[SOLVED|different segment regs] Why expression not resolved?

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ProMiNick

Joined: 24 Mar 2012
Posts: 806
Location: Russian Federation, Sochi

ProMiNick 19 Apr 2022, 05:15

Why expression [ebp+eax] not resolved into [eax+ebp]?
for example in mov instruction

Code:

mov     eax,dword[ebp+eax] ; 4 bytes: $8B,$44,$05,$00

not resolved into

Code:

mov     eax,dword[eax+ebp] ; 3 bytes: $8B,$04,$28

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Last edited by ProMiNick on 20 Apr 2022, 03:54; edited 1 time in total

19 Apr 2022, 05:15

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20459
Location: In your JS exploiting you and your system

revolution 19 Apr 2022, 05:37

The first register is taken as the base, and the second register as the index.

EBP can only be encoded as a base register with an offset.

fasm doesn't reorder the registers, even when the offset is zero.

19 Apr 2022, 05:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20459
Location: In your JS exploiting you and your system

revolution 19 Apr 2022, 07:33

I forgot to add that the reason for not swapping the register order is because of the different segment register used. EBP uses SS, EAX uses DS.

For Win32 this makes no difference normally, but fasm isn't only for Win32.

19 Apr 2022, 07:33

macomics

Joined: 26 Jan 2021
Posts: 1043
Location: Russia

macomics 19 Apr 2022, 16:08

ProMiNick wrote:

not resolved into

Code:

mov     eax,dword[eax+ebp] ; 3 bytes: $8B,$04,$28

Code:

mov     eax,dword[eax+ebp] ; 4 bytes: $17,$8B,$04,$28

19 Apr 2022, 16:08

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20459
Location: In your JS exploiting you and your system

revolution 20 Apr 2022, 03:45

If you define the segment register then you get this:

Code:

use32
mov eax,[ds:ebp+eax]    ; 0x8b, 0x04, 0x28
mov eax,[ds:eax+ebp]    ; 0x8b, 0x04, 0x28

mov eax,[ss:ebp+eax]    ; 0x8b, 0x44, 0x05, 0x00
mov eax,[ss:eax+ebp]    ; 0x8b, 0x44, 0x05, 0x00

20 Apr 2022, 03:45

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