I was reading about how programs get loaded into memory. I wanted to understand how different sections (like text,data,rodata etc.) of a PE/ELF file are mapped at different places in the virtual address space of a process. I particularly wanted to understand how does the linker know where will a particular section (say for example rodata) will be mapped onto the address space, so that it can resolve correct addresses for all the symbols before creating the executable.
Do operating systems (eg. Windows) have a fixed range of virtual addresses for a process where they load/map a particular section? If not, then how will the linker resolve the correct addresses for the symbols in different sections?
It doesn't. Linker can only propose the executable image be loaded at certain VA, usually 0x00400000 for PE or 0x10000000 for DLL. Virtual adresses of sections (.text, .rodata, .data etc) are aligned by the section alignment (usually 0x00001000) and their proposed VA are therefore 0x00401000, 0x00402000 etc. Linker then fixes adresses of symbols to those assumed VAs.
The default ImageBase address (taken from linker script or linker arguments) is not required by OS loader, but I don't see a reason to change it, its a good habit to see nice rounded addresses during debugging.
In rare cases the Windows loader may find out that part of the proposed address space is occupied, so it will load the image at a different VA and fix position-dependent (absolute) references to their new VA. Special PE section relocs contains addresses of references to program symbols which need relocation at load-time.