Concatenate a string, and return it in EAX

flat assembler > Main > Concatenate a string, and return it in EAX

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NEASM

Joined: 13 Apr 2018
Posts: 13

Sorry for the trouble,

I've been looking for a long way to concatenate two strings and return their value to EAX (ie: ECX = "hel"; EDX = "lo"; EAX = "hello", for example), but I've never been able to find nothing about this. Could you give me a function in assembly that concatenates two strings? Thanks in advance.

22 Aug 2018, 01:38

NEASM

Joined: 13 Apr 2018
Posts: 13

(i haven't found a good manner to do this in C as well)

22 Aug 2018, 01:39

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16618
Location: In your JS exploiting you and your system

One thing to realise is that the register EAX is 32-bits wide, so it can only hold a maximum of 4 bytes. So assuming you want just 4 bytes you can do this:

Code:

mov ecx,'hel' ;ECX=0x006c6568
mov edx,'l' ;EDX=0x0000006c
;...
mov eax,edx ;EAX=0x0000006c
shl eax,24 ;EAX=0x6c000000
or eax,ecx ;EAX=0x6c6c6568 = 'hell'

22 Aug 2018, 04:22

redsock

Joined: 09 Oct 2009
Posts: 306
Location: Australia

lest we forget AMD64 and do the same:

Code:

mov ecx, 'hel'
mov edx, 'lo'
mov eax, edx
shl rax, 24
or rax, rcx

_________________
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22 Aug 2018, 10:00

NEASM

Joined: 13 Apr 2018
Posts: 13

revolution wrote:

One thing to realise is that the register EAX is 32-bits wide, so it can only hold a maximum of 4 bytes. So assuming you want just 4 bytes you can do this:
Code:
mov ecx,'hel' ;ECX=0x006c6568
mov edx,'l' ;EDX=0x0000006c
;...
mov eax,edx ;EAX=0x0000006c
shl eax,24 ;EAX=0x6c000000
or eax,ecx ;EAX=0x6c6c6568 = 'hell'    

Thanks. But, for concatenate strings like this?

Code:

buffer   db 'Hello', 0
buffer2 db ', World!', 0
buffer3 rb 100

.....

mov ecx, [buffer]
mov edx, [buffer2]
.....  ; something
mov [buffer3], eax

22 Aug 2018, 10:42

DimonSoft

Joined: 03 Mar 2010
Posts: 532
Location: Belarus

NEASM wrote:

Thanks. But, for concatenate strings like this?

Code:

buffer   db 'Hello', 0
buffer2 db ', World!', 0
buffer3 rb 100

.....

mov ecx, [buffer]
mov edx, [buffer2]
.....  ; something
mov [buffer3], eax

First describe how would you store the resulting string in 4 bytes.

22 Aug 2018, 12:47

Walter

Joined: 26 Jan 2013
Posts: 133

The C runtime functions can be used if you are using pointers to strings.

Code:

;**************
;* Concat.asm *
;**************

format pe console 4.0
entry start

include 'win32a.inc'

section '.data' data readable writeable

    strPart1 db 'Hel',0
    strPart2 db 'lo.',0
    strFormat db '%s',13,10,0
    strResult rb 128

section '.code' code readable executable

    start:

        cinvoke strcpy,strResult,strPart1
        cinvoke strcat,strResult,strPart2
        cinvoke printf,strFormat,strResult
        invoke ExitProcess,0

section '.idata' import data readable writeable

  library kernel32,'kernel32.dll',\ 
          msvcrt,'msvcrt.dll' 

  import kernel32,\ 
         ExitProcess,'ExitProcess' 

  import msvcrt,\ 
         printf,'printf',\ 
         strcpy,'strcpy',\ 
         strcat,'strcat'

[/code]

22 Aug 2018, 16:54

DimonSoft

Joined: 03 Mar 2010
Posts: 532
Location: Belarus

C runtime functions should be supplied with obligatory warning about possible security vulnerabilities (buffer overflow) caused by insufficient buffer sizes (in case source string lengths are not known in advance) which has been the problem for C/C++ programs for a few decades now. Not sure if msvcrt.dll provides safe versions.

P.S. The functions still don’t let one put an arbitrary string into 4-byte register.

22 Aug 2018, 18:04

NEASM

Joined: 13 Apr 2018
Posts: 13

DimonSoft wrote:

NEASM wrote:
Thanks. But, for concatenate strings like this?
Code:
buffer   db 'Hello', 0
buffer2 db ', World!', 0
buffer3 rb 100

.....

mov ecx, [buffer]
mov edx, [buffer2]
.....  ; something
mov [buffer3], eax

    
First describe how would you store the resulting string in 4 bytes.

Excuse me, i'm not so expert.

24 Aug 2018, 13:49

DimonSoft

Joined: 03 Mar 2010
Posts: 532
Location: Belarus

NEASM wrote:

DimonSoft wrote:
First describe how would you store the resulting string in 4 bytes.

Excuse me, i'm not so expert.

It’s not about expertise, it’s about proper task formulation and common sense.

24 Aug 2018, 16:37

ManOfSteel

Joined: 02 Feb 2005
Posts: 1149

Perhaps NEASM wants eax to hold the pointer to the resulting string, not the actual resulting string?!

24 Aug 2018, 19:29

DimonSoft

Joined: 03 Mar 2010
Posts: 532
Location: Belarus

ManOfSteel wrote:

Perhaps NEASM wants eax to hold the pointer to the resulting string, not the actual resulting string?!

Which is the most obvious case but that contradicts with

NEASM wrote:

(ie: ECX = "hel"; EDX = "lo"; EAX = "hello", for example)

and code given later.

24 Aug 2018, 19:53

Furs

Joined: 04 Mar 2016
Posts: 1420

NEASM wrote:

Sorry for the trouble,

I've been looking for a long way to concatenate two strings and return their value to EAX (ie: ECX = "hel"; EDX = "lo"; EAX = "hello", for example), but I've never been able to find nothing about this. Could you give me a function in assembly that concatenates two strings? Thanks in advance.

EAX is 4 bytes.

'hel' is 3 bytes, but 'hello' is 5 bytes so it won't fit.

What you ask for is impossible.

25 Aug 2018, 15:57

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16618
Location: In your JS exploiting you and your system

Furs wrote:

EAX is 4 bytes.

'hel' is 3 bytes, but 'hello' is 5 bytes so it won't fit.

We could try to use a different character set. A character set with 6 bits per character would allow 5 characters within 30 bits.

25 Aug 2018, 16:01

DimonSoft

Joined: 03 Mar 2010
Posts: 532
Location: Belarus

revolution wrote:

Furs wrote:
EAX is 4 bytes.

'hel' is 3 bytes, but 'hello' is 5 bytes so it won't fit.
We could try to use a different character set. A character set with 6 bits per character would allow 5 characters within 30 bits.

Or we could use Huffman encoding with custom frequency table. 14 bits ought to be enough for greeting (and even zero-termination).

25 Aug 2018, 16:52

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 16618
Location: In your JS exploiting you and your system

At least two ways to encode in 14 bits:

Code:

l = 00
e = 01
h = 10
o = 110
_ = 111
-------
14 bits

Code:

l = 0
e = 100
h = 101
o = 110
_ = 111
-------
14 bits

25 Aug 2018, 17:01

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