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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 2.0 specification at https://www.gnu.org/software/grub/manual/multiboot2/multiboot.html 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.


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)


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