qemu and chroot

[Erez D]

ok, it now works
/proc/sys/fs/binfmt_misc/qemu-arm was missing,
internet searc told me to look for 'binfmt-support' pkg, however i
could not find none for centos6
so as chrooted systems share the same kernel (just need to mount /proc
under the chroot dir), I chroot to my wheezy_i686 (i have some chroots
for testing other disros), and there i did apt-get install
binfmt-support qemu-user-static, and update-binfmts --display

now i have /proc/sys/fs/binfmt_misc/qemu-arm. amd everything works again


(i do not know if this is permenent or will require redoing after
reboot), but i will check it at next reboot (somthing like in 6 months
;-)

thanks
erez

On Tue, May 20, 2014 at 11:22 AM, Tzafrir Cohen <tzafrir at cohens.org.il> wrote:
> On Tue, May 20, 2014 at 09:14:16AM +0300, Erez D wrote:
>> I am using centos 6 and  developing for an armel platform
>>
>> i created a rootfs using multistrap/debbootstrap
>>
>> i copied qemu-arm-static to rootfs/usr/bin/qemu-arm-static
>
> There's something missing from your description. I suspect you forgot to
> mention it: debootstrap's run can be broken to two parts: one that
> downloads everything, and the second stage that needs to run inside the
> chroot. In that case:
>
>   debootstrap --foreign [--arch=] [rest of parameters]
>   chroot to/chroot
>   ./debootstrap --second-stage
>
> At least in Debian, the package qemu-user-static includes the wrapper
> qemu-debootstrap to do just that, and also copy the required
> qemu-user-static.
>
>>
>> and i was astonished that doing just 'chroot rootfs' worked, without
>> explicitly telling 'chroot' to use qemu-arm-static - somehow it decided
>> automatically to run everything under qemu-arm-static without me telling it
>> to.
>>
>>
>> after a restart of the server. rootfs does not work anymore automatically,
>> i get a "chroot: failed to run command `/bin/bash': Exec format error"
>> doing "chroot rootfs /usr/bin/qemu-arm-static /bin/bash" does chroot, but
>> i get : "bash: /bin/cat: cannot execute binary file" (although
>> rootfs/bin/cat is a perfectly ok armel binary, tested on the armel target).
>> i also checked the md5sum of the rootfs/qemu-arm-static binary, and it is ok
>
> A chroot does not replace the kernel. It's running on your kernel and
> that kernel does not natively support the armel binaries.
>
> In Debian, the package qemu-user-static registers foreign Linux ELF
> formats. So maybe you forgot this is needed. Specifically:
>
> $ cat /proc/sys/fs/binfmt_misc/qemu-arm
> enabled
> interpreter /usr/bin/qemu-arm-static
> flags:
> offset 0
> magic 7f454c4601010100000000000000000002002800
> mask ffffffffffffff00fffffffffffffffffeffffff
>
> --
> Tzafrir Cohen         | tzafrir at jabber.org | VIM is
> http://tzafrir.org.il |                    | a Mutt's
> tzafrir at cohens.org.il |                    |  best
> tzafrir at debian.org    |                    | friend
>
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