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[[!meta copyright="Copyright © 2008, 2009, 2013 Free Software Foundation,
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[[!meta title="mtab"]]
In traditional monolithic system, the kernel keeps track of all mounts; the
information is available through `/proc/mounts` (on Linux at least), and in a
very similar form in `/etc/mtab`.
The Hurd on the other hand has a totally
[[decentralized_file_system|hurd/virtual_file_system]]. There is no single
entity involved in all mounts. Rather, only the parent file system to which a
mountpoint ([[hurd/translator]]) is attached is involved. As a result, there
is no central place keeping track of mounts.
As a consequence, there is currently no easy way to obtain a listing of all
mounted file systems. This also means that commands like `df` can only work on
explicitly specified mountpoints, instead of displaying the usual listing.
One possible solution to this would be for the translator startup mechanism to
update the `mtab` on any `mount`/`unmount`, like in traditional systems.
However, there are some problems with this approach. Most notably: what to do
with passive translators, i.e., translators that are not presently running, but
set up to be started automatically whenever the node is accessed? Probably
these should be counted among the mounted filesystems; but how to handle the
`mtab` updates for a translator that is not started yet? Generally, being
centralized and event-based, this is a pretty inelegant, non-hurdish solution.
A more promising approach is to have `mtab` exported by a special translator,
which gathers the necessary information on demand. This could work by
traversing the tree of translators, asking each one for mount points attached
to it. (Theoretically, it could also be done by just traversing *all* nodes,
checking each one for attached translators. That would be very inefficient,
though. Thus a special interface is probably required, that allows asking a
translator to list mount points only.)
There are also some other issues to keep in mind. Traversing arbitrary
translators set by other users can be quite dangerous -- and it's probably not
very interesting anyways what private filesystems some other user has mounted.
But what about the global `/etc/mtab`? Should it list only root-owned
filesystems? Or should it create different listings depending on what user
contacts it?...
That leads to a more generic question: which translators should be actually
listed? There are different kinds of translators: ranging from traditional
filesystems ([[disks|hurd/libdiskfs]] and other actual
[[stores|hurd/translator/storeio]]), but also purely virtual filesystems like
[[hurd/translator/ftpfs]] or [[hurd/translator/unionfs]], and even things that
have very little to do with a traditional filesystem, like a
[[gzip_translator|hurd/translator/storeio]],
[[mbox_translator|hurd/translator/mboxfs]],
[[xml_translator|hurd/translator/xmlfs]], or various device file translators...
Listing all of these in `/etc/mtab` would be pretty pointless, so some kind of
classification mechanism is necessary. By default it probably should list only
translators that claim to be real filesystems, though alternative views with
other filtering rules might be desirable.
After taking decisions on the outstanding design questions, the student will
implement both the actual [[mtab_translator|hurd/translator/mtabfs]], and the
necessary interface(s) for gathering the data. It requires getting a good
understanding of the translator mechanism and Hurd interfaces in general.
Possible mentors: Olaf Buddenhagen (antrik), Carl Fredrik Hammar (cfhammar)
Exercise: Make some improvement to any of the existing Hurd translators.
Especially those in [hurdextras](http://www.nongnu.org/hurdextras/) are often
quite rudimentary, and it shouldn't be hard to find something to improve.
# Related Discussion
## IRC, freenode, #hurd, 2013-04-17
<kuldeepdhaka> thinking how to get the listing. traversing would be
ineffecient, trying to come up with something better
<braunr> what listing ?
<braunr> and traversing what ?
<kuldeepdhaka> mtab
<braunr> well i assumed so
<braunr> be more precise please
<kuldeepdhaka> when the translator is done initalized <translation
info> are written to /etc/mtab <translation info> will be provided
by the translator, and when some one want to read the info just read it
this way if their is some credentials like ftp sites pass username can be
masked by the translator
<kuldeepdhaka> if some trans dont want to list them, no need to write to
file | while unmounting (sorry i couldnt find the right word) , it
will pass the mount node address | <translation info> will have special
structure to remove/add mounts example "a /mount-to /mount-from" = add
, "r /mount-to" = remove here "/mount-to" will be unique for every
mount
<kuldeepdhaka> this have a draw back , we would have to trust trans for the
listed data | also "/mount-to" + "/mount-from" could be used a
combination for making sure that other trans unable remove others trans
mount data
<kuldeepdhaka> sorry but "also "/mount-to" + "/mount-from" could be used a
combination for making sure that other trans unable remove others trans
mount data" this is a bad idea if we had to print the whole thing
<kuldeepdhaka> braunr, whats ur opinion?
<pinotree> you don't need a mtab to "unmount" things on hurd
<braunr> kuldeepdhaka: hum, have you read the project idea ?
<braunr>
http://darnassus.sceen.net/~hurd-web/community/gsoc/project_ideas/mtab/
<braunr> A more promising approach is to have mtab exported by a special
translator, which gathers the necessary information on demand. This could
work by traversing the tree of translators, asking each one for mount
points attached to it.
<kuldeepdhaka> pinotree, not to unmount, i mean is to remove the
<translation data>
<braunr> for a first implementation, i'd suggest a recursive traversal of
root-owned translators
<kuldeepdhaka> braunr, hum, but it did stated it as inefficient
<braunr> where ?
<kuldeepdhaka> para 5 , line 3
<kuldeepdhaka> and line 6
<braunr> no
<braunr> traversing "all" nodes would be inefficient
<braunr> translators which host the nodes of other translators could
maintain a simple list of active translators
<braunr> ext2fs, e.g. (if that's not already the case) could keep the list
of the translators it started
<braunr> we can already see that list with pstree for example
<braunr> but this new list would only retain those relevant for mtab
<braunr> i.e. root-owned ones
<pinotree> i would not limit to those though
<braunr> and then filter on their type (e.g. file system ones)
<braunr> pinotree: why ?
<pinotree> this way you could have proper per-user /proc/$pid/mounts info
<braunr> we could also very easily have a denial of service
<kuldeepdhaka> but how will the mount point and source point will be
listed?
<braunr> they're returned by the translator
<kuldeepdhaka> k
<braunr> you ask /, it returns its store and its options, and asks its
children recursively
<braunr> a /home translator would return its store and its options
<braunr> etc..
<braunr> each translator would build the complete path before returning it
<braunr> sort of, it's very basic
<braunr> but that would be a very hurdish way to do it
<kuldeepdhaka> shall /etc/mtab should be made seek-able and what should be
the filesize? content are generated on demand so, it could arise problem
(fsize:0 , seek-able:no), ur opinions?
<braunr> kuldeepdhaka: it should have all the properties of a regular file
<braunr> the filesize would be determined after it's generated
<braunr> being empty doesn't imply it's not seekable
<kuldeepdhaka> content is generated on demand so, could cause problem while
seeking and filesize, shall i still program as regular file?
<kuldeepdhaka> in two different read, it could generate different content,
though same seek pos is used...
<braunr> what ?
<braunr> the content is generated on open
<kuldeepdhaka> ooh, ok
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