[fasm2] Problem with 'format ELF64'

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Jessé

Joined: 03 May 2025
Posts: 71
Location: Brazil

Jessé 05 Jan 2026, 00:22

Hello there, noob question!

Today, I'm getting myself out of comfort zone, and trying to use fasm2 in a more "standard" (i.e., assembly) way, and try to make an externally linked program work. But I have the following problem:

Test code:

Code:

format ELF64 

extrn printf
extrn exit
extrn stdout
public _start

section '.text' executable

        align 4
        _start:          
                        endbr64
                        mov         rdi, [plt.stdout]
                        lea         rsi, [msg0]
                        xor         al, al
                        call        plt.printf
                        
                        sub         rsp, 8
                        xor         eax, eax
                        jmp         plt.exit
                        
section '.data' writeable

        msg0:           db 'This is a fasm2 program, compiled in'
                        db ' a traditional way!',10,0

It compiles successfully, and correctly generates the .o file:

Quote:

flat assembler version g.l04h
4 passes, 0.1 seconds, 872 bytes.

But when I link it by using 'ld' command, I get the following:

Quote:

LANG=EN_us ld -o asmextern -pie -lc --dynamic-linker=/lib64/ld-linux-x86-64.so.2
ld: warning: cannot find entry symbol _start; not setting start address

The final executable is broken, and apparently devoid of any section filled in.

My goal is to make PIE executables, and also start doing some shared libraries using fasm2.

Can one help figure out what is wrong?

_________________
jesse6

05 Jan 2026, 00:22

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20816
Location: In your JS exploiting you and your system

revolution 05 Jan 2026, 01:53

I've had success with linking in the past by using

Code:

ld ... -e start ...

Code:

public  start

start:
;...

This creates the _start label in the .so file. The linker adds the leading "_" automatically.

05 Jan 2026, 01:53

Jessé

Joined: 03 May 2025
Posts: 71
Location: Brazil

Jessé 05 Jan 2026, 04:00

Oooh, shame on me!
Sorry guys to annoy you with my lack of competence, it seems that my attention has blown away a fuse today!

Clearly my own 'ld' command line is missing the 'asmextern.o' file as the source object.

The correct is:

Quote:

ld -o asmextern asmextern.o -pie -lc --dynamic-linker=/lib/ld-linux-x86-64.so.2

With optional '-e _start' also working normally.

Change it here and it worked.

Consider solved.

Thanks anyways, revolution.

05 Jan 2026, 04:00

Jessé

Joined: 03 May 2025
Posts: 71
Location: Brazil

Jessé 05 Jan 2026, 04:09

And here, I let the correct working code, because the above one also has errors (if anyone wanna test it):

Code:

format ELF64 

extrn fprintf
extrn exit
extrn stdout
public _start

section '.text' executable

        align 4
        _start:          
                        endbr64
                        mov         rdi, [got.stdout]
                        mov         rdi, [rdi]
                        lea         rsi, [msg0]
                        xor         al, al
                        call        plt.fprintf
                        
                        sub         rsp, 8
                        xor         edi, edi
                        jmp         plt.exit
                        
section '.data' writeable

        msg0:           db 'This is a fasm2 program, compiled in'
                        db ' a traditional way!',10,0

05 Jan 2026, 04:09

duanebonas6822

Joined: 06 Dec 2024
Posts: 62

duanebonas6822 09 Jan 2026, 00:22

Jessé wrote:

Hello there, noob question!

Today, I'm getting myself out of comfort zone, and trying to use fasm2 in a more "standard" (i.e., assembly) way, and try to make an externally linked program work. But I have the following problem:

Test code:
Code:
format ELF64 

extrn printf
extrn exit
extrn stdout
public _start

section '.text' executable

        align 4
        _start:          
                        endbr64
                        mov         rdi, [plt.stdout]
                        lea         rsi, [msg0]
                        xor         al, al
                        call        plt.printf
                        
                        sub         rsp, 8
                        xor         eax, eax
                        jmp         plt.exit
                        
section '.data' writeable

        msg0:           db 'This is a fasm2 program, compiled in'
                        db ' a traditional way!',10,0
    
It compiles successfully, and correctly generates the .o file:

Quote:

flat assembler version g.l04h
4 passes, 0.1 seconds, 872 bytes.

But when I link it by using 'ld' command, I get the following:

Quote:

LANG=EN_us ld -o asmextern -pie -lc --dynamic-linker=/lib64/ld-linux-x86-64.so.2
ld: warning: cannot find entry symbol _start; not setting start address

The final executable is broken, and apparently devoid of any section filled in.

My goal is to make PIE executables, and also start doing some shared libraries using fasm2.

Can one help figure out what is wrong?

Hello mate, i have done exactly what you want but u have to manually define the binary, I have a main script , If you want PM me and ill show you how to create the binary, But u have to do it all manually . Like define the LibC functions, I have a premaid binary that has everything included, its quite large tho as it has nearly every LibC function built in. Send me a PM if you do ill show you my binary. format ELF64 executable, There is loads of examples of how to do it but its called dynamic shared library examples or summet on the forum. But yeah the full binary readelf - a (---) , shows as PIE position independant binary, you can load any function from a static memory location, best thing is it even loads commands when on a remote stack. FASM is the best bud. NASM cant create PIC/PIE binarys only FASM

_________________
d.bonas

09 Jan 2026, 00:22

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