Improper Opcode for flat binary file

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TightCoderEx

Joined: 14 Feb 2013
Posts: 58
Location: Alberta

TightCoderEx 24 Jul 2013, 02:11

When I compile this

Code:

        use64
        org     0x100000

        mov     edi, [Toolc]

   Toolc:        dw      32

I get this;

8B 3D 00 00 00 00 mov edi, [rel 0x6]

where what I want is this

8B 3C 25 07 00 10 00 mov edi, [0x100007]

What am I doing wrong?

24 Jul 2013, 02:11

tthsqe

Joined: 20 May 2009
Posts: 767

tthsqe 24 Jul 2013, 03:24

8B 3D 00 00 00 00 is correct for the code that you posted. I'm quite sure that in long mode direct memory references are RIP relative from the next instruction.

There might be some some encoding of mov eax,[blah blah] that are not relative to RIP but I was never able to deduce this from the documentation.

24 Jul 2013, 03:24

TightCoderEx

Joined: 14 Feb 2013
Posts: 58
Location: Alberta

TightCoderEx 24 Jul 2013, 07:50

Even though the first scenario is the better alternative albeit 1 byte, but I didn't expect to see position independence mixed with absolute like 0C compared to 07

Code:

00000007  BB78001000        mov ebx,0x100078
0000000C  488B3D3D000000    mov rdi,[rel 0x50]
00000013  488B7510          mov rsi,[rbp+0x10]

The other problem was the error that spawned this question had nothing to do with the assembly, but rather the way I mix 32 with 64 and one of my pointers had extraneous values in the high order 32 bits.

Anyway, problem solved, thanks

24 Jul 2013, 07:50

l_inc

Joined: 23 Oct 2009
Posts: 881

l_inc 24 Jul 2013, 19:49

TightCoderEx
You can force absolute addressing by putting a size operator into the brackets:

Code:

mov edi,[qword Toolc]

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24 Jul 2013, 19:49

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