path: root/documentation/GNU-efi

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##		BUILDING HELLOWORLD.EFI WIH GNU-EFI				##	
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#				SUMMARY						#
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Minimal working example:

	hello.c
---------------------------------------------------------------------------------
#include <efi.h>
#include <efilib.h>

EFI_STATUS
EFIAPI
efi_main (EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable) {

    InitializeLib(ImageHandle, SystemTable);
    Print(L"Hello, world!\n");

    return EFI_SUCCESS;
}


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

OBJS        = hello.o
TARGET        = hello.efi

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

CFLAGS        = $(EFFINCS) -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) $(EFI_CRT_OBJS)

all: $(TARGET)

hello.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) $^ $@

-----
Now expanded version in git

Get ImageBase:
	Run ./test.sh and then hello.efi...this prints ImageBase

Get Offsets:
	GDB
	file hello.efi
	-> get text and data offset
	file
	add-symbol-file hello.efi (ImageBase+text-off) -s .data (ImageBase+data-off)


#################################################################################
#		https://wiki.osdev.org/GNU-EFI					# 
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GNU-EFI is a very lightweight developing environment to create UEFI applications.
It is a set of libraries and headers for compiling UEFI applications with a
system's native GCC.

You can use host native compiler, then convert resulting ELF into UEFI-compatible
PE.
	OR
Use GCC Cross-Compiler generating PE directly.

*********************************************************************************
* $ git clone https://git.code.sf.net/p/gnu-efi/code gnu-efi			*
* $ cd gnu-efi									*
* $ make									*
*********************************************************************************

This should create
* crt0-efi-x86_64.o:
	A CRT0 (C runtime initialization code) that will call the 
	"efi_main" function

* libgnuefi.a:
	A library containing a single function (_relocate)
	that is used by the CRT0

* (optional) libefi.a:
	A library containing convenience functions like CRC computation, string
	length calculation, and easy text printing

HEADERS can be used from:
* /usr/include/efi   (updated to the latest)
* from EDK2 package
* Or from gnu-efi/inc

LINKER SCRIPT:
* gnu-efi/gnuefi/elf_x86_64_efi.lds
	OR
* /usr/lib/elf_x86_64_efi.lds

COMPILATION:
$ gcc
	-Ignu-efi-dir/inc		||	set this to the efi headers directory

	-fpic				||	UEFI PE executable must be relocatable

	-ffreestanding			||	there's no hosted gcc environment,
						we don't have libc
	-fno-stack-protector		=	
	-fno-stack-check		||	stack must be strictly used,
					||	no additional canaries or
					||	pre-allocated local variable
					||	space allowed
	-mno-red-zone			=

	-fshort-wchar			||	It is very important that UEFI
					||	uses 16bit characters
					||	(wide-characters or wchar_t,
					||	defined as CHAR16 in efi headers

	-maccumulate-outgoing-args	|| 	function calls must include the
					||	number of argumnets passed to the
					|| 	functions

	-c main.c -o main.o

LINKING:
$ ld 	-shared -Bsymbolic -Lgnu-efi-dir/x86_64/lib -Lgnu-efi-dir/x86_64/gnuefi \
	-Tgnu-efi-dir/gnuefi/elf_x86_64_efi.lds \
	gnu-efi-dir/x86_64/gnuefi/crt0-efi-x86_64.o \
	main.o -o main.so -lgnuefi -lefi

	-shared -Bsymbolic		|| tell GNU ld to create so (shared library)

	-L and -T			|| Where to find the static GNU-EFI libraries
					|| (.a) and the linker script

	.o				|| it is important to specify crt0 as the
					|| first. Should work as the last too, but
					|| some had problems

	-l				|| linking with gnuefi is a must, as that 
					|| contains the relocation code. Linking
					|| with efi is optional, but recommended

CONVERT CHARED OBJECT TO EFI EXECUTABLE
$ objcopy -j .text -j .sdata -j .data -j .dynamic -j .dynsym -j .rel -j .rela -j .rel.*
	-j .rela.* -j .reloc --target efi-app-x86_64 --subsystem=10 main.so main.efi

	-j				|| which sections to keep during convertion

	--target efi-app-x86_64		|| tells objcop to generate a PE32+ format,
					|| with architecture code 0x8664

	--subsystem=10:			|| most important. Sets file type to UEFI
					|| executable in the PE header

Now you can copy main.efi to your EFI System Partition, and after boot run it
from the EFI Shell. Or you can rename it to EFI\BOOT\BOOTX64.EFI and it should
be executed automatically on boot. 


LIBEFI.A

Has wrappers for the most common UEFI functions, but you might need to call
something not covered. For completeness, it provides:

uefi_call_wrapper(func, numarg, ...);

For example, the "Print" function used in our main.c and which accepts printf
compatible arguments, is under the hood nothing else than a call to:

uefi_call_wrapper(ST->ConOut->OutputString, 2, ST->ConOut, buffer);

The biggest advantage of 'uefi_call_wrapper_ is that doesn't matter what ABI
your gcc is using, it will always correctly translate thet into UEFI ABI. If,
and only if you've used the correct gcc options, then you should be able to
make the same call as:

ST->ConOut->OutputString(ST->ConOut, buffer);


#################################################################################
#	https://wiki.osdev.org/Debugging_UEFI_applications_with_GDB		#
#################################################################################

Makefile at:
https://sourceforge.net/p/ast-phoenix/code/ci/master/tree/kernel/boot/Makefile#l72

EFI firmware is unable to launch binaries with debug sections. What you need is
to create two EFI binaries - one with only required sections to upload it to
target system and another one with debug symbols to use with GDB. Actually you 
just need to run objcopy utility twice with different set of sections to copy and
different output files. (See Makefile example)

To load image with symbols to relocated addresses for .text and .data sections,
you need to add ImageBase address to their offsets:

*********************************************************************************
* # gdb hello.efi								*
* (gdb) info files								*
* ...										*
* Entry point: 0x3000								*
* 3000 - ... is .text								*
* c00 - ... is .data								*
* (gdb) file			|| unload file					*
* add-symbol-file hello.efi (ImageBase+text-off) -s .data (ImageBase+data-off)	*
*********************************************************************************

#################################################################################
#		https://www.rodsbooks.com/efi-programming/hello.html		#
#################################################################################

You should not normally include regular C header files, such as stdlib.h,
because most of  these header files define data types and functions that are used
by the C library. This library is not available in EFI.

************************************
* efi.h and efilib.h always needed *
************************************

Entry point: efi_main() in GNU-efi

CFLAGS:
* -fno-stack-protector:
	Stack protection isn't suppoerted by EFI, so there's no point in
	building a binary with this feature active.

* -fpic:
	EFI requires that code be position-independet, hence the use
	of this option.

* -fshort-wchar:
	GCC defines the wchar_t type to be 32 bits by default, but EFI requires
	it to be 16 bits for 16-bit strings to work correctly.

* fmno-red-zone:
	On x86-64 systems, the red zone is an area that follows the stack pointer
	that can be used for temporary variables. The EFI may modify this area,
	though, so it's not safe to use, and you must compile EFI binaries with
	this option.

* -Wall:
	When developing EFI applications, you might want to pay extra attention to
	compiler warnings, and this switch (which causes warnings to be treated as
	errors) can help.

* -DEFI_FUNCTION_WRAPPER:
	This option is required on the x86_64 platform, but is not defined on the
	32-bit x86 platform. It relates to th calling conventions for EFI functions,
	described on the Using EFI Services page.

LDFLAGS:
* -nostdlib:
	An EFI application should not be linked against standard libraries, and this
	argument accomplishes this goal.

* -nocombreloc:
	This argument causes the linker to not combine relocation sections.

* -T $(EFI_LDS):
	To create an EFI binary, a non-standard linker script must be used, and this
	option tells ld where to find it.

* -shared:
	Even with GNU-EFI's new linker script, ld can't create the final executable.
	Instead, it creates a shared library, which is subsequently
	turned into the final binary.

* -Bsymbolic:
	This option causes references to global symbols to be bound to the
	definitions within the shared library.