Debugging linux kernel with gdb via QEMU issue

1
votes

I am newbie in Linux kernel and I'm trying to debug it with gdb via QEMU. My problem is gdb doesn't stop after break start_kernel. More details is below.

My host system is ArchLinux 5.0.10-arch1-1-ARCH x86_64.

Qemu is QEMU emulator version 4.0.0

gdb is GNU gdb (GDB) 8.2.1.

Debugged kernel is linux-4.20.12

I have done following steps:

  1. Compile kernel with 
CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_GDB_SCRIPTS=y
# CONFIG_DEBUG_INFO_REDUCED is not set
  1. Try to run LFS-8.4 system with it kernel via QEMU
qemu-system-x86_64 lfs-8.4-08052019.raw

System starts good.

  1. Run this system with stop CPU and gdbserver options via QEMU
qemu-system-x86_64 lfs-8.4-08052019.raw -S -s
  1. Start gdb
$ gdb
  1. Load symbols
(gdb) file /mnt/lfs/sources/linux-4.20.12/vmlinux
Reading symbols from /mnt/lfs/sources/linux-4.20.12/vmlinux...done.
  1. Connect to QEMU
(gdb) target remote localhost:1234
Remote debugging using localhost:1234
0x000000000000fff0 in cpu_hw_events ()
  1. Set break point
(gdb) break start_kernel
Breakpoint 1 at 0xffffffff82761ab5: file init/main.c, line 538.

(I have tried hardware break point hbreak, but result is the same)

  1. Continue
(gdb) c
Continuing.

Further system loads login prompt without any breaks and gdb doesn't show any new messages. What I do wrong?

UPD: starting QEMU with embed -kernel facility give the same result

qemu-system-x86_64 -kernel /mnt/lfs/sources/linux-4.20.12/arch/x86/boot/bzImage -append 'root=/dev/sda3' -drive file=lfs-8.4-08052019.raw -S -s

UPD2: I have tried to start QEMU without -S key and run target remote localhost:1234 in gdb immediately. QEMU have stopped loading at

Decompressing Linux... Parsing ELF... Performing relocations... done.
Booting kernel.

When I type s in gdb it says

(gdb) s
Cannot find bounds of current function

I suspect that debug symbols in vmlinux isn't related to bzImage. Maybe they was made wrong, but I don't how to make them another way.

UPD3: I have built kernel 2.6 in LFS-8.4 chroot environment. System isn't loaded, but kernel is successfully debugged by method described above! So, I think that it is problem of new kernels. Maybe I should switch off/on something in my 4.20.12 kernel, but I don't know what is exactly. For my purpose (reading Robert Love "Linux Kernel Development") kernel version 2.6 is enough.

1
linuxlinux-kernelgdb

1 Answers

1
votes

You need to pass nokaslr to kernel cmdline.

qemu-system-x86_64 -kernel /mnt/lfs/sources/linux-4.20.12/arch/x86/boot/bzImage -append 'root=/dev/sda3 nokaslr' -drive file=lfs-8.4-08052019.raw -S -s