A Deep Dive into the ELF File Format
Debugging a Handmade ELF Image
Writing ELF files by hand is error prone, and the readelf utility is useful for checking the validity of ELF files. A corrupted ELF file will often crash readelf, but it usually provides some helpful information before crashing. Table 2 shows some important switches when using readelf.
Table 2
readelf Switches
| Option | Description |
|---|---|
| -h |
Prints the ELF header. |
| -l |
Prints the program headers and the relationships between sections and segments. |
| -t |
Prints detailed information about the sections. |
| -s |
Prints the symbol tables. |
| -r |
Prints the relocations. |
| -x |
Prints a section's contents in hexadecimal. |
| -d |
Prints the contents of the .dynamic section. |
Conclusion
I can now build and run the ELF file to confirm that it works (Figure 2).
Figure 2: Building and running an ELF file.
You could easily expand the ELF image I've just described with more handwritten assembly code placed in its .text section. All that's needed is to add references to any extra shared libraries and functions to the relevant sections (although certain features, such as thread-local storage (TLS), and accessing the program's arguments may require additional boilerplate code). The main use case, however, might be to combine the ~500 lines of assembler code with the output of a compiler for a higher level language. Fasm gives you ultimate control over the contents of the final binary without having to resort to a blunt instrument such as a hex editor.
Infos
- FreeBSD: https://www.freebsd.org
- flat assembler: https://www.flatassembler.net
- Git repository for this article: https://github.com/SanctaMaria1997/elf.git
- GNU Hash ELF Sections: https://blogs.oracle.com/solaris/post/gnu-hash-elf-sections
The Author
Alan Pilbeam is a programmer and writer from Oxfordshire, England. His passion for computers began when he was gifted a Commodore 64 (together with the legendary Simons' BASIC cartridge) in 1987. He has an endless fascination for compilers (especially BASIC-flavored), assemblers, debuggers, software, and hardware synths, and the history of Unix and Linux. He also likes vegan food and cats.
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