In order for certain programs to function properly, the proc file system must be mounted and available from within the chroot'ed environment as well. It's not a problem to mount the proc file system (or any other file system for that matter) twice or even more than that.
If you're still logged in as user "lfs", you should log out and log in again as user root. The reason for this is simple: only root is allowed to mount filesystems and to run chroot.
The proc file system is mounted under $LFS/proc by running the following command. We'll also chown it to user root/group root while we're at it (the rest of the filesystem is chown'ed to root.root in a minute when we start with chapter 6).
chown root.root $LFS/proc && mount proc $LFS/proc -t proc |