Workshop 2.3

Let's run the bootloader/OS

Content

• coreboot loader
• Payload types
• Libpayload
• Native Graphics Init
• Hands-on: LinuxBoot
• Hands-on: SeaBIOS with secondary payload
• Hands-on: TianoCore

Payload types

• Static ELF binaries
• Flat binaries
• bzImage
• uImage (FIT)
• libpayload based

Static ELF binaries

• Converted to Simple ELF (SELF) by cbfstool
• Fixed position in physical address space
• Not relocatable
• SELF segments can be compressed
• Possible payloads:
  • SeaBIOS
  • GRUB
  • Tianocore
  • depthcharge
  • memtest86+

Flat binaries

• Simple entry point
• No conversion
• Supported payloads:
  • uboot

bzImage

• x86 only
• Is already compressed
• Requires trampoline code
• Follows Linux aarch64 calling conventions
• Has a special loader
• vmlinux, vmlinuz, Image isn't supported

bzImage

• Graphics init in coreboot is optional
• Graphics init in kernel is done late
• No BIOS or EFI services
• Flash as size limitation
• Kernel update requires flashing a new firmware

but ...

uImage (FIT)

• ARM64 only
• Multiple sections (kernel, initrd, devicetree, config, ...)
• Has optional compression
• Includes a DTB
• Follows Linux aarch64 calling conventions
• Has a special loader

libpayload

• Bare metal library used by some payloads
• Custom USB driver
• Custom PS/2 driver
• Simple framebuffer driver
• Understands CBFS/CBMEM/CBTABLES ...
• Provides basic libc functions
• ASM stub, payload can be written in pure C

libpayload

•  Don't expect anything
  • No FPU initialized
  • No SSE enabled
  • Single threaded
  • No interrupts
• Has to deal with hardware bugs
• No filesystem drivers
• on x86 32-bit protected mode only
• Single threaded
• No ACPI support
• Simple powermangement in the works
• No filesystem driver

but ...

libpayload

• depthcharge
• nvramcui
• tint
• coreinfo

Payloads that use libpayload

Native graphics init

Native graphics init

• 64 KiB VGA Option ROM BLOB
• 16-bit x86 instructions
• No signature
• Optional YABEL emulator in coreboot
• not redistributable

Situation in comercial BIOS:

Situation in comercial EFI:

• GOP driver BLOB
• Signed code
• Can't be run in coreboot
• not redistributable

Community efforts in coreboot:

• Reverse engineered graphics init using YABEL
• Only for integrated LVDS panels
• Only for some panels ...

Native graphics init

• Written in ADA SPARK
• Supports all platforms starting from gm45
• Supports all connector types

Replaced by libgfxinit on x86:

• Requires coreboot support in the payload or OS
• No resolution switching at runtime
• Can be skipped if payload does graphics init
  • SeaBIOS using VGA Option ROM
  • TianoCore using GOP driver
  • Linux using KMS and VBT

In general:

coreboot framebuffer info

1. Provides a framebuffer base address and metadata
2. Only readable by payloads that know about coreboot
3. No support in Microsoft/Apple
4. Payloads that support reading the framebuffer info:
   • Libpayload
   • GRUB
   • SeaBIOS
   • TianoCore
   • GNU/Linux

#define LB_TAG_FRAMEBUFFER    0x0012
struct lb_framebuffer {
    uint32_t tag;
    uint32_t size;

    uint64_t physical_address;
    uint32_t x_resolution;
    uint32_t y_resolution;
    uint32_t bytes_per_line;
    uint8_t bits_per_pixel;
    uint8_t red_mask_pos;
    uint8_t red_mask_size;
    uint8_t green_mask_pos;
    uint8_t green_mask_size;
    uint8_t blue_mask_pos;
    uint8_t blue_mask_size;
    uint8_t reserved_mask_pos;
    uint8_t reserved_mask_size;
};

LinuxBoot

LinuxBoot

1. Requires go compiler
2. Compiles the kernel and u-root
3.  

facts

LinuxBoot

• Well tested drivers for anything
  • Graphics
  • USB
  • Input device
  • Block devices
  • SCSI devices
  • Network
  • EC/SuperIO
• Support for (almost) all filesystems
• Easy to write application programms
  • You don't need firmware engineers
• Don't have to deal with hardware errata
• Multithreaded
• Power managment
• Memory management
• ...

pro

con

• Requires external toolchain (non x86)
• No EFI boot support (yet)
• Big payload size
• Bigger attack surfaces

Hands-on: Build LinuxBoot

SeaBIOS

• x86 only
• CSM for TianoCore
• Supports secondary payloads
  • coreinfo
  • tint
  • nvramcui
  • memtest86
• Build using coreboot toolchain

facts

• payloads stored in CBFS
• loaded by a payload

secondary payloads:

SeaBios

• CSM for TianoCore
• Build using coreboot toolchain
• TPM support
• PS/2 driver
• Provides all common 16bit BIOS services
• SeaVGABios for "Native graphics Init"

pro

• Only text menu
• Runs unsigned Option ROMs
• BIOS services are 16bit real mode
• No Secure Boot
• Runs unsigned code from MBRs
• No mouse support

con

SeaVGABios

• 16bit VGA Option ROM
• Provides VBE 2.0 interface
• Advertises coreboot's framebuffer info

facts

SeaVGABios

• Code doesn't interact with hardware
• Legacy application do work with coreboot's graphics init

pro

con

• No mode switching
• No legacy VGA modes
• RGB8 pallette only

Hands-on: Build SeaBIOS with secondary payloads

Hands-on: Build SeaBIOS with secondary payloads

Hands-on: Build SeaBIOS with secondary payloads

Hands-on: Build SeaBIOS with secondary payloads

Note: memtest86 is VGA text mode only

Hands-on: Build SeaBIOS with secondary payloads

enable CMOS support for nvramcui

• Run coreinfo from SeaBIOS
• Run nvramtool from SeaBIOS in "high-resolution" framebuffer
• Run memtest86+ from SeaBIOS in text mode

Task:

Hands-on: Build SeaBIOS with secondary payloads

TianoCore

• Intel Open Source UEFI reference implementation
• integrated into x86 coreboot build system
• Only graphical menu
• SeaBIOS as CSM
• Build using an external toolchain
• Includes CorebootPkg
• No Secure Boot support yet for coreboot
• No persistent NVRAM support for coreboot

Hands-on: Build Tianocore in QEMU

Hands-on: Build Tianocore in QEMU

Select Tianocore as payload and select "debug build":

Select "high-resolution" frambuffer in Devices > Display :

Hands-on: Build Tianocore in QEMU

Remove the file:

$ rm payloads/external/tianocore/patches/05_CorebootPayloadPkg_noserial.patch

Build coreboot.rom

Run it in qemu:

 $ make

$ qemu-system-x64_86 -m 2048 -M q35 -bios build/coreboot.rom