When a program imports a certain function or variable, the linker will include a string with the function or variable’s name in the .dynstr section.
A symbol (Elf Sym) that refers to the function or variable’s name in the .dynsym section, 
and a relocation (Elf Rel) pointing to that symbol in the .rela.plt section.
.rela.dyn and .rela.plt are for imported variables and functions,respectively.