Assembly infinite loop (jmp -2)

1

I heard that with a jmp -2 we could make an infinite loop. It sounded weird but after the explanation that a relative jump would take one byte, the next instruction address (eip) would be 2 bytes less than the jmp address. So, I decided to implement that but I can't make it work.
That's my source code:

SECTION .text

global main
main:
   push   ebp
   mov    esp, ebp
   jmp    0x-2
   leave
   ret

And used this to compile:

nasm -f elf asmloop.asm -o objasmloop.o 

 ld -m elf_i386 -o execasmloop -e main objasmloop.o  

I also tried to use the hex value of -2 (FE) but still get segmentation fault. After all, I took a look at the disassembly with GDB:

Dump of assembler code for function main:
   0x08048060 <+0>: push   %ebp
   0x08048061 <+1>: mov    %ebp,%esp
   0x08048063 <+3>: jmp    0xfffffffe
   0x08048068 <+8>: leave  
   0x08048069 <+9>: ret  

I was able to see on this dump that the actual difference between jmp address and leave address is in fact 5 bytes (it's not using the short reference jmp). But I tried with this to and the result was the same (segmentation fault). How can I perform that kind of infinite-loop?

assembly
nasm
infinite-loop
asked on Stack Overflow Jun 29, 2016 by Luiz Guilherme Ribeiro • edited Jul 2, 2016 by Fifoernik

1 Answer

5

jmp -2 will jump to the address -2 (or 0ff..fffeh), at least in NASM.
As far as I know, the operand of a direct jump is always the target address, leaving the computation of the relative immediate, encoded in the opcode, to the assembler.

In short, jmp SHORT -2 if placed at offset 0, is assembled as EB FC which can be seen, with a new syntax, as jmp <-4> since 2+(-4) = -2.

If you want to craft opcodes, you need to revert to the pseudo instructions db, dw and so on.

If you just want to loop without a label, you can always use the $ symbol, that refer to the offset/address/counter of the current instruction/symbol.
So a jmp <-2> is simply a jmp $, that for a jump placed at zero is equivalent to jmp 0 and assembled as EB FE as you expected.

Being $ a symbol, you can perform ordinary arithmetic on it: jmp $-2 is EB FC or jmp <-4>.

answered on Stack Overflow Jun 29, 2016 by Margaret Bloom

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