Ciro Santilli @cirosantilli  40

If the number of sections is greater than or equal to SHN_LORESERVE (0xff00), e_shnum has the value SHN_UNDEF (0) and the actual number of section header table entries is contained in the sh_size field of the section header at index 0 (otherwise, the sh_size member of the initial entry contains 0).

There are also other magic sections detailed in Figure 4-7: Special Section Indexes.

 Read the full article

ELF Hello World Tutorial / .data section Updated 2025-07-16

 View more

.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.

 Read the full article

ELF Hello World Tutorial / .text section Updated 2025-07-16

 View more

Now that we've done one section manually, let's graduate and use the readelf -S of the other sections:

  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 2] .text             PROGBITS         0000000000000000  00000210
       0000000000000027  0000000000000000  AX       0     0     16

.text is executable but not writable: if we try to write to it Linux segfaults. Let's see if we really have some code there:

objdump -d hello_world.o

gives:

hello_world.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_start>:
   0:       b8 01 00 00 00          mov    $0x1,%eax
   5:       bf 01 00 00 00          mov    $0x1,%edi
   a:       48 be 00 00 00 00 00    movabs $0x0,%rsi
  11:       00 00 00
  14:       ba 0d 00 00 00          mov    $0xd,%edx
  19:       0f 05                   syscall
  1b:       b8 3c 00 00 00          mov    $0x3c,%eax
  20:       bf 00 00 00 00          mov    $0x0,%edi
  25:       0f 05                   syscall

If we grep b8 01 00 00 on the hd, we see that this only occurs at 00000210, which is what the section says. And the Size is 27, which matches as well. So we must be talking about the right section.

This looks like the right code: a write followed by an exit.

The most interesting part is line a which does:

movabs $0x0,%rsi

to pass the address of the string to the system call. Currently, the 0x0 is just a placeholder. After linking happens, it will be modified to contain:

4000ba: 48 be d8 00 60 00 00    movabs $0x6000d8,%rsi

This modification is possible because of the data of the .rela.text section.

 Read the full article

ELF Hello World Tutorial / SHT_STRTAB Updated 2025-07-16

 View more

Sections with sh_type == SHT_STRTAB are called string tables.

They hold a null separated array of strings.

Such sections are used by other sections when string names are to be used. The using section says:

which string table they are using
what is the index on the target string table where the string starts

So for example, we could have a string table containing:

Data: \0 a b c \0 d e f \0
Index: 0 1 2 3  4 5 6 7  8

The first byte must be a 0. TODO rationale?

And if another section wants to use the string d e f, they have to point to index 5 of this section (letter d).

Notable string table sections:

.shstrtab
.strtab

 Read the full article

ELF Hello World Tutorial / .shstrtab Updated 2025-07-16

 View more

Section type: sh_type == SHT_STRTAB.

Common name: "section header string table".

The section name .shstrtab is reserved. The standard says:

This section holds section names.

This section gets pointed to by the e_shstrnd field of the ELF header itself.

String indexes of this section are are pointed to by the sh_name field of section headers, which denote strings.

This section does not have SHF_ALLOC marked, so it will not appear on the executing program.

readelf -x .shstrtab hello_world.o

outputs:

Hex dump of section '.shstrtab':
  0x00000000 002e6461 7461002e 74657874 002e7368 ..data..text..sh
  0x00000010 73747274 6162002e 73796d74 6162002e strtab..symtab..
  0x00000020 73747274 6162002e 72656c61 2e746578 strtab..rela.tex
  0x00000030 7400                                t.

The data in this section has a fixed format: www.sco.com/developers/gabi/2003-12-17/ch4.strtab.html

If we look at the names of other sections, we see that they all contain numbers, e.g. the .text section is number 7.

Then each string ends when the first NUL character is found, e.g. character 12 is \0 just after .text\0.

 Read the full article

 There are unlisted articles, also show them or only show them.