.data section

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.data section by Ciro Santilli 34 Updated 2024-12-15 +Created 1970-01-01
.data is section 1:
00000080  01 00 00 00 01 00 00 00  03 00 00 00 00 00 00 00  |................|
00000090  00 00 00 00 00 00 00 00  00 02 00 00 00 00 00 00  |................|
000000a0  0d 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000b0  04 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
  • 80 0: sh_name = 01 00 00 00: index 1 in the .shstrtab string table
    Here, 1 says the name of this section starts at the first character of that section, and ends at the first NUL character, making up the string .data.
    .data is one of the section names which has a predefined meaning according to www.sco.com/developers/gabi/2003-12-17/ch4.strtab.html:
    These sections hold initialized data that contribute to the program's memory image.
  • 80 4: sh_type = 01 00 00 00: SHT_PROGBITS: the section content is not specified by ELF, only by how the program interprets it. Normal since a .data section.
  • 80 8: sh_flags = 03 7x 00: SHF_WRITE and SHF_ALLOC: www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html#sh_flags, as required from a .data section
  • 90 0: sh_addr = 8x 00: TODO: standard says:
    If the section will appear in the memory image of a process, this member gives the address at which the section's first byte should reside. Otherwise, the member contains 0.
    but I don't understand it very well yet.
  • 90 8: sh_offset = 00 02 00 00 00 00 00 00 = 0x200: number of bytes from the start of the program to the first byte in this section
  • a0 0: sh_size = 0d 00 00 00 00 00 00 00
    If we take 0xD bytes starting at sh_offset 200, we see:
    00000200  48 65 6c 6c 6f 20 77 6f  72 6c 64 21 0a 00        |Hello world!..  |
    AHA! So our "Hello world!" string is in the data section like we told it to be on the NASM.
    Once we graduate from hd, we will look this up like:
    readelf -x .data hello_world.o
    which outputs:
    Hex dump of section '.data':
  0x00000000 48656c6c 6f20776f 726c6421 0a       Hello world!.
    NASM sets decent properties for that section because it treats .data magically: www.nasm.us/doc/nasmdoc7.html#section-7.9.2
    Also note that this was a bad section choice: a good C compiler would put the string in .rodata instead, because it is read-only and it would allow for further OS optimizations.
    • a0 8: sh_link and sh_info = 8x 0: do not apply to this section type. www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html#special_sections
    • b0 0: sh_addralign = 04 = TODO: why is this alignment necessary? Is it only for sh_addr, or also for symbols inside sh_addr?
    • b0 8: sh_entsize = 00 = the section does not contain a table. If != 0, it means that the section contains a table of fixed size entries. In this file, we see from the readelf output that this is the case for the .symtab and .rela.text sections.
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