[LLVMdev] Inspecting target-specific opcodes in machine function pass

[Peter Edelstein]

Hello,

thank you very much for answering. I am trying to do the following: get the
encoding for each instruction and if that encoding contains a C3 byte,
insert a NOP instruction (or multiple NOP instructions, or any other
instructions) before that instruction. The idea behind this is to protect
against ROP (Return Oriented Programming) attacks. By inserting a NOP the
attacker can no longer abuse alignment to get a useful gadget. I thought a
machine function pass would be sufficient to accomplish this. However, now
I realize that this was probably a rather naive thought.

Can you think of any other approaches for achieving this?

Cheers.

2015-03-08 21:21 GMT+01:00 Tim Northover <t.p.northover at gmail.com>:

> Hi,
>
> > As the comment suggests I want to inspect the target-specific opcode of
> each
> > instruction. By opcode I mean the actual machine code (=encoding of that
> in
> > struction as an array of bytes), not the integer descriptor returned by
> > I->getOpcode().
>
> That's not generally possible. The best you might be able to do would
> be to lower each MachineInstr to an MCInst and emit its encoding via
> an MCCodeEmitter. But that has quite a few problems:
>
>   + There's virtually no hope before register allocation. You'd
> probably get an assertion if you're lucky.
>   + Many targets don't have really separate "opcode" and "operand"
> bytes. Just 32-bit (say) instructions that have
>   + A MachineInstr can become 0, 1, or many real target .instructions
>   + What you get back may or may not be what's emitted finally anyway.
>   + It's slow and redundant work.
>
> What are you trying to do with the opcode? There may be a better way.
>
> Cheers.
>
> Tim.
>
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