Lite My Fuse: File System in User Space

The FUSE filesystem allows users to mount all sorts of storage and "filesystems" for per-process use. Obvious uses are encrypted filesystems or remote storage, but accessing a phone or camera, layered filesystems, or even an XML document as a tree are available. The "User" space makes the FUSE systems especially flexible and easy to use since they don't require superuser access to install into the kernel.

This talk describes the basics of *NIX filesystems, how FUSE fits into the system, and gives a few examples of using the filesystems.

Transcript

1. FUSE:
   Being where you aren't, seeing what I can't.
   Steven Lembark
   Workhorse Computing
   lembark@wrkhors.com
   

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2. In the beginning was System7
   And it was good... enough.
   One big innovation: mount points.
   No device names, just "absolute paths"
   One "filesystem" to rule them all.
   

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3. I nodes, you nodes, we all need...
   Another innovation: "node" vs "link".
   Directory is a flat file of inodes + names.
   "Inode" has ownership, mods, allocation.
   Allows for symlinks.
   

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4. Directory as indirection.
   One aside...
   

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5. Directory as indirection.
   Directories
   Directories
   are
   are
   not
   not
   Folders
   Folders
   

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6. UNIX Directories
   Dir contents are universal.
   dirent = basename -> inode
   

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7. UNIX Directories
   Dir contents are universal.
   dirent = basename -> inode
   “link”
   

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8. UNIX Directories
   Inodes defined by filesystem.
   Owner, group, mods, size, type...
   Can vary by filesystem type.
   

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9. UNIX Directories
   Inodes defined by filesystem.
   Owner, group, mods, size, type...
   Can vary by filesystem type.
   stat( inode ) returns data.
   

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10. UNIX Directories
    Inodes defined by filesystem.
    Owner, group, mods, size, type...
    Can vary by filesystem type.
    stat can be expensive.
    

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11. Directory as indirection.
    Filesystem requires kernel support.
    Indirection across physical devices.
    

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12. Directory as indirection.
    Filesystem requires kernel support.
    Hey, what about remote devices?
    NFS extended "inode" to "vnode".
    ”virtual node”
    

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13. "vnode" abstracts device
    Replace single "filesystem".
    More OO-ish: vnode has "handler".
    Allows for multiple filesystem types.
    Semantics are specific to handler.
    

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14. One thing didn't change
    Q: What do: LVM, NFS, XFS, BTRFS,
    F2FS, ext2, ext3, ext4, proc, sysfs, tmpfs
    have in common?
    

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15. One thing didn't change
    Q: What do: LVM, NFS, XFS, BTRFS,
    F2FS, ext2, ext3, ext4, proc, sysfs, tmpfs
    have in common?
    A: /etc/fstab
    The only way to get there from here.
    

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16. /etc/fstab makes mounts SU-only
    Only SU can "mount" or "umount".
    "users" allows mounting by non-SU
    UID's.
    Only at locations defined by /etc/fstab.
    Defined by SU.
    

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17. Getting personal
    Some filesystems are personal:
    Only make sense to one UID at a time.
    Possibly only one process.
    Examples:
    Encryption.
    Access via ssh.
    

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18. Breaking the tyranny: FUSE
    "fusermount" allows non-SU mounts.
    May be private to process or UID
    mounting.
    May be invisible to other proc's or UIDS.
    

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19. A bit of indirection
    "Normal" mounts go into the kernel.
    FUSE mounts come back out again:
    

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21. Available for any number of systems
    Fuse for FreeBSD
    Fuse4X (now merged with OSXFuse.)
    MacFUSE
    OSXFuse successor to MacFUSE
    Dokan Windows user mode
    NetBSD starting with NetBSD-6.0
    MINIX 3 starting with version 3.2.0
    

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22. Example: sshfs
    Replace NFS with ssh.
    Secure.
    Less chatty: single mountpoint.
    User mounts in their own space.
    

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23. Example: sshfs
    ssh connection is specific to a process.
    Or process group.
    sshfs not well suited to general mounts.
    

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24. Mounting sshfs
    Step 1: Make sure ssh works.
    $ ssh-add;
    $ ssh jeeves;
    +lembark@dizzy ~ $
    

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25. Executing sshfs mount
    One approach: /etc/fstab.
    Saves remembering it all.
    Fine for a desktop: only one user.
    

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26. /etc/fstab entry for sshfs
    jeeves:/images /mnt/remote/images \
    fuse.sshfs \
    user,noauto,nonempty,reconnect 0 0
    Filesytem type "fuse.sshfs"
    Delegates mount.
    "users" allows non-SU mount.
    

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27. Do it manually
    "sshfs" is user-land mount utility:
    $ sshfs jeeves:/var/tmp /var/tmp/11061/
    $ sshfs -u jeeves:/var/tmp /var/tmp/11061/
    

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28. Make it magical
    afuse is a userland automounter:
    $ afuse -o mount_template='sshfs \
    -o ServerAliveInterval=10 \
    -o reconnect %r:/ %m' \
    -o unmount_template= \
    'fusermount -u -z \ %m' ~/mnt/ssh ;
    

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29. FUSE mounts are private
    Non-SU proc's mount for themselves.
    sshfs option: "allow_other".
    Makes mounts visible to other users.
    Without even SU cannot see contents.
    

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30. Hide your porn
    encfs == encrypted FUSE.
    Passphrase required to mount volume.
    SU cannot access deciphered content.
    SU can back up enciphered space.
    

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31. Example: My notebook
    ~lembark/.bash_profile:
    cd /var/tmp;
    /opt/bin/extmount $HOME;
    cd $HOME;
    exec bash --login
    

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32. It takes two to tango
    Or mount encfs: one enciphered, one not.
    drwxr-s--- 71 lembark lembark 12288 Mar
    9 17:56 lembark
    drwxr-s--- 71 lembark lembark 12288 Mar
    9 17:56 .lembark
    

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33. The enchpered portion is visible to others
    But not very useful:
    $ ls -1
    total 262689
    0d9jdsFuZmhxlsqwQ7GMV,Pt
    0KvCQ2RXsi2YTGe7K0G3OHtG
    0NzQCAtLUiL1XTAfFjzPfBID
    ...
    

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34. Mounting the encfs
    #!/bin/bash
    mount=${1-$HOME};
    shadow=$(dirname $mount)/.$(basename $mount);
    /usr/bin/encfs –ondemand \
    --extpass=/opt/bin/extpass -i 60 $shadow \
    $mount -o nonempty ;
    

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35. Getting the password
    Encfs wants md5, not text.
    Fix: Grab the input and output md5_hex:
    #!/bin/perl
    my $phrase = shift || acquire_password;
    say md5_base64 $phrase;
    

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36. Backing up
    # ls /home/lembark
    ls: cannot access /home/lembark:
    Permission denied
    SU can back up /home/.lembark.
    Backups are enciphered.
    

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37. More examples
    adbfs Android via USB.
    CloudFusion DropBox, Google Drive...
    fuse-archive Read-only compressed.
    gphotofs Mount a camera.
    s3fs AWS S3 as a filesytem
    

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38. Bedside Reading
    The Design and Implementation of the
    4.3BSD UNIX Operating System
    Sam Leffler, Kirk McKusick, Michael
    Karels & John Quartermann.
    1989, Addison-Wesley. ISBN 0-201-06196-1.
    

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39. Bedside Reading
    https://www.usenix.org/legacy/events/
    usenix99/full_papers/zadok/zadok.pdf
    Extending File Systems Using Stackable
    Templates
    

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40. Bedside Reading
    https://github.com/pcarrier/afuse/
    Userland fuse automounter.
    

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41. Bedside Reading
    $ man mount.sshfs;
    $ man -k encfs;
    

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42. Bedside Reading
    https://wiki.archlinux.org/title/FUSE
    Nice list of fuse systems.
    

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