Boot a Linux kernel in a VM without a dedicated root filesystem.
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README.md
eudyptula-boot
minimal-configuration

README.md

eudyptula-boot

eudyptula-boot boots a Linux kernel in a VM without a dedicated root filesystem. The root filesystem is the underlying root filesystem (or some pre-built chroot). This is a convenient way to do quick tests with a custom kernel.

The name comes from Eudyptula which is a genus for penguins. This is also the name of a challenge for the Linux kernel.

This utility is aimed at development only. This is a hack. It relies on AUFS/overlayfs and 9P to build the root filesystem from the running system.

Unfortunately, a change in overlayfs in Linux 4.2 prevents its use with 9P (change 4bacc9). This has been fixed in Linux 4.6 (change b403f0). The fix is also present in Linux 4.4.17+. As a workaround for affected kernels, see the --readonly option.

Also see this blog post for a quick presentation of this tool.

Usage

It is preferable to have a kernel with AUFS or OverlayFS enabled. Ubuntu and Debian kernels are patched to support AUFS. Since 3.18, vanilla kernels have OverlayFS built-in. Ubuntu kernels also come with OverlayFS support. Check you have one of those options:

CONFIG_AUFS_FS=y
CONFIG_OVERLAY_FS=y
CONFIG_OVERLAYFS_FS=y

Ensure you have the following options enabled (as a module or builtin):

CONFIG_9P_FS=y
CONFIG_NET_9P=y
CONFIG_NET_9P_VIRTIO=y
CONFIG_VIRTIO=y
CONFIG_VIRTIO_PCI=y
CONFIG_VIRTIO_CONSOLE=y

To get a somewhat minimal configuration, have a look at the minimal-configuration script.

Once compiled, the kernel needs to be installed in some work directory:

$ make modules_install install INSTALL_MOD_PATH=$WORK INSTALL_PATH=$WORK

Then, boot your kernel with:

$ eudyptula-boot --kernel $WORK/vmlinuz-3.15.0~rc5-02950-g7e61329b0c26

Use --help to get additional available options.

Before booting the kernel, the path to GDB socket will be displayed. You can use it by running gdb on vmlinux (which is somewhere in the source tree):

$ gdb vmlinux
GNU gdb (GDB) 7.4.1-debian
Reading symbols from /home/bernat/src/linux/vmlinux...done.
(gdb) target remote | socat UNIX:/path/to/vm-eudyptula-gdb.pipe -
Remote debugging using | socat UNIX:/path/to/vm-eudyptula-gdb.pipe -
native_safe_halt () at /home/bernat/src/linux/arch/x86/include/asm/irqflags.h:50
50  }
(gdb)

Starting from 4.12, KASLR may be troublesome when debugging. The easy way is to disable it by providing nokaslr flag to the kernel with -c nokaslr.

If you have modules, you also need to manually load debug symbols for them. In guest:

$ grep . /sys/module/vxlan/sections/{.text,.data,.bss}
/sys/module/vxlan/sections/.text:0xffffffffc0370000
/sys/module/vxlan/sections/.data:0xffffffffc0378000
/sys/module/vxlan/sections/.bss:0xffffffffc0378900

In GDB:

(gdb) add-symbol-file /usr/lib/debug/lib/modules/$(uname -r)/kernel/drivers/net/vxlan.ko \
                      0xffffffffc0370000 \
             -s .data 0xffffffffc0378000 \
             -s .bss  0xffffffffc0378900

This can be automated with lx-symbols command if you source vmlinux-gdb.py from a compiled kernel.

A serial port is also exported. It can be convenient for remote debugging of userland processes. More details can be found in this blog post (which also covers debugging the kernel).

QEMU monitor is also attached to a UNIX socket. You can use the following command to interact with it:

$ socat - UNIX:/path/to/vm-eudyptula-console.pipe
QEMU 2.0.0 monitor - type 'help' for more information
(qemu)

You can also get something similar to guestfish:

$ eudyptula-boot --qemu="-drive file=someimage.qcow2,media=disk,if=virtio"

With --extra-gettys, you can allocate additional consoles. To access one of them, use:

$ socat STDIO,echo=0,icanon=0 UNIX:/tmp/tmp.oCshB5ryj4/getty-1.pipe

Alternatives

Similar projects exist: