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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">There has been previous NOP insertion work, for example:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><a href="http://reviews.llvm.org/D3392">http://reviews.llvm.org/D3392</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvmdev-bounces@cs.uiuc.edu [mailto:llvmdev-bounces@cs.uiuc.edu]
<b>On Behalf Of </b>Peter Edelstein<br>
<b>Sent:</b> Sunday, March 08, 2015 1:57 PM<br>
<b>To:</b> Tim Northover<br>
<b>Cc:</b> LLVM Developers Mailing List<br>
<b>Subject:</b> Re: [LLVMdev] Inspecting target-specific opcodes in machine function pass<o:p></o:p></span></p>
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<p class="MsoNormal">by the way, everything mentioned in the last mail is of course X86 specific, because that is what I am currently targeting. <o:p></o:p></p>
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<p class="MsoNormal">Cheers<o:p></o:p></p>
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<p class="MsoNormal">2015-03-08 21:49 GMT+01:00 Peter Edelstein <<a href="mailto:peter.c.edelstein@gmail.com" target="_blank">peter.c.edelstein@gmail.com</a>>:<o:p></o:p></p>
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<p class="MsoNormal">Hello,<o:p></o:p></p>
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<p class="MsoNormal">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. <o:p></o:p></p>
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<p class="MsoNormal">Can you think of any other approaches for achieving this?<o:p></o:p></p>
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<p class="MsoNormal">Cheers.<o:p></o:p></p>
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<p class="MsoNormal">2015-03-08 21:21 GMT+01:00 Tim Northover <<a href="mailto:t.p.northover@gmail.com" target="_blank">t.p.northover@gmail.com</a>>:<o:p></o:p></p>
<p class="MsoNormal">Hi,<br>
<br>
> As the comment suggests I want to inspect the target-specific opcode of each<br>
> instruction. By opcode I mean the actual machine code (=encoding of that in<br>
> struction as an array of bytes), not the integer descriptor returned by<br>
> I->getOpcode().<br>
<br>
That's not generally possible. The best you might be able to do would<br>
be to lower each MachineInstr to an MCInst and emit its encoding via<br>
an MCCodeEmitter. But that has quite a few problems:<br>
<br>
+ There's virtually no hope before register allocation. You'd<br>
probably get an assertion if you're lucky.<br>
+ Many targets don't have really separate "opcode" and "operand"<br>
bytes. Just 32-bit (say) instructions that have<br>
+ A MachineInstr can become 0, 1, or many real target .instructions<br>
+ What you get back may or may not be what's emitted finally anyway.<br>
+ It's slow and redundant work.<br>
<br>
What are you trying to do with the opcode? There may be a better way.<br>
<br>
Cheers.<br>
<span style="color:#888888"><br>
Tim.</span><o:p></o:p></p>
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