Why data and stack segments are executable?

I have just noticed that my simple program has its data and stack segments executable. I saw it in /proc/[pid]/maps, and simple code confirmed it.

For example:

; prog.asm
section .data
    code:   db 0xCC    ;int3

section .text
global _start
_start:
    jmp    code

    mov    rax, 60    ; sys_exit
    mov    rdi, 0
    syscall

then

nasm -f elf64 prog.asm
ld -o prog prog.o
./prog

causes prog to execute int3 instruction.

Programs written in C and built with gcc have their data, stack and heap non-executable, so why those written in assembly behave in a different manner?

On modern Linux systems, the linker will mark stack/data non-executable IFF all objects that participate in the link have a special "marker" section .note.GNU-stack.

If you compile e.g. int foo() { return 1; } into assembly (with gcc -S foo.c), you'll see this:

    .section    .note.GNU-stack,"",@progbits

For nasm, the syntax is shown in section 8.9.2 of the manual; you want something like this:

 section .note.GNU-stack noalloc noexec nowrite progbits

Note

This has to be done for every .o file that goes into the executable. If any object file needs executable stack or data, then it's set for the entire segment.