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Dysfunctional Programming

Inside Specs: ELF Segments and Sections

The ELF data format divides object files into segments and sections, which has for long caused confusion. Both terms segment and section can be used interchangeably in almost all cases in the English language ([1], [2]). What is often overlooked is that the ELF specification explicitely meant both to mean almost the same. They merely provide two views of the same data, but use different terms to allow referring to them more easily.

When we look at the defining specification (gABI: System V Application Binary Interface) we find this quote in the introduction:

Object files participate in program linking (building a program) and program execution (running a program). For convenience and efficiency, the object file format provides parallel views of a file’s contents, reflecting the differing needs of those activities.

This is, in my opinion, a crucial detail often overlooked. The ELF data format explicitly provides two views of the same data. The difference between segments and sections is thus not what data they contain, but how they index the same data. The specification goes a step further:

A program header table tells the system how to create a process image. Files used to build a process image (execute a program) must have a program header table; relocatable files do not need one.

A section header table contains information describing the file’s sections. Every section has an entry in the table; each entry gives information such as the section name, the section size, and so on. Files used during linking must have a section header table; other object files may or may not have one.

Keep in mind that the program header table is effectively a segment header table. Therefore, the specification explicitly says that these two data views do not have to be present in a specific file. Depending on the use case, the format allows for only segments or only sections.

To summarize, an ELF object file contains data and machine code of a program, which itself is divided into many parts. The ELF format then provides two different views of this same content: segments and sections. However, these are views of the data present in the file, they do not define the content, but merely index it.

As a closing note, we must acknowledge how all this evolved over time, though. While the ELF specification provides this neat dual-view, a lot of this freedom is not actually used in most ELF files. Instead, most files are effectively split into many small sections, and the segments merely provide a grouping of sequential sections in the file. Sections have become the tool that drives the data in ELF files, and segments have become a view of that data. But this was a purely artifical interpretation and is not rooted in the ELF data format.

Written by David Rheinsberg, on April 26, 2020.