I'm following along with https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html, a guide on decreasing the size of ELF files. I find that my executable today, 20 years later, is ten times larger. I want to learn why.
The program under discussion is below. I've modified it slightly, to be 64-bit (having also tested under 32-bit mode, this adds no more than 40 bytes):
section .text
bits 64
global _start
_start:
mov rax, 60
mov rdi, 42
syscall
This calls the syscall 'exit' (ABI number 60, currently) with the exit code 42. Why? Because that's what the original author thought was a very simple program.
Our build process:
nasm -f elf64 teensy.asm
ld -s teensy.o -o teensy
./teensy; echo $? # Echoes 42 as expected
However, it looks like file sizes have changed since the article was written:
$ wc -c a.out # 1999
368 a.out
$ wc -c teensy # 2022
4320 teensy
Why the difference? Looking at the output of objdump in 1999:
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000007 08048074 08048074 00000074 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .data 00000000 0804907c 0804907c 0000007c 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 0804907c 0804907c 0000007c 2**2
ALLOC
vs in 2022:
$ objdump -x teensy
teensy: file format elf64-x86-64
teensy
architecture: i386:x86-64, flags 0x00000102:
EXEC_P, D_PAGED
start address 0x0000000000401000
Program Header:
LOAD off 0x0000000000000000 vaddr 0x0000000000400000 paddr 0x0000000000400000 align 2**12
filesz 0x00000000000000b0 memsz 0x00000000000000b0 flags r--
LOAD off 0x0000000000001000 vaddr 0x0000000000401000 paddr 0x0000000000401000 align 2**12
filesz 0x000000000000000c memsz 0x000000000000000c flags r-x
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 0000000c 0000000000401000 0000000000401000 00001000 2**4
CONTENTS, ALLOC, LOAD, READONLY, CODE
SYMBOL TABLE:
no symbols
So File off has changed, which may account for the difference. But why?