Ciro Santilli @cirosantilli 37

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ELF Hello World Tutorial / Section header table Updated 2025-07-16

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Array of Elf64_Shdr structs.

Each entry contains metadata about a given section.

e_shoff of the ELF header gives the starting position, 0x40 here.

e_shentsize and e_shnum from the ELF header say that we have 7 entries, each 0x40 bytes long.

So the table takes bytes from 0x40 to 0x40 + 7 + 0x40 - 1 = 0x1FF.

Some section names are reserved for certain section types: www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html#special_sections e.g. .text requires a SHT_PROGBITS type and SHF_ALLOC + SHF_EXECINSTR

Running:

readelf -S hello_world.o

outputs:

There are 7 section headers, starting at offset 0x40:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0
  [ 1] .data             PROGBITS         0000000000000000  00000200
       000000000000000d  0000000000000000  WA       0     0     4
  [ 2] .text             PROGBITS         0000000000000000  00000210
       0000000000000027  0000000000000000  AX       0     0     16
  [ 3] .shstrtab         STRTAB           0000000000000000  00000240
       0000000000000032  0000000000000000           0     0     1
  [ 4] .symtab           SYMTAB           0000000000000000  00000280
       00000000000000a8  0000000000000018           5     6     4
  [ 5] .strtab           STRTAB           0000000000000000  00000330
       0000000000000034  0000000000000000           0     0     1
  [ 6] .rela.text        RELA             0000000000000000  00000370
       0000000000000018  0000000000000018           4     2     4
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
  I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
  O (extra OS processing required) o (OS specific), p (processor specific)

The struct represented by each entry is:

typedef struct {
    Elf64_Word  sh_name;
    Elf64_Word  sh_type;
    Elf64_Xword sh_flags;
    Elf64_Addr  sh_addr;
    Elf64_Off   sh_offset;
    Elf64_Xword sh_size;
    Elf64_Word  sh_link;
    Elf64_Word  sh_info;
    Elf64_Xword sh_addralign;
    Elf64_Xword sh_entsize;
} Elf64_Shdr;

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Quartz clock Updated 2025-07-16

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Video 1.

How a quartz watch works by Steve Mould (2017)

Source. Mentions feedback loop loop with the quartz tuning fork for the piezoelectricity and an amplifier. Also mentions the choice of 32768 Hertz (

2^{15}

) as the first power of 2 that is outside of the human hearing range, and then how a frequency divider is used to reduce the frequency to get the second counter.

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Animal-in-the-loop Updated 2025-07-16

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Ciro Santilli invented this term, it refers to mechanisms in which you put an animal in a virtual world that the animal can control, and where you can measure the animal's outputs.

MouseGoggles www.researchsquare.com/article/rs-3301474/v1 | twitter.com/hongyu_chang/status/1704910865583993236
Fruit fly setup from Penn State: scitechdaily.com/secrets-of-fly-vision-for-rapid-flight-control-and-staggeringly-fast-reaction-speed/

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Heavy water Updated 2025-07-16

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Cody'sLab had a nice 5 video series on making it at home! But the United States Government asked him to take it down as suggested at Video "What's Been Going On With Cody'sLab? by Cody'sLab (2019)" at youtu.be/x1mv0vwb08Y?t=84.

Here's a copy online as of 2020: www.youtube.com/watch?v=bCXB6BdMh9Y

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Here's to the crazy ones Updated 2025-07-16

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en.wikipedia.org/w/index.php?title=Think_different&oldid=990983100#Television

Because the people who are crazy enough to think they can change the world are the ones who do.

Video 1. Source.

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Hyperbolic gemoetry Updated 2025-07-16

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ELF Hello World Tutorial / Section vs segment Updated 2025-07-16

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We will get into more detail later, but it is good to have it in mind now:

section: exists before linking, in object files.
One ore more sections will be put inside a single segment by the linker.
Major information sections contain for the linker: is this section:
- raw data to be loaded into memory, e.g. .data, .text, etc.
- or metadata about other sections, that will be used by the linker, but disappear at runtime e.g. .symtab, .srttab, .rela.text
segment: exists after linking, in the executable file.
Contains information about how each segment should be loaded into memory by the OS, notably location and permissions.

See also:

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III-V semiconductor Updated 2025-07-16

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Most notable example: gallium arsenide, see also: gallium arsenide vs silicon.

An important class of semiconductors, e.g. there is a dedicated III-V lab at: École Polytechnique: www.3-5lab.fr/contactus.php

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ELF Hello World Tutorial / Specified file formats Updated 2025-07-16

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The ELF standard specifies multiple file formats:

Object files (.o).
Intermediate step to generating executables and other formats:
```
Source code

    |
    | Compilation
    |
    v

Object file

    |
    | Linking
    |
    v

Executable
```
Object files exist to make compilation faster: with make, we only have to recompile the modified source files based on timestamps.
We have to do the linking step every time, but it is much less expensive.

Executable files (no standard Linux extension).
This is what the Linux kernel can actually run.

Archive files (.a).
Libraries meant to be embedded into executables during the Linking step.

Shared object files (.so).
Libraries meant to be loaded when the executable starts running.

Core dumps.
Such files may be generated by the Linux kernel when the program does naughty things, e.g. segfault.
They exist to help debugging the program.

In this tutorial, we consider only object and executable files.

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

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ELF is specified by the LSB:

The LSB basically links to other standards with minor extensions, in particular:

Generic (both by SCO):
- System V ABI 4.1 (1997) www.sco.com/developers/devspecs/gabi41.pdf, no 64 bit, although a magic number is reserved for it. Same for core files. This is the first document you should look at when searching for information.
- System V ABI Update DRAFT 17 (2003) www.sco.com/developers/gabi/2003-12-17/contents.html, adds 64 bit. Only updates chapters 4 and 5 of the previous document: the others remain valid and are still referenced.
Architecture specific (by the processor vendor):
- IA-32: refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-IA32/LSB-Core-IA32/elf-ia32.html, points mostly to www.sco.com/developers/devspecs/abi386-4.pdf
- AMD64: refspecs.linuxfoundation.org/LSB_4.1.0/LSB-Core-AMD64/LSB-Core-AMD64/elf-amd64.html, points mostly to www.x86-64.org/documentation/abi.pdf

A handy summary can be found at:

man elf

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

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Entry 1 has ELF64_R_TYPE == STT_FILE. ELF64_R_TYPE is continued inside of st_info.

Byte analysis:

10 8: st_name = 01000000 = character 1 in the .strtab, which until the following \0 makes hello_world.asm
This piece of information file may be used by the linker to decide on which segment sections go: e.g. in ld linker script we write:
```
segment_name :
{
    file(section)
}
```
to pick a section from a given file.
Most of the time however, we will just dump all sections with a given name together with:
```
segment_name :
{
    *(section)
}
```
10 12: st_info = 04
Bits 0-3 = ELF64_R_TYPE = Type = 4 = STT_FILE: the main purpose of this entry is to use st_name to indicate the name of the file which generated this object file.
Bits 4-7 = ELF64_ST_BIND = Binding = 0 = STB_LOCAL. Required value for STT_FILE.
10 13: st_shndx = Symbol Table Section header Index = f1ff = SHN_ABS. Required for STT_FILE.
20 0: st_value = 8x 00: required for value for STT_FILE
20 8: st_size = 8x 00: no allocated size