summaryrefslogtreecommitdiffstats
path: root/documentation/GNU-efi
blob: 1efe81cabe216bb8c82d4a049edab810c4c9dcfe (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
##################################################################################
##										##
##		BUILDING HELLOWORLD.EFI WIH GNU-EFI				##	
##										##
##################################################################################

#################################################################################
#				SUMMARY						#
#################################################################################

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) $^ $@




#################################################################################
#		https://wiki.osdev.org/GNU-EFI					# 
#################################################################################

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-table hello.efi (ImageBase+text-off) -s .data (ImageBase+data-off)	*
*********************************************************************************