The loopback file system device allows new, virtual file systems to be created, which provide access to existing files using alternate pathnames. Once the virtual file system is created, other file systems can be mounted within it, without affecting the original file system. However, file systems which are subsequently mounted onto the original file system are visible to the virtual file system, unless or until the corresponding mount point in the virtual file system is covered by a file system mounted there.

virtual is the mount point for the virtual file system. dir is the pathname of the existing file system. mflag specifies the mount options; the MS_DATA bit in mflag must be set. If the MS_RDONLY bit in mflag is not set, accesses to the loop back file system are the same as for the underlying file system. Otherwise, all accesses in the loopback file system will be read-only. All other mount(2) options are inherited from the underlying file systems.

A loopback mount of '/' onto /tmp/newroot allows the entire file system hierarchy to appear as if it were duplicated under /tmp/newroot, including any file systems mounted from remote NFS servers. All files would then be accessible either from a pathname relative to '/' or from a pathname relative to /tmp/newroot until such time as a file system is mounted in /tmp/newroot, or any of its subdirectories.

Loopback mounts of '/' can be performed in conjunction with the chroot(2) system call, to provide a complete virtual file system to a process or family of processes.

Recursive traversal of loopback mount points is not allowed. After the loopback mount of /tmp/newroot, the file /tmp/newroot/tmp/newroot does not contain yet another file system hierarchy; rather, it appears just as /tmp/newroot did before the loopback mount was performed (for example, as an empty directory).

Examples

lofiadm(1M), mount(1M), chroot(2), mount(2), sysfs(2), vfstab(4), lofi(7D)

Loopback mounts must be used with care; the potential for confusing users and applications is enormous. A loopback mount entry in /etc/vfstab must be placed after the mount points of both directories it depends on. This is most easily accomplished by making the loopback mount entry the last in /etc/vfstab.

Files can be modified on a read-only loopback mounted file system, and a loopback mounted file system can be unmounted even if there is an open regular file on that file system. The loopback file system works by shadowing directories of the underlying file system. Because no other file types are shadowed, the loopback file system can not enforce read-only access to non-directory files located on a read-only mounted loopback file system. Thus, write access to regular files located on a loopback mounted file system is determined by the underlying file system. In addition, the loopback file system can not correctly determine whether a loopback mounted file system can be unmounted or not. It can only detect when a directory is active or not, not when a file within a directory is active. Thus, a loopback mounted file system may be unmounted if there are no active directories on the file system, even if there are open files on the file system.