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[ASM] Calculating a Relative Address

 
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samuri25404
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PostPosted: Mon Dec 24, 2007 8:13 pm    Post subject: [ASM] Calculating a Relative Address Reply with quote
How might one go about calculating a relative address?

I've tried Google, but nothing showed up; I was going to try CEF, but something strayed me away... (Look at the picture if you don't get it)
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sponge
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PostPosted: Mon Dec 24, 2007 8:21 pm    Post subject: Reply with quote
Obtain base address.
Code:
add base, OFFSET


Anyways, you weren't being specific enough.
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samuri25404
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PostPosted: Mon Dec 24, 2007 8:28 pm    Post subject: Reply with quote
Ok, well what I meant was:

If you are given an opcode, for example

Code:

call 01002ff5


The way you would find its array of bytes is to get the opcode's defined bytes for those specified parameters (in that case, parameter), which is

Code:

e8


then still the rel32 bytes in reverse on the back.

How do you calculate the rel32 from that?
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appalsap
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PostPosted: Mon Dec 24, 2007 9:10 pm    Post subject: Reply with quote
Code:

(Destination - (Base + 5))


so if we wanted to do call 01002ff5 from 00500000...

01002ff5 - 00500005 = 00B02FF0

expressed in little endian byte notation is:

Code:

e8 f0 2f b0 00

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Flyte
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PostPosted: Mon Dec 24, 2007 11:15 pm    Post subject: Reply with quote
Damn it appalsap, I get back to the computer and I see this thread and feel like helping, and you beat me too it. Razz

Anyways, to OP: Its best to make an array such as this:
Code:
unsigned char jump[] = { 0xe8 /*0xe9 for JMP*/, 0x00, 0x00, 0x00, 0x00 };


Then constructing the jump using memcpy (or the intrinsic function __movsb()).

Oh, and for the sake of simplicity you could just reduce the formula to: (To - From - 5). You be surprised how many people don't realize this when all it is is basic math. Confused
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DoomsDay
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PostPosted: Tue Dec 25, 2007 9:39 am    Post subject: Reply with quote
Flyte wrote:
Damn it appalsap, I get back to the computer and I see this thread and feel like helping, and you beat me too it. Razz
Welcome to the club Razz
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samuri25404
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PostPosted: Wed Dec 26, 2007 2:09 pm    Post subject: Reply with quote
Alright, thanks a ton everyone. The dll should be finished by the end of today.
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"A"
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PostPosted: Wed Dec 26, 2007 7:09 pm    Post subject: Reply with quote
i no this hummm gold on lol
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