[LLVMdev] Re: RE:RE: Question about inserting instructions

[Qiuyu Zhang]

Hi,

One more thing I just realized for the method Chris mentioned.

>Actually that's not true.  You can make instructions with an asmstring of:
>
>   ".byte 123\n .byte 56\n .byte 86" and those bytes will get emitted to 
>the code stream.

It seems to work on generating binary file since it uses asmstring, right?

There are two steps related with binary file operation,

1) opt -load passname < *.bc > *.bc which is the first one.
2) the second one is 'as' or 'gcc'  
    as -o *.o *.s
    gcc -c *.s -o *.o
 
Is it true the method you mention should work on the first one?

Another question, when emitting the asmstring to the code stream ( binary code string,right), we need adjust some instruction operation like jmp jne call, for example

jmp/jne 0xxxx ( address of memory or offset). If we put asmstring in code stream, we should adjust the value/offset of such instruction, it makes complicated. So that is why I am trying find a way to insert meaningless code when generating assemble code by llc. ( assemble code generated work on label for those instruction).

Actually, I think I can write a code which read assemble file generated by 'LLC' and find the tag of the dummy BB and put some meanless assemble code like 'push %eax' etc. I am supposed that it should work well. However, I prefer to implement the whole thing within LLVM. 

Anyway, I do appreciate if you can give me some hint about how to emit asmstring to code

Thanks




  ----- Original Message ----- 
  From: Qiuyu Zhang 
  To: llvmdev at cs.uiuc.edu 
  Sent: Wednesday, May 11, 2005 7:32 PM
  Subject: RE:RE: Question about inserting instructions


  Hi,

  Thanks again.
   
  >>> during step 2, we read *.bc code and find dummy BB and put some
  >>> meaningless machinecode, here, we cannot put some illegal machince
  >>> code, otherwise, step 3 goes to fail.
  >>
  >> Yes, you are correct -- if you want to create illegal code you need to
  >> not use system as.  What you need is the ability for llc to create
  >> object files with native code directly, without using the system
  >> assembler.  I think someone is working on it, but I'm not sure as to the
  >> status.  Otherwise, you will just have some random one-byte
  >> instructions.
  >
  >Actually that's not true.  You can make instructions with an asmstring of:
  >
  >   ".byte 123\n .byte 56\n .byte 86" and those bytes will get emitted to 
  >the code stream.

  Yes, you are right. The way is like buffer overflow code in which the instructions exist as asm string in memory
  \x123\x35\86......However, I have no idea how to make it . Would you like giving me idea? Is there any way I can follow?
   
  CodeEmitGen.cpp MachineCodeEmitter.cpp ... which one is response for it? 
   
  Thanks.
  Qiuyu

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