I was trying to study the debugging of kernel using QEMU. I tried initially and failed due to the fact that there was no virtual file system. The answers to this post suggests that there should be a virtual file system. But it doesn't talk about how to create virtual FS for kernel debugging and how to pass it over to the qemu. Can you help me out?
Depending on the distribution you'd like to use, there are various ways to create a file system image, e.g. this article walks you through the laborious way to a "Linux from Scratch" system.
In general, you'd either create a QEMU image using
qemu-img, fetch some distribution's installation media and use QEMU with the installation medium to prepare the image (this page explains the process for Debian GNU/Linux) or use an image prepared by someone else.
This section of the QEMU Wikibook contains all the information you need.
As Gilles' answer to the linked question suggests, you don't need a full-blown root file system for testing, you could just use an
initrd image (say, Arch Linux's initrd like here)
QEMU + GDB step-by-step procedure tested on Ubuntu 16.10 host
To get started from scratch quickly I've made a minimal fully automated QEMU + Buildroot example at: https://github.com/cirosantilli/linux-kernel-module-cheat Major steps are covered below.
First get a root filesystem
rootfs.cpio.gz. If you need one, consider:
- a minimal
init-only executable image: Custom Linux Distro that runs just one program, nothing else | Unix & Linux Stack Exchange
- a Busybox interactive system: What is the smallest possible Linux implementation? | Unix & Linux Stack Exchange
Then on the Linux kernel:
git checkout v4.9 make mrproper make x86_64_defconfig cat <<EOF >.config-fragment CONFIG_DEBUG_INFO=y CONFIG_DEBUG_KERNEL=y CONFIG_GDB_SCRIPTS=y EOF ./scripts/kconfig/merge_config.sh .config .config-fragment make -j"$(nproc)" qemu-system-x86_64 -kernel arch/x86/boot/bzImage \ -initrd rootfs.cpio.gz -S -s
On another terminal, supposing you want to start debugging from
gdb \ -ex "add-auto-load-safe-path $(pwd)" \ -ex "file vmlinux" \ -ex 'set arch i386:x86-64:intel' \ -ex 'target remote localhost:1234' \ -ex 'break start_kernel' \ -ex 'continue' \ -ex 'disconnect' \ -ex 'set arch i386:x86-64' \ -ex 'target remote localhost:1234'
and we are done!!
For kernel modules see: How to debug Linux kernel modules with QEMU? | Stack Overflow
For Ubuntu 14.04, GDB 7.7.1,
hbreak was needed,
break software breakpoints were ignored. Not the case anymore in 16.10. See also: https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/901944
disconnect and what come after it are to work around the error:
- https://sourceware.org/bugzilla/show_bug.cgi?id=13984 might be a GDB bug
- gdb - Remote 'g' packet reply is too long | Stack Overflow
- http://wiki.osdev.org/QEMU_and_GDB_in_long_mode osdev.org is as usual an awesome source for these problems
- https://github.com/torvalds/linux/blob/v4.9/Documentation/dev-tools/gdb-kernel-debugging.rst official Linux kernel "documentation"
- How to debug the Linux kernel with GDB and QEMU? | Stack Overflow
- the Linux kernel does not support (and does not even compile without patches) with
-O0: How to de-optimize the Linux kernel to and compile it with -O0? | Stack Overflow
- GDB 7.11 will blow your memory on some types of tab completion, even after the
max-completionsfix: Tab completion interrupt for large binaries | Stack Overflow Likely some corner case which was not covered in that patch. So an
ulimit -Sv 500000is a wise action before debugging. Blew up specifically when I tab completed
sys_execveas in: Can the sys_execve() system call in the Linux kernel receive both absolute or relative paths? | Stack Overflow