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