Operand sizes do not match

Index > Main > Operand sizes do not match

Author

Thread

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 10:24

Hi guys. I have troubles with assembling when fasm tries to optimize the instruction size (Code can't be generated). So i used explicit instruction size specifier. This works god with JMP/JXX instructions but i have troubles with x64 arithmetic instructions (operand sizes do not match):

Code:

format PE64 console

sub rax, dword 248

How to force to use specified operand size here?

Thank in advance!

01 Feb 2024, 10:24

Overclick

Joined: 11 Jul 2020
Posts: 669
Location: Ukraine

Overclick 01 Feb 2024, 10:54

If your variable can be larger than the dword, use a mask instead of the size:

Code:

sub rax, <operand> and 0xFFFFFFFF

Otherway you don't even need to mark it anyhow

01 Feb 2024, 10:54

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8357
Location: Kraków, Poland

Tomasz Grysztar 01 Feb 2024, 10:54

As fasm's syntax focuses on the actual operation of the instruction and not on the encoding, the size prefixes are expected to match the actual sizes of the operands. You could forcefully disable the optimization by using a non-optimizable value and then rewriting it afterwards:

Code:

sub rax,07FFFFFFFh
store dword 248 at $-4

With fasm2 it can be achieved directly:

Code:

{imm32} sub rax,248

The new syntax I designed there reflects my hope of separating the instruction operation layer and instruction encoding layer into clearly separated areas.

01 Feb 2024, 10:54

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 11:09

Thank you guys! I love fasm because it contains FASMW editor.

@Overclick i can't use your approach because i have the labels calculations so fasm can't resolve it because if the operand size occupies 1 byte the labels expression contains one number if it occupies 4 bytes it contains another.

@Tomasz Grysztar, can i adapt FASMW to fasm2? I like the second syntax.

Last edited by thetrick on 01 Feb 2024, 11:09; edited 1 time in total

01 Feb 2024, 11:09

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20431
Location: In your JS exploiting you and your system

revolution 01 Feb 2024, 11:09

248 is larger than 127 so it always encodes as a dword.

Code:

sub rax,248 ; always uses dword encoding

01 Feb 2024, 11:09

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 11:12

@revolution the actual value is calculated in assembly stage so it can have any value which is suitable as for 1 byte operand as for 4 bytes. FASM can't generate code because it can't resolve the sizes. This is the reason i need to use only specified size of operand encoding.

01 Feb 2024, 11:12

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20431
Location: In your JS exploiting you and your system

revolution 01 Feb 2024, 11:16

If you use the PE format then all relocatable addresses use qword encoding. Are you sure you end up with byte encodings?

Normally RIP-relative addresses are more efficient.

01 Feb 2024, 11:16

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 11:20

My code operates the relative labels offsets not absolute.

01 Feb 2024, 11:20

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20431
Location: In your JS exploiting you and your system

revolution 01 Feb 2024, 11:23

thetrick wrote:

My code operates the relative labels offsets not absolute.

That is what RIP-relative does. fasm uses it by default. No special tricks needed.

01 Feb 2024, 11:23

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 11:31

Yes you're right but this isn't suitable for my purpose.

01 Feb 2024, 11:31

Overclick

Joined: 11 Jul 2020
Posts: 669
Location: Ukraine

Overclick 01 Feb 2024, 12:18

Hmm

Description:
Filesize:	18.75 KB
Viewed:	3280 Time(s)

01 Feb 2024, 12:18

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 13:10

Quote:

Hmm

When i uncomment Tomasz' approach it works good.

Description:
Filesize:	9.45 KB
Viewed:	3271 Time(s)

01 Feb 2024, 13:10

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 13:19

revolution wrote:

thetrick wrote:
My code operates the relative labels offsets not absolute.
That is what RIP-relative does. fasm uses it by default. No special tricks needed.

Thank you for suggestion. I see LEA also uses RIP for displacement too it's suitable for me! So i can use lea RAX, [label] without adding relocation information to binary. Jut the same instruction in x86 add the relocation information so i cannot to calculate valid code-crc after code loaded from non-base address. Thank you!

01 Feb 2024, 13:19

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20431
Location: In your JS exploiting you and your system

revolution 01 Feb 2024, 13:57

If you use RIP-relative addressing then you don't need any relocation information, and the binary doesn't change when it is loaded to another address.

I can't help but think you are over-thinking something, when a normal RIP-relative binary wouldn't change any CRC, and wouldn't need any manual address computations, the CPU does it all automatically at runtime.

01 Feb 2024, 13:57

Overclick

Joined: 11 Jul 2020
Posts: 669
Location: Ukraine

Overclick 01 Feb 2024, 14:54

Maybe you just need an org directive?..

01 Feb 2024, 14:54

thetrick

Joined: 19 Feb 2020
Posts: 18

thetrick 01 Feb 2024, 20:12

revolution wrote:

If you use RIP-relative addressing then you don't need any relocation information, and the binary doesn't change when it is loaded to another address.

I can't help but think you are over-thinking something, when a normal RIP-relative binary wouldn't change any CRC, and wouldn't need any manual address computations, the CPU does it all automatically at runtime.

Yes i used your suggestion and LEA instruction (which always 4 byte operand encoding).

01 Feb 2024, 20:12

< Last Thread | Next Thread >

Forum Rules:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum