Problem with large immediate constants in x64 mode

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murder

Joined: 03 Nov 2011
Posts: 14

murder 03 Nov 2011, 17:01

When I try to compile code like this

Code:

mov qword[RegKey+14+rax],'\CLSID'

I`ve got "value out of range" (in 64-bit mode)

but this

Code:

mov    rdx,'\CLSID' 
mov    qword[RegKey+14+rax],rdx

still work

sorry for my english Embarassed

03 Nov 2011, 17:01

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 03 Nov 2011, 18:20

You can't define a large constant like '\CLSID' with mov m64,imm.

See the Intel manual for operand bounds. The largest is this:

Intel manual wrote:

MOV r/m64,imm32

BTW: Are you aware that '\CLSID' is not a pointer to a memory location, but is in actually an immediate constant?

03 Nov 2011, 18:20

murder

Joined: 03 Nov 2011
Posts: 14

murder 04 Nov 2011, 13:56

Quote:

Intel manual wrote:
MOV r/m64,imm32

ok, thanks.

I didnt know about this restriction. Sorry.

04 Nov 2011, 13:56

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 07 Nov 2011, 18:35

CPUs limit instruction lengths to 15 bytes.
If 64-bit architecture would allow full 64-bit immediates then
MOV m64,imm64 could be inflated to 20 bytes by only using

Code:

mov qword[rax+rax*8+123456789ABCDEF0h],123456789ABCDEF0h

I did a simple calculation from the following line that takes 12 bytes

Code:

mov qword[rax+rax*8+12345678h],12345678h

Thx revolution!

Last edited by Madis731 on 07 Nov 2011, 19:23; edited 1 time in total

07 Nov 2011, 18:35

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 07 Nov 2011, 19:09

Madis731 wrote:

CPUs limit instruction lengths to 16 bytes.

Erm ... 15 bytes, right?

07 Nov 2011, 19:09

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20448
Location: In your JS exploiting you and your system

revolution 07 Nov 2011, 19:13

Madis731 wrote:

If 64-bit architecture would allow full 64-bit immediates then
MOV m64,imm64 could be inflated to 20 bytes by only using
Code:
mov dqword[rax+rax*8+123456789ABCDEF0h],123456789ABCDEF0h    
I did a simple calculation from the following line that takes 12 bytes
Code:
mov qword[rax+rax*8+12345678h],12345678h    

Do you mean:

Code:

mov qword[rax+rax*8+123456789ABCDEF0h],123456789ABCDEF0h

dqword would be 128 bits!

07 Nov 2011, 19:13

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 07 Nov 2011, 19:25

Sorry, wasn't thinking straight. I was not referencing any documents like I should've done Smile

and like I usually do. Fixed!

07 Nov 2011, 19:25

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