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Index > Windows > Win64 shortest pass control instruction

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kandamun



Joined: 20 Jul 2005
Posts: 25
kandamun
Hello all
my problem is to find the shortest sequence of instructions that will "jump" across 64 bit .
For example
Code:
mov rax, imm64
jmp rax
    

can do the job in 12 bytes.
Is there a shorter way ?
What do you think ?
Post 13 Feb 2008, 14:54
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Tomasz Grysztar
Assembly Artist


Joined: 16 Jun 2003
Posts: 7724
Location: Kraków, Poland
Tomasz Grysztar
AFAIK, there's no shorter way.
Post 13 Feb 2008, 15:08
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 17278
Location: In your JS exploiting you and your system
revolution
kandamun: You mean to suggest that you have so much memory that you need to use a 64bit jump and yet you still need to save a few bytes because you are running out of memory? Confused
Post 13 Feb 2008, 15:28
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f0dder



Joined: 19 Feb 2004
Posts: 3170
Location: Denmark
f0dder
revolution wrote:
kandamun: You mean to suggest that you have so much memory that you need to use a 64bit jump and yet you still need to save a few bytes because you are running out of memory? Confused

It can be useful if you're building (a lot of) trampolines...

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Post 13 Feb 2008, 15:44
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kandamun



Joined: 20 Jul 2005
Posts: 25
kandamun
Actually I'm not sure , if that is possible in win64, that is some chunks of code (DLLs or I don't know what else) to be so far they cannot address each other with 32bit address.
OK, then.
You can use data from memory too Smile
I will try now to calculate the final jump with single 64bit memory cell and 8/16 bit offsets.
Post 13 Feb 2008, 15:54
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bitRAKE



Joined: 21 Jul 2003
Posts: 2914
Location: [RSP+8*5]
bitRAKE
ret Very Happy
Post 13 Feb 2008, 16:03
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MHajduk



Joined: 30 Mar 2006
Posts: 6034
Location: Poland
MHajduk
bitRAKE
do you mean something like this:
Code:
push rax
retn    
Question
Post 13 Feb 2008, 16:12
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bitRAKE



Joined: 21 Jul 2003
Posts: 2914
Location: [RSP+8*5]
bitRAKE
Could calculate address on the stack or in a register - all depends on the situation. Maybe, it's easier to change the RSP register (i.e. BIOS code, thread). Anyhow RET is the smallest instruction to change RIP in a flexible, usable manner, imho. (INT3 and such are not as flexible. Laughing )
Code:
mov rsp,Table
jmp SomeTest
Table:
    dq OptionA
    dq OptionB
    dq OptionC

SomeTest:
    and rax,1
    jne .0
    retn 8
.0: retn

OptionA:
    retn

OptionB:
    retn

OptionC:
    ; end up here    
Post 13 Feb 2008, 17:05
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bitRAKE



Joined: 21 Jul 2003
Posts: 2914
Location: [RSP+8*5]
bitRAKE
Another example:
Code:
call MutiBranch


MutiBranch:
  mov rbp,COMMON

; branch to multiple


.0:     call rbp
        ;...
        retn


.1:     call rbp
        ;...
        retn


COMMON:
        pop rbp

        ; common code

        call rbp

        ; common code

        retn    
"call the common" is a way to put other addresses on the stack for a common routine/loop, or to store for use later. Still need to branch to the different options with Jcc or a jump table; but the address data is generated by the CPU.

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Post 25 Feb 2008, 16:48
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