freeloader/notes.md

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2024-08-01 01:20:06 +00:00
# Loader notes
## Modinfo
Loader must provide modinfo to kernel, a TLV structure
* Dump from normal FreeBSD with `sysctl debug.dump_modinfo`
* Tag is `MODINFO_*` or `MODINFO_METADATA | MODINFOMD_*`
* Tag and length are 4 bytes native endian
* Value is padded to align to `sizeof(size_t)`
* Strings are null-terminated
* Encodes multiple modules in sequence, separated by `MODINFO_NAME` string
### Fields
* `MODINFO_NAME`: string with path to file if available
* `MODINFO_TYPE`: string with type, like `elf kernel`
* `MODINFO_ADDR`: usize with base virtual address of object
* `MODINFO_SIZE`: usize with size of object, for elf based on end of final load section
* `MODINFOMD_EFI_MAP`: UEFI memory map, as comes from `GetMemoryMap`
* `MODINFOMD_EFI_FB`: Some structure describing UEFI framebuffer
* `MODINFOMD_KEYBUF`: `struct keybuf` object with cached keys, don't really need it
* `MODINFOMD_FW_HANDLE`: physical address of RuntimeServices system table
* `MODINFOMD_MODULEP`: Base physical address of modinfo, probably
* `MODINFOMD_KERNEND`: Last physical address of kernel, should be free after
* `MODINFOMD_HOWTO`: u32 with a bunch of bitflags that start with `RB_` in `sys/sys/reboot.h`
* `MODINFOMD_ELFHDR`: copy of the elf header
* `MODINFOMD_DYNAMIC`: base virtual address of `PT_DYNAMIC` segment
* `MODINFOMD_SSYM`: start virtual address of symtab/strtab
* symtab is section with `SHT_SYMTAB`, strtab is linked section
* Both have length prepended (as usize) and are aligned to usize
* `MODINFOMD_ESYM`: end virtual address of symbols
* `MODINFOMD_SHDR`: the section header table
## Kernel
Kernel must be loaded into memory at offset divisible by 2 MiB
* yes, the loader zeroes bss
## Architecture specific
Tons of arch-specific restrictions
### `x86_64`
* Pointers are passed as 32-bit, must be within the first 4G
* First 4G of virtual memory must be identity mapped
* Kernel virtual address must also be mapped to kernel (normally `0xffff_ffff_8000_0000`)