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 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 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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