GoKawiilThe Teensy Executable Revisited
(or, "Thunderclouds Gather on the Horizon")
On a couple of occasions, people have responded to my original essay with the comment that what I've created by the end isn't really an ELF executable. Rather, it is a file that the Linux kernel, in its current incarnation, happens to mistake for an ELF executable.
It's a fair point. That 45-byte file clearly doesn't conform to numerous requirements of the ELF specification. But can you blame me? How could I have stopped at the point just before I tossed the ELF specification out the window, knowing what might still be possible?
But to satisfy these purists, and the puritan side in all of us, I've created this sequel.
So. We have an executable that we whittled down to 45 bytes. We now want to bring it into rigid conformance with published standards, while still keeping it as small as possible.
The point at which we strayed from straight and narrow path was when we started fiddling with "unused" fields in the ELF header. So let's back up to before that point:
BITS 32 org 0x08048000 ehdr: ; Elf32_Ehdr db 0x7F, "ELF", 1, 1, 1 ; e_ident times 9 db 0 dw 2 ; e_type dw 3 ; e_machine dd 1 ; e_version dd _start ; e_entry dd phdr - $$ ; e_phoff dd 0 ; e_shoff dd 0 ; e_flags dw ehdrsz ; e_ehsize dw phdrsz ; e_phentsize dw 1 ; e_phnum dw 0 ; e_shentsize dw 0 ; e_shnum dw 0 ; e_shstrndx ehdrsz equ $ - ehdr phdr: ; Elf32_Phdr dd 1 ; p_type dd 0 ; p_offset dd $$ ; p_vaddr dd $$ ; p_paddr dd filesz ; p_filesz dd filesz ; p_memsz dd 5 ; p_flags dd 0x1000 ; p_align phdrsz equ $ - phdr _start: xor eax, eax inc eax mov bl, 42 int 0x80 filesz equ $ - $$
This was our ninety-one-byte version. So: are we stuck with this as our best size? No, not quite. We violated no rules when we overlapped the ELF header and the program header table by eight bytes. The ELF specification explicitly permits overlap of different data structures within the file. So let's do that here:
; tiny.asm BITS 32 org 0x08048000 ehdr: db 0x7F, "ELF", 1, 1, 1 ; e_ident times 9 db 0 dw 2 ; e_type dw 3 ; e_machine dd 1 ; e_version dd _start ; e_entry dd phdr - $$ ; e_phoff dd 0 ; e_shoff dd 0 ; e_flags dw ehdrsz ; e_ehsize dw phdrsz ; e_phentsize phdr: dd 1 ; e_phnum ; p_type ; e_shentsize dd 0 ; e_shnum ; p_offset ; e_shstrndx ehdrsz equ $ - ehdr dd $$ ; p_vaddr dd $$ ; p_paddr dd filesz ; p_filesz dd filesz ; p_memsz dd 5 ; p_flags dd 0x1000 ; p_align phdrsz equ $ - phdr _start: xor eax, eax inc eax mov bl, 42 int 0x80 filesz equ $ - $$
... continue reading