Debugging Linux kernel under QEMU with GDB: breakpoints are not hit

Question

I am using Ubuntu 24.04 system under qemu-system-aarch64 +gdb-multiarch to debug the kernel. But setting breakpoint doesn't make it to stop.

System information:

Ubuntu version: 22.04
Debug kernel version: 5.0.0

Qemu version:

qemu-system-aarch64 --version
QEMU emulator version 8.2.2 (Debian 1:8.2.2+ds-0ubuntu1.2)
Copyright (c) 2003-2023 Fabrice Bellard and the QEMU Project developers

GDB version:

gdb-multiarch --version
GNU gdb (Ubuntu 15.0.50.20240403-0ubuntu1) 15.0.50.20240403-git
Copyright (C) 2024 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Steps to reproduce:

Boot the system using qemu:

qemu-system-aarch64 -machine virt -cpu cortex-a57 -machine type=virt -m 1024 -smp 4 -kernel arch/arm64/boot/Image --append "noinitrd root=/dev/vda rw console=ttyAMA0 loglevel=8" -nographic -drive if=none,file=rootfs_ext4.img,id=hd0 -device virtio-blk-device,drive=hd0 --fsdev local,id=kmod_dev,path=$PWD/kmodules,security_model=none -device virtio-9p-device,fsdev=kmod_dev,mount_tag=kmod_mount -S -s

Use gdb-multiarch to connect and debug:

gdb-multiarch vmlinux
(gdb) target remote localhost:1234
Remote debugging using localhost:1234

0x0000000040000000 in ?? ()

(gdb) b start_kernel
Breakpoint 1 at 0xffff2000126704ec: file init/main.c, line 538.
(gdb) c
Continuing.

I tried replacing the debugged Linux kernel with version 5.10.9, but the problem still exists. I tried replacing the gdb tool with GNU gdb (Arm GNU Toolchain 13.3.Rel1 (Build arm-13.24)) 14.2.90.20240526-git, but the problem still exists.

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community
– Community Bot, Commented Sep 23, 2024 at 20:14
I have adjusted the title of your question so that it may be easier to find and more useful in the future. — Marco Bonelli
– Marco Bonelli, Commented Sep 29, 2024 at 20:49

Marco Bonelli · Accepted Answer · 2024-09-24 21:03:56Z

0

That's because you forgot to disable KASLR. The breakpoint you place will never be hit because the symbol address you get from vmlinux does not match the real one at runtime, as the kernel image will be moved somewhere else in memory when KASLR is applied at boot, before start_kernel() is invoked. Add nokaslr to your kernel command line (the -append option of QEMU) and you will hit the breakpoint as expected.

answered Sep 24, 2024 at 21:03

Marco Bonelli

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1 Comment

jie ren Over a year ago

Thank you very much. Adding kaslr successfully solved the problem.

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