In previous post coreboot was configured and installed. Here we try to
establish good debugging environment for it. To create a good emulated
environment to debug, research and learn coreboot few tricks are required.
First of all we need to know how to run our emulated environment (qemu). What I
mean by that?
- load coreboot image (-bios option),
- freeze CPU at startup (-S),
- get appropriate feedback about virtual machine state (-d in_asm,cpu),
- set up remote gdb server to run qemu step by step (-s). So finally we get:
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qemu -bios src/coreboot/build/coreboot.rom -s -S -d in_asm,cpu -nographic
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We don’t need graphics so it also could be disable (-nographic). Run above
command and prepare debugging environment as described below.
- load bootblock file in gdb:
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file path/to/coreboot/build/bootblock.elf
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- use objdump to find out at what address .text, .bss and .data sections are:
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objdump -h src/coreboot/build/coreboot_ram|grep -E "text|bss|.data"
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my output looks like that:
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0 .text 00010810 00100000 00100000 00001000 2**2 3 .data 000004d8 001174e8
001174e8 000184e8 2**2 4 .bss 0000080c 001179c0 001179c0 000189c0 2**3
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- use above addresses to load symbols from
coreboot_ram file in gdb:
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add-symbol-file src/coreboot/build/coreboot_ram 0x00100000 -s .data
0x001174e8 -s .bss 0x001179c0
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- In another terminal or screen window
(use :e to reload qemu.log file after every instruction), in this file we will
get information about all registers of virtual machine
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EAX=00000000 EBX=00000000 ECX=00000000 EDX=00000633
ESI=00000000 EDI=00000000 EBP=00000000 ESP=00000000
EIP=0000fff0 EFL=00000002 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =0000 00000000 0000ffff 00009300
CS =f000 ffff0000 0000ffff 00009b00
SS =0000 00000000 0000ffff 00009300
DS =0000 00000000 0000ffff 00009300
FS =0000 00000000 0000ffff 00009300
GS =0000 00000000 0000ffff 00009300
LDT=0000 00000000 0000ffff 00008200
TR =0000 00000000 0000ffff 00008b00
GDT= 00000000 0000ffff
IDT= 00000000 0000ffff
CR0=60000010 CR2=00000000 CR3=00000000 CR4=00000000
DR0=00000000 DR1=00000000 DR2=00000000 DR3=00000000
DR6=ffff0ff0 DR7=00000400
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it means that your debugging environment was set correctly.
Piotr Król
Founder of 3mdeb, a passionate advocate for open-source firmware solutions, driven by a belief in transparency, innovation, and trustworthiness. Every day is a new opportunity to embody the company's vision, emphasizing user liberty, simplicity, and privacy. Beyond business, a casual chess and bridge player, finding peace in nature and nourishment in theology, philosophy, and psychology. A person striving to foster a healthy community, grounded in collaboration and shared growth, while nurturing a lifelong curiosity and a desire to deeply understand the world.
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