[LLVMdev] Disassembly arbitrary machine-code byte arrays

[Aidan Steele]

Hi Kev and James,

Thanks to both of you for responding. I had looked at the otool
release published for 10.7.2 (cctools-800), but it seems that it only
snuck in after that and by the cctools-809 release!

In any case, both that and llvm-mc should be more than adequate! A
follow-up question: is the C interface to LLVM a second-class citizen
or should I reasonably be able to expect to do everything with it that
I could do as a consumer of the C++ API?

Regards,
Aidan

On Tue, Dec 20, 2011 at 7:14 AM, Kevin Enderby <enderby at apple.com> wrote:
> Hi Aiden,
>
> The 'C' based interface you could use in is llvm/include/llvm-c/Disassembler.h, which in there is:
>
> /**
>  * Disassemble a single instruction using the disassembler context specified in
>  * the parameter DC.  The bytes of the instruction are specified in the
>  * parameter Bytes, and contains at least BytesSize number of bytes.  The
>  * instruction is at the address specified by the PC parameter.  If a valid
>  * instruction can be disassembled, its string is returned indirectly in
>  * OutString whose size is specified in the parameter OutStringSize.  This
>  * function returns the number of bytes in the instruction or zero if there was
>  * no valid instruction.
>  */
> size_t LLVMDisasmInstruction(LLVMDisasmContextRef DC, uint8_t *Bytes,
>                             uint64_t BytesSize, uint64_t PC,
>                             char *OutString, size_t OutStringSize);
>
> This is used in darwin's otool(1) which is an objdump(1) like tool.  It ends up in the libLTO shared library.
>
> Kev
>
> On Dec 19, 2011, at 1:23 AM, James Molloy wrote:
>
>> Hi Aiden,
>>
>> The easiest thing I can do is to point you to the source of the "llvm-mc" tool, which does exactly what you ask in its "-disassemble" mode. The code is rather small, so it should be easy to work out.
>>
>> tools/llvm-mc
>>
>> Cheers,
>>
>> James
>>
>> -----Original Message-----
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Aidan Steele
>> Sent: 19 December 2011 04:30
>> To: llvmdev at cs.uiuc.edu
>> Subject: [LLVMdev] Disassembly arbitrary machine-code byte arrays
>>
>> Hi,
>>
>> My apologies if this appears to be a very trivial question -- I have
>> tried to solve this on my own and I am stuck. Any assistance that
>> could be provided would be immensely appreciated.
>>
>> What is the absolute bare minimum that I need to do to disassemble an
>> array of, say, ARM machine code bytes? Or an array of Thumb machine
>> code bytes? For example, I might have an array of unsigned chars --
>> how could I go about decoding these into MCInst objects? Does such a
>> decoding process take place in one fell swoop or do I parse the stream
>> one instruction at a time? Can I ask it to "decode the next 10 bytes"?
>> What follows is my (feeble) attempt at getting started. It probably
>> doesn't help that I am only familiar with C and Objective-C and find
>> C++ syntax absolutely bewildering.
>>
>> Kind regards,
>> Aidan Steele
>>
>> int main (int argc, const char *argv[])
>> {
>> LLVMInitializeARMTargetInfo();
>> LLVMInitializeARMTargetMC();
>> LLVMInitializeARMAsmParser();
>> LLVMInitializeARMDisassembler();
>>
>> const llvm::Target Target;
>>
>> llvm::OwningPtr<const llvm::MCSubtargetInfo>
>> STI(Target.createMCSubtargetInfo("", "", ""));
>> llvm::OwningPtr<const llvm::MCDisassembler>
>> disassembler(Target.createMCDisassembler(*STI));
>>
>> llvm::OwningPtr<llvm::MemoryBuffer> Buffer;
>> llvm::MemoryBuffer::getFile(llvm::StringRef("/path/to/file.bin"), Buffer);
>> llvm::MCInst Inst;
>> uint64_t Size = 0;
>>
>> disassembler->getInstruction(Inst, Size, *Buffer.take(), 0,
>> llvm::nulls(), llvm::nulls());
>>
>> //  llvm::StringRef TheArchString("arm-apple-darwin");
>> //  std::string normalized = llvm::Triple::normalize(TheArchString);
>> //
>> //  llvm::Triple TheTriple;
>> //  TheTriple.setArch(llvm::Triple::arm);
>> //  TheTriple.setOS(llvm::Triple::Darwin);
>> //  TheTriple.setVendor(llvm::Triple::Apple);
>> //  llvm::Target *TheTarget = NULL;
>>
>> return 0;
>> }
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