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Contents
  1. 1. EFI overview
  2. 2. EFI boot stub
  3. 3. Build EFI shell App
  • EFI overview
  • Boot ubuntu in EFI shell.
  • Build EFI shell App.

EFI overview

EFI boot stub

  • More info

  • Preparation

    • build kernel

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      make-kpkg -j 8 --rootcmd fakeroot --initrd --append-to-version=-custom kernel_image kernel_headers

    • copy efi, in EFI shell only /boot/efi/ will be accessed

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      sudo cp arch/x86/boot/bzImage /boot/efi/EFI/BOOT/bzImage.efi

  • Boot into initramfs in EFI shell

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    FS1:
    cd EFI
    cd BOOT
    bzImage.efi initrd=\EFI\BOOT\initrd.img-4.13.0-custom

  • Boot rootfs in EFI shell

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    FS1:
    cd EFI
    cd BOOT
    bzImage.efi initrd=\EFI\BOOT\initrd.img-4.13.0-custom root=/dev/sda2
    • by default the sda is separated as below in Ubuntu
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      /dev/sda1       2048   1050623   1048576   512M EFI System
      /dev/sda2    1050624 217819135 216768512 103.4G Linux filesystem
      /dev/sda3  217819136 234440703  16621568   7.9G Linux swap

Build EFI shell App

  • Create main.c

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    #include <efi.h>
    #include <efilib.h>

    EFI_STATUS
    efi_main (EFI_HANDLE image_handle, EFI_SYSTEM_TABLE *systab)
    {
    	EFI_STATUS status;
    	EFI_GRAPHICS_OUTPUT_PROTOCOL *gop;
    	EFI_GUID gop_guid = EFI_GRAPHICS_OUTPUT_PROTOCOL_GUID;

    	InitializeLib(image_handle, systab);

    	status = uefi_call_wrapper(systab->BootServices->LocateProtocol,
    			3,
    			&gop_guid,
    			NULL,
    			&gop);

    	Print(L"Framebuffer base is at %lx\n", gop->Mode->FrameBufferBase);

    	return status;
    }

  • Create Makefile(take care of in the file)

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    ARCH            = $(shell uname -m | sed s,i[3456789]86,ia32,)

    OBJS            = main.o
    TARGET          = check_fb

    EFIINC          = /usr/include/efi
    EFIINCS         = -I$(EFIINC) -I$(EFIINC)/$(ARCH) -I$(EFIINC)/protocol
    LIB             = /usr/lib
    EFILIB          = /usr/lib
    EFI_CRT_OBJS    = $(EFILIB)/crt0-efi-$(ARCH).o
    EFI_LDS         = $(EFILIB)/elf_$(ARCH)_efi.lds

    CFLAGS          = $(EFIINCS) -fno-stack-protector -fpic \
    		-fshort-wchar -mno-red-zone -Wall 
    ifeq ($(ARCH),x86_64)
    	CFLAGS += -DEFI_FUNCTION_WRAPPER
    endif

    LDFLAGS         = -nostdlib -znocombreloc -T $(EFI_LDS) -shared \
    		  -Bsymbolic -L $(EFILIB) -L $(LIB) $(EFI_CRT_OBJS) 

    		  all: $(TARGET).efi

    $(TARGET).so: $(OBJS)
    	ld $(LDFLAGS) $(OBJS) -o $@ -lefi -lgnuefi

    %.efi: %.so
    	objcopy -j .text -j .sdata -j .data -j .dynamic \
    		-j .dynsym  -j .rel -j .rela -j .reloc \
    		--target=efi-app-$(ARCH) $^ $@

  • make it

  • copy it to /boot/BOOT/efi or an USB storage, any of which you can access in EFI shell

  • run it directly

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    check_fb.efi
Contents
  1. 1. EFI overview
  2. 2. EFI boot stub
  3. 3. Build EFI shell App