How to mount a local directory without root

Question

Say I have a folder which is completely readable by my user. I want it to be mounted to my home folder. I can't use a symlink because I want the files to be exposed at ~/ and I want other programs "not to know" that it's a mount. Is there a fuse program that implements this? The issue with mount is that it requires root privileges.

I'd also appreciate, if I'd be able to mount this directory, not on boot, but on demand - that's because my ~/ is encrypted unless I'm logged in.

Answer 1

Various FUSE utilities can do that. For instance, bindfs can mount a directory to a different location, similarly to what mount --bind would do.

bindfs --no-allow-other /source/directory /mount/point

The --no-allow-other option is needed unless you uncomment user_allow_other in /etc/fuse.conf (but see, below, the note about libfuse for the security implications).

Unmount with:

fusermount -u /mount/point

Note that FUSE file systems currently have known issues/limitations. It's worth mentioning, among other things, the performance cost, the impossibillity to use inotify to monitor file system events occurring on the source file system and the security implications listed in the libfuse GitHub repository's README.

On Linux, a bind mount performed as mount --bind would not have these downsides. But it would require root privileges at least once, to edit your fstab.
Refer to the linked Q/A for a much more detailed coverage of the subject.

Answer 2

You could set up /etc/fstab once, with the following entry:

/path/to/original/dir  /path/to/bind/dir  none  bind,rw,user,noauto  0 0 

The mount options specify the following things, in order:

• bind indicates that this entry is a bind mount.
• rw specifies that the entry will be mounted in read-write mode.
• user allows any non-root user to mount the filesystem.
• noauto specifies that this entry should not be automatically mounted with mount -a and at boot time.

You'll need to set this up once using root privileges. Once the entry is in place, you can perform the mount as a non-root user. Simply run mount /path/to/bind/dir.

A couple of points to note:

• With the user option, only the same user account that originally mounted the filesystem can perform the unmount. If multiple users are involved, you can look at the users option instead. See man 8 mount for details.
• The user option implies three other options: noexec (do not permit execution of binaries), nosuid (do not honor setuid/setgid bits) and nodev (do not interpret devices). If you want to restore any of these functionality, append the corresponding option to the end of the list. For example, bind,rw,user,noauto,exec. Keep in mind that there are security implications for these options. See man 8 mount for details.

Answer 3

See this answer https://unix.stackexchange.com/a/767296/139357 which shows how to do bind mounts as non-root using unshare and nsenter

That answer will be improved upon.