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As part of trying to write a compiler completely from scratch, I'm currently working on the part the handles ELF files.
After skimming through several articles and specifications about them, I still don't quite understand where section to segment mappings are stored. When observing small executables generated by NASM+ld, I can see that the .text section is somehow mapped onto a LOAD-type program header, but how?
A small piece of readelf's output when given a small (working) executable as input:
Program Headers:
Type Offset VirtAddr PhysAddr
FileSiz MemSiz Flags Align
LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
0x0000000000000084 0x0000000000000084 R E 200000
Section to Segment mapping:
Segment Sections...
00 .text
Is this mapping even required to have a working executable? Or can they be omitted completely and you would still have a valid executable?
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Segments, which are commonly known as Program Headers, break down the structure of an ELF binary into suitable chunks to prepare the executable to be loaded into memory. In contrast with Section Headers, Program Headers are not needed on linktime.
The segments contain information that is necessary for runtime execution of the file, while sections contain important data for linking and relocation. Any byte in the entire file can be owned by at most one section, and there can be orphan bytes which are not owned by any section.
The ELF header is always located at the beginning of the ELF file. It describes the ELF file organization and contains the actual sizes of the object file control structures. The initial bytes of an ELF header specify how the file is to be interpreted.
I still don't quite understand where section to segment mappings are stored.
They are not stored anywhere.
Rather, readelf computes the mapping by looking at file offset and size of sections and segments.
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