x86 Assembly Questions

$ cat bork.asm

section .data
fuck: dd 0xDEADBEEF

section .text
global _start
_start:
mov dword [fuck],0xABADC0DA

mov eax,1
mov ebx,0
int 0x80


Why doesn't writing to the .data segment segfault? Shouldn't Linux map it to a read-only page?

$ nasm -f elf bork.asm
$ gold -s -o bork-gold bork.asm
$ ld -s -melf_i386 -o bork-ld bork.o
$ du -bh *
432 bork-gold
132 bork.asm
340 bork-ld
608 bork.o


How does such a minimal binary take up 300+ bytes?

>>33
You need to be careful that you only use each segment for its intended use. The text segment (.text) can contain both code and data (keeping in mind that any data you place in the text segment cannot be changed by your program during execution). The initialized part of the data segment (.data) should contain just data, not code. And the uninitialized part of the data segment (.bss) cannot have values placed in it as assembly time at all -- all you can do here is allocate memory (that is, add to the total amount of memory to be included as uninitialized data). In all three types of segment, you can define symbolic labels; such a label has as its value an address within that segment.