by Brandon Rhodes • Home

Mounting Windows shares in Linux userspace

Date: 9 June 2010
Tags:computing, emacs

A current project has forced me into the clunky world of Windows, to verify that a Python program compiles and runs there just like it runs under Linux. Instead of trying to port my entire development environment to Windows — which includes two decades of customizations and a small empire of tools like Emacs, pyflakes, and Rope — I want to simply mount my Windows home directory under Linux so that I can run my normal editor and version control tools in a more familiar environment.

I have worked out an elegant solution by combining two of the powerful user-space filesystems available through the FUSE mechanism in the Linux kernel. Let me take you through the story of how I put them together!

Mounting and unmounting a "cifs" mount point — the most basic and low-level way of mounting a Windows share under the Linux filesystem — unfortunately requires root privileges under Ubuntu, even if one is careful to specify the "user" option in /etc/fstab when creating the share. The only way, therefore, for all users to be given CIFS mounting superpowers is through a manual intervention like:

# Even with the "user" option in fstab, user
# mounting of CIFS shares under Ubuntu require:

chmod u+s /sbin/mount.cifs
chmod u+s /sbin/umount.cifs

But I want a safe solution, that requires neither root permissions nor any editing of the master fstab each time the details about a particular Windows share change. Plus, the error messages from the system mounting logic are a bit cryptic for my taste! Can you guess the meaning of the following error message, which was deposited unceremoniously into my syslog file?

CIFS VFS: cifs_mount failed w/return code = -22

It means that I forgot to install the smbfs Ubuntu package before attempting the mount. I should have guessed.

So I turned my attention to userspace filesystems, those wonderful inventions that let normal users create and delete mount points under their home directories. Linux keeps the scheme secure by imposing a few reasonable restrictions on the owners and modes of the files inside the filesystem, so that users cannot elevate their privileges through indiscretions like SUID binaries. For years, as an example, I have been using the userspace sshfs to make remote source trees visible to my editor here on my laptop, and now I just needed to do the same magic for a Windows filesystem.

My first Google searches on the subject lead me to old pages mentioning something called fusesmb, which, it turns out, is still available as an Ubuntu package. But after several frustrating attempts resulted only in empty mount points, I did further some investigation and discovered that a new project named smbnetfs is now carrying the torch and maintaining compatibility with the most recent kernel versions. It also is available as an Ubuntu package, so I installed it and was immediately up and running!

$ mkdir ~/win
$ smbnetfs ~/win
$ ls ~/win
ALAN/   KB/    TOSH/
IRMA/   SAM/   W2K/
KAREN/  TEST/

As you can see, this filesystem is far more dynamic than a single-share CIFS mount point: it makes your entire Network Neighborhood visible, and lets you browse both workgroups and individual machines by simply visiting the directories beneath it!

The virtual Windows machine that contains my development environment is not actually visible to the master browser that generated the listing shown above, but that is okay: we can list any machine's shares, whether it appears in the directory listing or not, by simply naming it and seeing what happens:

$ ls ~/win/BRANDON-PC
Brandon/  Python26/  project/

There! We can see both of the shares that my virtual machine makes available, and can start editing files with equal ease:

$ cd ~/win/BRANDON-PC/project/src
$ svn st
?       trace.out
M       setup.py
$ svn up
At revision 3677.
$ emacs setup.py

After a minute of work in my new playground, however, I was stymied when Emacs suddenly ground to a halt — then, after a minute, continued running again. By judiciously running strace against the editor during these morbid pauses, I discovered the problem. The Emacs version control logic often climbs up the directory tree from the current file, looking for version control directories that might provide context to the current buffer. This usually happens so fast that, actually, I had never known that Emacs was performing this check — after all, how much time can it take to quickly verify whether a containing directory contains an .svn, .git, or {arch}/=tagging-method file?

Well, it turns out that it can take quite some time, when one of the containing directories is a virtual filesystem that scans the network for Windows machines each time an unknown filename is accessed! Take a look at how long two sample file accesses take when invoked from the command line:

# Looking for a machine named ".git": 20 seconds!

$ time ls win/.git
ls: cannot open directory ...: Input/output error
... 20.701 total

# Looking for a share named ".git": >1 second

$ time ls win/BRANDON-PC/.git
ls: cannot open directory ...: No such file or directory
... 1.227 total

I probably wasted a half hour playing with the Emacs setting vc-ignore-dir-regexp in a vain attempt to explain to my editor that the ~/win directory was not an appropriate place to search for version control files but, despite my considerable talent with Emacs regular expressions, I could never get the setting to have any effect. And, anyway, I soon noticed that other Emacs mechanisms were also filesystem recursive — such as the search for .dir-locals.el files that is now a feature of Emacs 23 — and I determined that trying to subvert them all would be a long and losing battle.

I toyed with the idea of tackling the problem from the other end, because smbnetfs supports a “workaround depth” setting in its configuration file by which you can protect its top few directory levels from user agents that check for special file names. Unfortunately, though, this list of files is hard-coded in the binary, and includes only one of the many version control directories for which Emacs searches (because it is only trying to protect Konqueror and Gnome Terminal from hanging, and they search for only three particular files).

Having failed to solve the problem from either direction, I saw clearly that the only way out was to hide the ~/win directory from Emacs altogether. My first experiment, which failed, was to create a symlink and then ask Emacs to edit files under the link instead:

# Does not actually fix the problem

$ ln -s win/BRANDON-PC/project/src ~/winsrc
$ cd ~/winsrc
$ emacs setup.py

Unfortunately, Emacs is not so easily fooled. To help programmers avoid all of the problems that arise when you try to edit the same file under several different pathnames, Emacs always resolves any symlinks in a file's path during the process of opening it — so that I found myself, in Emacs, editing the setup.py file deep inside of the ~/win directory anyway. So I removed the symlink and took a different approach.

The user filesystem toolbox has a feature that addresses exactly this case — where you need a given directory to appear somewhere else on your filesystem. This kind of mount is called a bind because it simply creates another name for an already-mounted file, rather than connecting to a new share or block device. And it is supported by a FUSE-powered bindfs command which, as usual, has already been packaged by the Ubuntu folks! Here is what it looks like in action:

$ mkdir ~/winsrc
$ bindfs ~/win/BRANDON-PC/project/src ~/winsrc
$ cd ~/winsrc
$ svn up
At revision 3677.
$ emacs setup.py

Success! Emacs now behaves normally, my editing operations finish quickly, and the editor's version control features can be invoked and run without hanging. All thanks to a simple user-mode command that lets me mount a directory from inside of another mount and thereby draw my tools' attention away from the powerful but expensive virtual directory that dynamically searches the network for Windows computers and shares.

In case I have managed to obscure the actual solution by describing all of my wrong turns, here is the entire procedure for mounting my development directory on a newly installed Ubuntu system:

# The whole shebang

$ sudo aptitude install smbnetfs bindfs
$ mkdir -p ~/win ~/winsrc
$ smbnetfs ~/win
$ bindfs ~/win/BRANDON-PC/project/src ~/winsrc
$ cd ~/winsrc
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