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To loot you first need to boot
43
Way back in 2.5.64
bzImage
`dd if=bzImage of=/dev/sda`
wait, what?
_start()
bootsect.o
setup_sects
- Relocate itself at 0x90000
- Load setup.o at 0x90200
- Load system at 0x100000
To loot, you first need to boot
start_of_setup()
setup.o
- Check memory layout
- Setup idt/gdt
- Switch to protected mode
- Jump at 0x100000
proot
startup_32()
decompress_kernel()
{head,misc}.o
- Decompress the kernel in-place
- Re-jump at 0x100000 into startup_32
The case of piggy.o
As of 3.2.55
setup_sects
start_of_setup()
main()
setup_video()
startup_32()
decompress_kernel()
- dummy bootsect.S
- setup.S moved to main.c
- creation of header.S
linux!
Booting like normal people
- Load real-mode code at an offset in low-memory (below 1Mo)
- Load command-line right behind the real-mode code
- Load the vmlinux.bin.gz file above 0x100000
- Jump over the dummy MBR into _start()
- Jump directly into start_of_setup(), which calls main
How do I do?
Basic concepts
The UEFI boot stub
Booting for men, by men
UEFI: Introduction
Encapsulated in Portable Executable file (PE)
Easy development possible thanks to gnu-efi
Sample application:
Since linux 3.3
gnu-efi is for noobs
hand-made PE headers
Legacy Boot: Conclusion
Three types of application:
- Simple app (type=10)
- Boot Services (type=11)
- Runtime Services (type=12)
Services implement protocols (drivers) and handles (devices)
Apps stored on EFI fat32 partition of type 0xEF
- EFI by Intel in 2001 for IA64
- Goals: get rid of 16 to 32 to 64 bits booting, provide user-friendly API, unify boot procedure
- UEFI in 2005 by major actors: Intel, AMD, Dell, Apple, IBM, Microsoft, Phoenix...
Boot manager using NvRAM:
Boot order defaults to:
- Try to boot on floppy
- Try to boot on hard drive
- Try to boot on NIC0
- Run a shell application
Editable via `efibootmgr` and efivars or efivarfs modules
EFI_STATUS efi_main(EFI_HANDLE image_handle, EFI_SYSTEM_TABLE *systab)
{
SIMPLE_TEXT_OUTPUT_INTERFACE *conout;
InitializeLib(image_handle, systab);
conout = systab->ConOut;
uefi_call_wrapper(conout->OutputString, 2, conout, (CHAR16 *)L"Hello World!\n\r");
}
echo "CONFIG_EFI_STUB=y" >> .config
make -j8
mv arch/x86/boot/bzImage /boot/efi/EFI/bzImage.efi
cp /boot/initramfs-linux.img /boot/efi/EFI/initramfs.img
reboot
bzImage.efi root=/dev/sda2 initrd=EFI\initramfs.img