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Re: Booting uncompressed kernel images

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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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Thread

Booting uncompressed kernel images Tiago Ilieve <tiago.myhro@gmail.com> - 2016-02-01 09:30 +0100
  Re: Booting uncompressed kernel images Ian Campbell <ijc@debian.org> - 2016-02-01 11:10 +0100
    Re: Booting uncompressed kernel images Ian Campbell <ijc@debian.org> - 2016-02-01 11:40 +0100
  Re: Booting uncompressed kernel images Jose R R <Jose.r.r@metztli-it.com> - 2016-02-01 11:40 +0100
    Re: Booting uncompressed kernel images Tiago Ilieve <tiago.myhro@gmail.com> - 2016-02-02 02:50 +0100
      Re: Booting uncompressed kernel images Ian Campbell <ijc@debian.org> - 2016-02-02 12:30 +0100
        Re: Booting uncompressed kernel images Tiago Ilieve <tiago.myhro@gmail.com> - 2016-02-02 18:40 +0100
          Re: Booting uncompressed kernel images Ian Campbell <ijc@debian.org> - 2016-02-02 18:40 +0100
            Re: Booting uncompressed kernel images Tiago Ilieve <tiago.myhro@gmail.com> - 2016-02-04 19:00 +0100
              Re: Booting uncompressed kernel images Ian Campbell <ijc@debian.org> - 2016-02-05 12:10 +0100
                Re: Booting uncompressed kernel images Ben Hutchings <ben@decadent.org.uk> - 2016-02-05 16:10 +0100
                Re: Booting uncompressed kernel images Andreas Ziegler <ml2013@andreas-ziegler.de> - 2016-02-05 16:10 +0100

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