to ship controversial features like OK Google (aka hotwords).

One resource I have found is a technical description of the SFI mechanism:

NaCl SFI model on x86-64 systems

Which gives some helpful hints on (e.g.) what R15 does, it's mostly about the what restrictions NaCL assembly code has to comply with, rather than how things work.

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