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JMP Offset

 
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ZeroAinz
How do I cheat?
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Joined: 03 Sep 2021
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PostPosted: Fri Sep 03, 2021 4:59 am    Post subject: JMP Offset Reply with quote

hi, i'm trying to create code cave using python Rolling Eyes
my problem for now is the jmp offset...i can't calculate it's relative...
the situation is this:
Code:
instruction address:
0x004ECF25
allocated address:
0x05190000


the calculated offset for e9 is 4CA 30DB
Code:
allocAdr - instrAddr - 5(bytes for instruction)=4CA 30DB


but when i modify the bytes...i see in the memory viewer:
Code:
E9 04 CA 30 DB      jmp DB7F992E


but the exact bytes(wich calculate CE are diferent):
Code:
E9 D6 30 CA 04      jmp 05190000



where i am wrong?
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ParkourPenguin
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Joined: 06 Jul 2014
Posts: 3073

PostPosted: Fri Sep 03, 2021 10:58 am    Post subject: Reply with quote

First of all, the jump is relative to RIP, which points to the next instruction to execute. You need to account for the instruction size (5 bytes) before you assemble the offset. Therefore the jump effectively starts at 0x004ECF2A, and the offset would be 0x05190000 - 0x004ECF2A = 0x4CA30D6.

Second, you're writing the offset in big endian when it should be in little endian. i.e. bytes for 0x4CA30D6 = D6 30 CA 04.

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