qemu and chroot

[Tzafrir Cohen]

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

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