gravechapa / multiboot2-elf64

multiboot2-elf64

An experimental Multiboot 2 crate for ELF-64 kernels. It's still incomplete, so please open an issue if you're missing some functionality. Contributions welcome!

It uses the Multiboot 1.6 specification at http://nongnu.askapache.com/grub/phcoder/multiboot.pdf and the ELF 64 specification at http://www.uclibc.org/docs/elf-64-gen.pdf.

Below is the draft for a blog post about this. I don't plan to finish it but maybe it's helpful as documentation.

The Multiboot 2 Information Structure

The Multiboot information structure looks like this:

Field	Type
total size	u32
reserved	u32
tags	variable
end tag = (0, 8)	(u32, u32)

There are many different types of tags, but they all have the same beginning:

Field	Type
type	u32
size	u32
other fields	variable

All tags are 8-byte aligned. The last tag must be the end tag, which is a tag of type 0 and size 8.

Tags

We are interested in two tags, the Elf-symbols tag and the memory map tag. For a full list of possible tags see section 3.4 in the Multiboot 2 specification (PDF).

The Elf-Symbols Tag

The Elf-symbols tag contains a list of all sections of the loaded ELF kernel. It has the following format:

Field	Type
type = 9	u32
size	u32
number of entries	u32
entry size	u32
string table	u32
section headers	variable

Note that this format differs from the description in the Multiboot specification because it seems to be wrong for ELF 64 kernels: The number of entries, entry size, and string table fields seem to be u32 instead of u16. The multiboot2.h file in the example section of the specification also specifies these fields as being u32, which suggests that the u16 fields are an editing error. The GRUB2 bootloader uses u32 fields, too.

The section headers are just copied from the ELF file, so we need to look at the ELF specification to find the corresponding structure definition. Our kernel is a 64-bit ELF file, so we need to look at the ELF-64 specification (PDF). According to section 4 and figure 3, a section header has the following format:

Field	Type	Value
name	u32	string table index
type	u32	0 (unused), 1 (section of program), 3 (string table), 8 (uninitialized section), etc.
flags	u64	0x1 (writable), 0x2 (loaded), 0x4 (executable), etc.
address	u64	virtual start address of section (0 if not loaded)
file offset	u64	offset (in bytes) of section contents in the file
size	u64	size of the section in bytes
link	u32	associated section (only for some section types)
info	u32	extra information (only for some section types)
address align	u64	required alignment of section (power of 2)
entry size	u64	contains the entry size for table sections (e.g. string table)

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.