Re: Booting uncompressed kernel images
On Mon, 2016-02-01 at 06:03 -0200, Tiago Ilieve wrote:
> Hi,
>
> I have a scenario[1] where the default Linux kernel compressed with XZ
> (from Jessie and up) cannot be booted. The first thing that I've tried
> was to uncompress it using "extract-linux"[2] and it didn't worked by
> the time, so I decided to rebuild the entire "linux-image-*" package
> changing "CONFIG_KERNEL_XZ=y" to "CONFIG_KERNEL_GZIP=y". This, of
> course, implies that it would be needed to recompile the kernel every
> time a new version of the package is released, which is an overkill
> for a such simple requirement.
>
> Yesterday, Ben Hutchings itself suggested[3] me to write a hook that
> decompresses the kernel at package installation time, something which
> I find a great idea. The problem is that, again, I couldn't boot a
> machine (tried on VirtualBox and Xen) after uncompressing its
> "/boot/vmlinuz-*" using "extract-linux".
What does file(1) say about the resulting file? It should be a plain ELF
file of some sort.
I wouldn't expect such a kernel to be bootable by very much other than Xen
PV. It won't work as native for sure, nor as Xen HVM.
> I can generate an initrd file
> from this uncompressed image, "update-grub" detects it fine, but if I
> reboot it the following error appears:
>
> Loading Linux 3.16.0-4-amd64 ...
> error: invalid magic number.
> Loading initial ramdisk ...
> error: you need to load the kernel first.
Is this booting via pvgrub[1], if not then do you know how? What does the
grub.cfg stanza look like?
> - Is "extract-linux" stripping some essential information (the script
> looks for an offset to start the decompression process) from the
> kernel image that is needed to boot it later? If so, is there a way to
> recover and insert it on the uncompressed image?
It should just be the raw ELF file payload form the bzImage. Xen requires
some particular ELF notes to be present, but I very much doubt extract-
linux would be stripping those (since that would involve diving into the
contents of the payload)
Ian.
[1] http://wiki.xen.org/wiki/PvGrub
>
> Regards,
> Tiago.
>
> [1]: https://lists.debian.org/debian-cloud/2016/01/msg00052.html
> [2]: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plai
> n/scripts/extract-vmlinux
> [3]: https://lists.debian.org/debian-cloud/2016/01/msg00060.html
> [4]: http://www.he1ix.org/2015/01/creating-a-xen-domu-on-debian-squeeze-6
> -0-6/
> [5]: http://noone.org/blog/English/Computer/Debian/Running%20a%20Sid%20Do
> mU%20on%20a%20Squeeze%20Dom0.html
>
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