SHN_ABS
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hello_world_len points to the special st_shndx == SHN_ABS == 0xF1FF.0xF1FF is chosen so as to not conflict with other sections.st_value == 0xD == 13 which is the value we have stored there on the assembly: the length of the string Hello World!.This means that relocation will not affect this value: it is a constant.
This is small optimization that our assembler does for us and which has ELF support.
If we had used the address of
hello_world_len anywhere, the assembler would not have been able to mark it as SHN_ABS, and the linker would have extra relocation work on it later.