Set Kernel Output as ELF

Updated on 17 Jun, 20259 mins read 24 views

Up to this point we had our kernel as binary file. Now we update it to an ELF executable targeting the 32-bit x86 architecture. Below is the updated linker.ld script.

OUTPUT_FORMAT(elf32-i386)
ENTRY(start)
phys = 0x00100000;
SECTIONS
{
	.text phys : AT(phys) {
		code = .;
		*(.text)
		*(.rodata)
		. = ALIGN(4096);
	}
	.data : AT(phys + (data - code))
	{
		data = .;
		*(.data)
		. = ALIGN(4096);
	}
	.bss : AT(phys + (bss - code))
	{
		bss = .;
		*(.bss)
		. = ALIGN(4096);
	}
	end = .;
	/DISCARD/ :
	{
		*(.comment)
		*(.eh_frame)
		*(.note.gnu.build-id)
	}
}

1 OUTPUT_FORMAT(elf32-i386):

Specifies the output format of the executable as ELF for the 32-bit x86 architecture.

2 ENTRY(start):

Specifies the entry point of the program as the symbol start. This is the address where execution of the program begins.

3 phys = 0x00100000;:

Defines a symbol phys with the value 0x00100000, indicating the physical address where the program will be loaded into memory. This is commonly referred to as the base address.

4 SECTIONS:

Marks the beginning of the section definitions.

5 .text phys : AT(phys):

Defines the .text section and specifies that it should be loaded at the physical address phys. The code = .; line marks the beginning of the .text section. The *(.text) directive includes all .text sections from input files. The *(.rodata) directive includes all .rodata sections from input files. The . = ALIGN(4096); directive aligns the location counter to the next 4096-byte boundary.

6 .data : AT(phys + (data - code)):

Defines the .data section and specifies that it should be loaded at the physical address phys plus the offset between the data and code symbols. The data = .; line marks the beginning of the .data section. The *(.data) directive includes all .data sections from input files. The . = ALIGN(4096); directive aligns the location counter to the next 4096-byte boundary.

7 .bss : AT(phys + (bss - code)):

Defines the .bss section and specifies that it should be loaded at the physical address phys plus the offset between the bss and code symbols. The bss = .; line marks the beginning of the .bss section. The *(.bss) directive includes all .bss sections from input files. The . = ALIGN(4096); directive aligns the location counter to the next 4096-byte boundary.

8 end = .;:

Defines a symbol end with the value of the current location counter, marking the end of all sections.

9 /DISCARD/:

Marks a discard section, which specifies sections to be discarded during linking. In this case, it discards .comment, .eh_frame, and .note.gnu.build-id sections.