About creating a DLL

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LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 09 Jul 2012, 20:48

hopcode, if you need to force relocations because the DLL uses no external functions and all addresses happen to be relative, then you can use this:

Code:

section '.reloc' fixups data readable discardable
if ~ $-$$ 
        dd      0,8 ;empty fixups section iff no other fixups 
end if

In your example, you could save some bytes by using just "dd ExitProcess" since the code is not reachable anyway. However, I think the correct way to have both, the import and relocations, is to use either mine or JohnFound's referencing method and the code block of this post.

09 Jul 2012, 20:48

hopcode

Joined: 04 Mar 2008
Posts: 563
Location: Germany

hopcode 10 Jul 2012, 10:44

LocoDelAssembly wrote:

...the code block of this post.

it's ok, a bit safer. nevertheless i dont use this code nor the other above,
because on conditional compilation some references to exp/imp functions may be skipped. one safer workaround could be using something always known as here:

Code:

DllEntryPoint:
 mov eax,DllEntryPoint
 mov eax,1
 ret

Cheers

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10 Jul 2012, 10:44

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 10 Jul 2012, 15:57

hopcode, although your last code works in 32-bit code, on 64-bit code "mov eax,DllEntryPoint" would use RIP-relative encoding so as a general solution using the complicated reloc section or "dd/dq ExitProcess" (or an internal label if you only want to force relocation) is safer.

10 Jul 2012, 15:57

hopcode

Joined: 04 Mar 2008
Posts: 563
Location: Germany

hopcode 10 Jul 2012, 17:00

LocoDelAssembly wrote:

on 64-bit code "mov eax,DllEntryPoint" would use RIP-relative encoding

touché!
it works perfectly, and not RIP

Code:

L_00401000:mov rax, 0x401000
L_0040100A:mov eax, 0x1
L_0040100F:ret

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10 Jul 2012, 17:00

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 10 Jul 2012, 23:12

Ups, sorry, I made a terrible mistake! There is no RIP-relative encoding here (as it would happen with LEA), not even if using EAX.

10 Jul 2012, 23:12

questlima

Joined: 27 Aug 2014
Posts: 37

questlima 27 Aug 2014, 13:23

This is nice now i know how to write a DLL in FASM:D thanks

27 Aug 2014, 13:23

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