What is the format of special section in ELF - elf

Question

I try to write a program to generate ELF(based on Arm and execute through qemu-arm). Most format in ELF has been well illustrated
on wiki. But I can't find any spec describe the format of special section(e.g. .text .data(especially what I want to know)).
I tried to put some initialized global variable in .data section. What format should I write in ELF(.data section) if I have global statement like: int a = 10;

Answer 1

There is not special format for .text and .data.
When the static linker links several .o file,
it simply concatenates the .text and .data segments (while resolving relocations)
and places them in the final .so or executable file according to the linker script (see gcc -Wl,-verbose /dev/null).
The .data segment simply contains the initial values of the instanciated global variables.
The .text segment simply contains the machine code of the routines/functions.
Let's take this simple C file:
char x[5] = {0xba, 0xbb, 0xbc, 0xbd, 0xbe};
char f(int i) {
return x[i];
}
Let's compile it:
$ gcc -c -o test.o test.c
Let's dump the .data section, using elfcat:
$ elfcat test.o --section-name .data | xxd
00000000: babb bcbd be .....
We can clearly explain the content of .data section.
Let's dump the .text section:
$ elfcat test.o --section-name .text | xxd
00000000: 5548 89e5 897d fc8b 45fc 4898 488d 1500 UH...}..E.H.H...
00000010: 0000 000f b604 105d c3
Let's decompile this:
$ elfcat test.o --section-name .text > test.text
$ r2 -a x86 -b 64 -qc pd test.text
0x00000000 55 push rbp
0x00000001 4889e5 mov rbp, rsp
0x00000004 897dfc mov dword [rbp - 4], edi
0x00000007 8b45fc mov eax, dword [rbp - 4]
0x0000000a 4898 cdqe
0x0000000c 488d15000000. lea rdx, qword [0x00000013] ; 19
0x00000013 0fb60410 movzx eax, byte [rax + rdx]
0x00000017 5d pop rbp
0x00000018 c3 ret
Again, there is nothing special in the text segment: it only contains the machine code of the routines/functions of my program.
Notice however the relocation and symbol informations in other segments:
$ readelf -a test.o
[ ... ]
Relocation section '.rela.text' at offset 0x1b8 contains 1 entry:
Offset Info Type Sym. Value Sym. Name + Addend
00000000000f 000800000002 R_X86_64_PC32 0000000000000000 x - 4
Relocation section '.rela.eh_frame' at offset 0x1d0 contains 1 entry:
Offset Info Type Sym. Value Sym. Name + Addend
000000000020 000200000002 R_X86_64_PC32 0000000000000000 .text + 0
[...]
Symbol table '.symtab' contains 10 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS test.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 3
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4
5: 0000000000000000 0 SECTION LOCAL DEFAULT 6
6: 0000000000000000 0 SECTION LOCAL DEFAULT 7
7: 0000000000000000 0 SECTION LOCAL DEFAULT 5
8: 0000000000000000 5 OBJECT GLOBAL DEFAULT 3 x
9: 0000000000000000 25 FUNC GLOBAL DEFAULT 1 f