difference between .text and .data

Question

I want to know what is the difference between the data section and text section in terms of the entire code in a specific section.

I was trying to run the code I have added here. In the data sectoin it runs and prints "af)a" If I change it to the text section in line 2, it seg fault.

I can't figure out what are the crucial differences. Thanks :)

global _start
section .data

_start: mov ecx, 3
    xor byte [_start + 1], 0x02
    pushad  
    mov eax, 4
    mov ebx, 1
    mov edx, ecx
    mov ecx, dword blah
    int 0x80
blah:   popad
    sub bx, ax
    loop _start
    mov eax, 1
    mov ebx, 2
    int 0x80
fin:

expected code to yield the same result, but was proven otherwise.

Answer 1

difference between .text and .data

I don't know about the MASM assembler but the GNU assembler makes two differences:

1. The name (.text in one case, .data in the other case). However, this name is ignored by the operating system.
2. The section flags in the executable file:
   • SHF_ALLOC | SHF_EXECINSTR for .text and
   • SHF_ALLOC | SHF_WRITE for .data

The "section flags" tell the operating system what kind of data is present in the section and what kind of operations are allowed:

• SHF_EXECINSTR means that the section contains code which can be executed. If this flag misses, the program will crash in most OSs if the section contains code. Older 32-bit CPUs did not support this, so this flag was ignored by many OSs when running 32-bit programs. Obviously, your OS ignores this flag; otherwise the .data variant of your program would crash because the code is located in the .data section.

• SHF_WRITE means that data in the section can be overwritten. If this flag is missing and you are trying to do a write operation to the data in the section, the program will crash.

Of course it would be possible to manipulate the executable file in a way that the section .text has the SHF_WRITE flag set. (And some assemblers allow setting this flag for the .text section directly.) In this case your program would not crash if you write to the .text section.