##################################################################################
## ##
## 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.
