# Kernel File Systems as Userspace Servers
This chapter describes the symbiosis of background knowledge on them, see the
respective web pages.
## On Kernel File System Servers
The integration of the [[puffs]] and [[rump]] technologies enables mounting
kernel file systems as userspace servers. Equivalent functionality to the
in-kernel option is maintained, and an application accessing files is unable to
tell the difference between the two.
A useful scenario for mounting a kernel file system as a userspace server is
accessing untrusted media. As file system code is generally authored assuming a
trusted and correct file system image, a corrupt or deliberately modified image
can easily cause a kernel crash or security compromise. By using a userspace
server, the compromise is limited to the userspace server program, not the whole
It also enables the mounting of kernel file systems which are not included as
part of the kernel and loading a module is not desired. Additionally, using
kernel file systems from different operating system versions is possible, since
interfacing with the kernel is done through the puffs protocol, not the kernel
binary interface. This is desired for example when wanting to take advantage of
new functionality without upgrading the operating system or wanting to do a
partial rollback to avoid a regression in a newer version.
![File System Access Comparison](/puffs/rump.png)
**File System Access Comparison**
## Mounting and Use
The use of kernel file systems as userspace servers requires the support for
[[puffs]] in the kernel.
To mount the file system, the relevant `rump_fs` command must be run. These
commands share the same suffix as the normal in-kernel `mount_fs` commands. For
instance, the equivalent of `mount_msdos` is `rump_msdos`. The usage is also
equal, so the manual page of the mount equivalent should be consulted. The only
exception is that rump file systems do not require the use of
to mount regular files. Rather, such images can be mounted directly by using the
image as the device file path.
To unmount, `umount` should be run as normal. While it is possible to violently
kill the server process, this does not give the file system an opportunity to
flush its caches and cleanly unmount and may result in data loss. In case the
server process is violently killed, puffs automatically performs a forced
unmount and no further action is necessary.
To mount a file system image as a read-only mount:
# rump_ffs -o ro ~/img/ffs.img /mnt
# mount -t ffs -o ro,rump ~/img/ffs.img /mnt
If you want to mount a disk device instead, the procedure is the same. This
example also instructs the file system to use journalling. Note: for reasons
beyond the scope of this document, it is highly recommended you use the raw
device instead of the usual block device.
# rump_ffs -o log /dev/rwd0e /mnt2
Mounting nfs works in a similar fashion. The command line flag `-p` makes sure a
non-root mount is succesful if the server allows them:
# rump_nfs -p host:/export /mnt3
Finally, the option `rump` in fstab signals that the file system should be
mounted using rump instead of the kernel file service. The following example is
for specifying an nfs share mount from a laptop over wireless:
server:/m/server /m/server nfs rw,noauto,-r=8192,-w=8192,rump
After this the file system will show up on the mountlist and you can access it
through the mountpath like any other mounted file system.
Internally, kernel file systems are implemented against the kernel virtual file
system layer. This layer defines a protocol which kernel file systems convert to
their backend storage protocol to satisfy the operation. The result is then
converted back to a format the virtual file system layer understands and passed
back to caller (most likely an application, although it can be for example the
Analogously, puffs file systems must interpret and convert the puffs file system
interface protocol. While the puffs interface is very similar to the kernel
virtual file system interface, there are differences in some parameters. For
example, the virtual file system passes information about I/O as a reference to
`struct uiomove`. In contrast, puffs passes this as explicit parameters.
Since a userspace kernel file system server attaches itself as a puffs file
system, protocol conversion from puffs to the kernel virtual file system must
happen, if the whole protocol stack is to work. This conversion of protocols is
done by the p2k, or puffs-to-kernel, component. It is installed as `libp2k` and
linked with all kernel file system servers.
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