[dm-crypt] ing rootfs without initramfs

Milan Broz gmazyland at gmail.com
Sun Jul 21 10:47:17 CEST 2013

On 21.7.2013 7:40, Bryan Kadzban wrote:
> Milan Broz wrote:
>> On 07/20/2013 09:36 PM, ebelcrom ebelcrom wrote:
>>> I played around with dm-crypt without using initramfs for
>>> en-/decryption of my root file system. The rootfs is encrypted
>>> plain with cryptsetup and the key is stored at the disk containing
>>> the rootfs between MBR and the partition. The kernel parameter
>>> given to it from the bootloader is configured as it should be
>>> (cryptdevice, cryptkey, root mapper). The disk driver (loaded
>>> before) is built-in as well as dm-crypt (loaded after). The message
>>> I got at boot time is this (cr_rootfs is the encrypted rootfs):
>>> VFS: Cannot open root device "mapper/cr_rootfs" or
>>> unknown-block(0,0)
>>> According to some hints in the web there is no need to have an
>>> initramfs. Is that true? If yes what are the steps to get there and
>>> what should I keep into account?
>> I think the only possibility is to use GRUB2 which should understand
>> LUKS directly and boot from it. (Not sure about plain dmcrypt
>> device).
> So I've never tried it myself (I'm using a pretty simple initramfs I
> wrote in shell for my luks-rootfs setup), but I'm not sure how this can
> work.
> Because no bootloader mounts the rootfs.  They only find the kernel code
> (and, if configured, the initramfs image), load it (or them) into
> memory, and jump to the kernel's init code, transferring control of the
> machine to the kernel.  (There's a protocol to tell the kernel about the
> initramfs if one is present.)
> The kernel either runs the initramfs's /init program, or mounts the
> rootfs itself and runs /sbin/init.  (Or whatever you set init= to on the
> kernel command line.)
> (Plus there's the fact that the kernel can't automount luks.)

Yes, GRUB2 solve just initial kernel boot load, you cannot map any device-mapper
device (that's include crypt but also LVM etc) without userspace tools...

Seems I anwered different question, sorry :)

Anyway, there were tries to add kernel boot parameters for DM
e.g. http://article.gmane.org/gmane.linux.kernel/988034

But this wil not work for LUKS either without in-kernel LUKS implementation.
And for plain crypt you have to provide key on kernel line (quite insecure).

I think using some initramfs is the only solution now for mapping
encrypted root fs (for now).


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