Recently I wrote about how to debug the Linux kernel running under qemu.. There I showed how to give the emulated kernel to access the host's filesystem. But that access was read-only, as the consequences of giving the guest kernel write access to the filesystem of the host could be drastic. On the other hand, not being able to write to the filesystem limits the kinds of activities that can be debugged.

Fortunately, it's easy to extend the approach to provide a writable filesystem, without going to the trouble of doing a full Linux guest install. The debootstrap tool quickly builds a debian base system in a directory (and you don't even have to be running debian to use it — it's in the fedora repos). As root, do:

# debootstrap --variant=minbase sid guest-root-dir

(The --variant=minbase option requests an absolutely minimal system. Skip it for a not-quite-so-minimal system, or use the --include=pkg1,pkg2,... option to include other debian packages in the system.)

Then the options to qemu are changed slightly to use this new filesystem, and to allow read-write access to it. Also note that you now need to run qemu as root, so that it can set ownership on files within the exposed filesystem

# qemu-system-x86_64 -s -nographic \
        -kernel kernel tree path/arch/x86/boot/bzImage \
        -fsdev local,id=root,path=guest-root-dir,security_model=passthrough \
        -device virtio-9p-pci,fsdev=root,mount_tag=/dev/root \
        -append 'root=/dev/root rw rootfstype=9p rootflags=trans=virtio console=ttyS0 init=/bin/sh'

Then you can connect gdb to qemu as in the previous post.