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  • 32-bit Chroot on CentOS

    August 21st, 2014
    tech, linux  [html]
    Let's say you want to build some 32-bit binaries for distribution but only have a 64-bit OS. You could try to set your build system up to make 32-bit builds, but if your build system is as complicated as ours is that sounds likely to go wrong. What if we just set up a 32-bit chroot? On Debian/Ubuntu this isn't very hard because this is how they generally build packages, and debootstrap and schroot can do most of the work for you. On CentOS/RHEL this is much more annoying, and I can't find anyone out there who has documented how to do this. So I figured I should at least write up what I did, in case someone (or me) later is trying to do the same thing:
    # setarch is a command that lets you make uname lie and say it's 32-bit
    sudo yum install setarch
    
    CHROOT_DIR=/var/chroot/centos_i386
    sudo mkdir -p $CHROOT_DIR
    sudo mkdir -p $CHROOT_DIR/var/lib/rpm
    
    # If there's a release since 5.10, you should update this url.
    CENTOS_RPM_URL="http://mirror.centos.org/centos/5/os/i386/CentOS/"
    CENTOS_RPM_NAME="centos-release-5-10.el5.centos.i386.rpm"
    wget "$CENTOS_RPM_URL$CENTOS_RPM_NAME"
    
    # When running on a 64-bit system, rpm will install 64-bit packages
    # even if you set $arch and $basearch like you're supposed to.
    # To force it to install 32-bit packages inside our chroot we have to
    # change /etc/rpm/platform.  While this is changed, don't run any
    # other system commands or they will get the architecture wrong.  I
    # don't know why rpm doesn't use uname like everything else; then we
    # could just use setarch.  For more details see this discussion.
    sudo cp /etc/rpm/platform /etc/rpm/platform.real
    echo i686-redhat-linux | sudo tee /etc/rpm/platform
    sudo rpm --rebuilddb --root=$CHROOT_DIR
    sudo rpm --root=$CHROOT_DIR --nodeps -i $CENTOS_RPM_NAME
    sudo yum --installroot=$CHROOT_DIR/ update
    sudo yum --installroot=$CHROOT_DIR/ install -y rpm-build yum
    sudo mv /etc/rpm/platform.real  /etc/rpm/platform
    
    # The base system is now installed on our chroot, and we've put the
    # platform file back.
    
    # Copying over resolv.conf makes networking work in the chroot.
    $ sudo cp /etc/resolv.conf $CHROOT_DIR/etc/resolv.conf
    
    # If you're setting up a chroot for security purposes, these next
    # steps are a bad idea.  I'm just using chroot for a build environment
    # so this is fine.  The users and groups within the chroot will be the
    # same as the ones outside, with the same passwords, and sudo will work.
    $ pushd $CHROOT_DIR/etc
    $ sudo rm group passwd
    $ for x in passwd shadow group gshadow hosts sudoers; do sudo ln /etc/$x; done
    $ popd
    
    # These mounts you'll need to do again on every reboot.  There's
    # probably also a way to do this with /etc/fstab.
    $ sudo mount -t proc proc $CHROOT_DIR/proc
    $ sudo mount -t sysfs none $CHROOT_DIR/sys
    $ sudo mount --bind /dev $CHROOT_DIR/dev
    $ sudo mount -t tmpfs tmpfs $CHROOT_DIR/dev/shm
    
    # Enter the chroot!
    $ sudo setarch i386 /usr/sbin/chroot $CHROOT_DIR/ /bin/bash -l
    
    # A few things to do now that we're in our chroot for the first time.
    # First, delete any rpm database files.  Otherwise you'll get errors
    # about them being corrput.  I think this is fine; as far as I can
    # tell they just get regenerated when they're needed.
    $ rm /var/lib/rpm/__db.00*
    
    # Install sudo, which isn't in the base install.
    $ yum install sudo
    
    # Switch to yourself.
    $ su yourusername
    
    # Now make yourself a homedir and go there:
    $ sudo mkdir $HOME
    $ sudo chown $USER $HOME
    $ cd ~
    
    # Give yourself a prompt that reminds you you're in the chroot.
    $ echo 'PS1='"'"'(32bit) \u@\h:\w\$ '"'" >> ~/.bashrc
    
    # Things are set up now.
    $ exit
    
    # Whenever you want to get back into the chroot, just run:
    $ sudo setarch i386 /usr/sbin/chroot $CHROOT_DIR/ /bin/bash -l
    $ su yourusername
    

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