Applications running in userspace utilize various file systems created by the kernel to communicate with the kernel itself. These file systems are virtual: no disk space is used for them. The content of these file systems resides in memory. These file systems must be mounted in the $LFS directory tree so the applications can find them in the chroot environment.
Begin by creating the directories on which these virtual file systems will be mounted:
mkdir -pv $LFS/{dev,proc,sys,run}
During a normal boot of an LFS system, the kernel automatically
mounts the devtmpfs file system on
the /dev directory; the kernel
creates device nodes on that virtual file system during the boot
process, or when a device is first detected or accessed. The udev
daemon may change the ownership or permissions of the device nodes
created by the kernel, and create new device nodes or symlinks, to
ease the work of distro maintainers and system administrators. (See
Section 9.3.2.2, “Device
Node Creation” for details.) If the host kernel supports
devtmpfs, we can simply mount a
devtmpfs at $LFS/dev and rely on the kernel to populate it.
But some host kernels lack devtmpfs
support; these host distros use different methods to create the
content of /dev. So the only
host-agnostic way to populate the $LFS/dev directory is by bind mounting the host
system's /dev directory. A bind mount
is a special type of mount that makes a directory subtree or a file
visible at some other location. Use the following command to do
this.
mount -v --bind /dev $LFS/dev
Now mount the remaining virtual kernel file systems:
mount -vt devpts devpts -o gid=5,mode=0620 $LFS/dev/pts mount -vt proc proc $LFS/proc mount -vt sysfs sysfs $LFS/sys mount -vt tmpfs tmpfs $LFS/run
The meaning of the mount options for devpts:
gid=5
This ensures that all devpts-created device nodes are owned
by group ID 5. This is the ID we will use later on for the
tty group. We use the group
ID instead of a name, since the host system might use a
different ID for its tty
group.
mode=0620
This ensures that all devpts-created device nodes have mode 0620 (user readable and writable, group writable). Together with the option above, this ensures that devpts will create device nodes that meet the requirements of grantpt(), meaning the Glibc pt_chown helper binary (which is not installed by default) is not necessary.
In some host systems, /dev/shm is a
symbolic link to a directory, typically /run/shm. The /run tmpfs was mounted above so in
this case only a directory needs to be created with the correct
permissions.
In other host systems /dev/shm is a
mount point for a tmpfs. In that case the mount of /dev above will
only create /dev/shm as a directory in the chroot environment. In
this situation we must explicitly mount a tmpfs:
if [ -h $LFS/dev/shm ]; then install -v -d -m 1777 $LFS$(realpath /dev/shm) else mount -vt tmpfs -o nosuid,nodev tmpfs $LFS/dev/shm fi