freeloader/notes.md

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)