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@@ -5,4 +5,63 @@ Contributions welcome! **If someone wants to maintain this crate, please contact
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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.
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-Note that the multiboot specification for the ELF-sections tag seems to be wrong for ELF 64 kernels: The `num`, `entsize`, and `shndx` fields seem to be `u32` instead of `u16` (but I'm not sure on this).
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+Below is the draft for a blog post about this. I don't plan to finish it but maybe it's helpful as documentation.
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+
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+## The Multiboot 2 Information Structure
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+The Multiboot information structure looks like this:
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+
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+Field | Type
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+---------------- | -----------
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+total size | u32
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+reserved | u32
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+tags | variable
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+end tag = (0, 8) | (u32, u32)
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+
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+There are many different types of tags, but they all have the same beginning:
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+Field | Type
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+------------- | -----------------
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+type | u32
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+size | u32
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+other fields | variable
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+
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+All tags are 8-byte aligned. The last tag must be the _end tag_, which is a tag of type `0` and size `8`.
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+
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+## Tags
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+
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+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][Multiboot 2]).
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+[Multiboot 2]: http://nongnu.askapache.com/grub/phcoder/multiboot.pdf
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+
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+### The Elf-Symbols Tag
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+The Elf-symbols tag contains a list of all sections of the loaded [ELF] kernel. It has the following format:
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+[ELF]: http://www.uclibc.org/docs/elf-64-gen.pdf
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+
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+Field | Type
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+--------------------------- | -----------------
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+type = 9 | u32
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+size | u32
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+number of entries | u32
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+entry size | u32
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+string table | u32
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+section headers | variable
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+
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+Note that this format differs from the description in the Multiboot specification because if 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` (but I'm not sure on this).
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+
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+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][ELF specification]). According to section 4 and figure 3, a section header has the following format:
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+
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+[ELF specification]: http://www.uclibc.org/docs/elf-64-gen.pdf
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+
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+Field | Type | Value
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+--------------------------- | ---------------- | -----------
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+name | u32 | string table index
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+type | u32 | `0` (unused), `1` (section of program), `3` (string table), `8` (uninitialized section), etc.
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+flags | u64 | `0x1` (writable), `0x2` (loaded), `0x4` (executable), etc.
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+address | u64 | virtual start address of section (0 if not loaded)
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+file offset | u64 | offset (in bytes) of section contents in the file
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+size | u64 | size of the section in bytes
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+link | u32 | associated section (only for some section types)
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+info | u32 | extra information (only for some section types)
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+address align | u64 | required alignment of section (power of 2)
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+entry size | u64 | contains the entry size for table sections (e.g. string table)
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Philipp Oppermann