Custom object loader (ELF64). LEA vs MOV

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system error

Joined: 01 Sep 2013
Posts: 670

system error 31 Mar 2016, 07:35

I am trying to write a custom object loader for ELF64 and should be able to execute the loaded object from memory they were loaded. It's working just fine until I ran into the old MOV vs LEA problem again;

Here's my pseudo program

Code:

format ELF64 executable 3
entry start

segment readable writeable
f db 'object.o',0               ;the object file to be loaded

segment readable executable
start:
        ;open the file
        ;get the object size
        ;create dynamic memory based on the size
        ;save/load the object to that memory. Pointer in RAX
        add rax,64      ;skip the header
        jmp rax

And here's the object file I'm loading

Code:

;compile to create object.o
format elf64

mov rdx,29      
lea rsi,[x]     ;address
;mov rsi,x      ;this won't work. Why?
mov edi,1       
mov eax,1       
syscall         ;print the string

xor edi,edi
mov rax,60
syscall         ;exit code

x db 'Hi, I am from the loaded file.'

See the commented line. That's the problem. I can't address the string by using MOV but it is working perfectly using LEA (the message got printed from the loaded object).

What's wrong here?

31 Mar 2016, 07:35

system error

Joined: 01 Sep 2013
Posts: 670

system error 31 Mar 2016, 07:57

Mom?

31 Mar 2016, 07:57

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 31 Mar 2016, 08:00

"mov rsi, x" works here as well as "lea".

31 Mar 2016, 08:00

system error

Joined: 01 Sep 2013
Posts: 670

system error 31 Mar 2016, 08:15

John, it doesn't work on my machine. Maybe you missed the fact that it is a loaded object, and not a linked / executable file. Of course both work if it was in an executable / linked object. That means I can't properly execute a string code which is addressed using MOV, this way?

31 Mar 2016, 08:15

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8466
Location: Kraków, Poland

Tomasz Grysztar 31 Mar 2016, 12:51

LEA in this case uses RIP-relative addressing and therefore does not need relocation (unless it refers to an address in some other section), while the opcode of MOV contains an absolute address that has to be relocated. So the fact that LEA works in your case but MOV doesn't, means that your loader has not applied the relocations correctly.

31 Mar 2016, 12:51

system error

Joined: 01 Sep 2013
Posts: 670

system error 31 Mar 2016, 22:35

Thanks Tomasz. Sounds like there's more to it than it seemed. I'll check that out.

31 Mar 2016, 22:35

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