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
I’m into open-source firmware, drawing inspiration from the classic hacker ethos of collaborative innovation and transparency. At 3mdeb, we’re excited about our two main projects: Zarhus OS, an Embedded Linux distribution based on Yocto, and Dasharo, a coreboot downstream. Both are about the value of open development, embedded firmware resilience, platform security transparency, the right to repair, and digital sovereignty.
Community is at the center of our activity. By being involved in the Dasharo Users Group, Dasharo Developers vPub, and events like the Qubes OS Summit, we connect with others who share our passion for open-source.
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