Re: quick scripting 'is /P/Q mounted'

[Mark Fletcher]

On Tue, Mar 13, 2018 at 03:56:00PM -0400, The Wanderer wrote:
> On 2018-03-13 at 15:39, Joe wrote:
> 
> > On Tue, 13 Mar 2018 14:49:56 +0100  wrote:
> 
> That test can be spoofed, however, by the creation of a directory with
> the same name (and/or other characteristics) under the mount point while
> the mount is not active.
> 

Yes, but in most use cases one would not be worried about malicious 
actions, you are trying to protect against cock-ups.

> Even if you don't think anything malicious is ever going to try to spoof
> this in whatever case is at hand, can you be sure no script (or, for
> that matter, user) will ever attempt to create that directory under the
> mistaken impression that the mount is active?
> 

Yeah, that's a fair point though.

Mark

Re: quick scripting 'is /P/Q mounted'

[Joe]

On Tue, 13 Mar 2018 15:56:00 -0400
The Wanderer  wrote:

> On 2018-03-13 at 15:39, Joe wrote:
> 
> > On Tue, 13 Mar 2018 14:49:56 +0100  wrote:  
> 
> >> On Tue, Mar 13, 2018 at 10:33:43PM +0900, Mark Fletcher wrote:  
> 
> >>> Unless I've misunderstood the question, you can tell if
> >>> something is mounted at a mount point by checking if anything is
> >>> present under the mount point [...]  
> >> 
> >> Not if the file system is really empty (and no, lost+found doesn't 
> >> really count, since not all file systems have that).  
> > 
> > But one use-case is unattended synchronisation, possibly over a
> > network. If the directory is mounted, but with some kind of problem,
> > then a check for mounting could succeed without any data being
> > visible to the synchronising application. In the worst case, the
> > backup directory might be synchronised to an apparently empty
> > one
> > 
> > I prefer to test for the existence of a known lower directory in
> > this case, which tests not only for mounting but for a successful
> > read.  
> 
> That test can be spoofed, however, by the creation of a directory with
> the same name (and/or other characteristics) under the mount point
> while the mount is not active.
> 
> Even if you don't think anything malicious is ever going to try to
> spoof this in whatever case is at hand, can you be sure no script
> (or, for that matter, user) will ever attempt to create that
> directory under the mistaken impression that the mount is active?
> 

Reasonably, because the only program likely to create it would be the
synchroniser itself, which should have been prevented from running
because it wasn't there to start with. If it did happen, nothing
would be getting wiped, just a spurious copy created on a drive that
doesn't contain any system stuff, so I wouldn't worry too much. The
wiping of one of the sync targets *might* be catastrophic (and might
lead to the other getting wiped on the next sync) and is completely
unacceptable.

Yes, synchronisers do have some protection against this kind of error,
but I like to use belt, braces, and skyhooks when available.

Should I worry about your scenario, I could take it further, with
utterly improbably-named directories or files containing specific data,
but so far I haven't.

-- 
Joe

-- 
Joe

Re: quick scripting 'is /P/Q mounted'

[tomas]

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On Tue, Mar 13, 2018 at 03:56:00PM -0400, The Wanderer wrote:
> On 2018-03-13 at 15:39, Joe wrote:

[...]

> > I prefer to test for the existence of a known lower directory in
> > this case, which tests not only for mounting but for a successful
> > read.
> 
> That test can be spoofed, however, by the creation of a directory with
> the same name (and/or other characteristics) under the mount point while
> the mount is not active.
> 
> Even if you don't think anything malicious is ever going to try to spoof
> this in whatever case is at hand, can you be sure no script (or, for
> that matter, user) will ever attempt to create that directory under the
> mistaken impression that the mount is active?

Seconded. That depends on what you want to achieve, but if this is some
kind of backup script, if it manages once (by mistake) to sync to the
mount point while no mount is active, it'll keep repeating that mistake
forever...

Cheers
- -- tomás
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Re: quick scripting 'is /P/Q mounted'

[The Wanderer]

On 2018-03-13 at 15:39, Joe wrote:

> On Tue, 13 Mar 2018 14:49:56 +0100  wrote:

>> On Tue, Mar 13, 2018 at 10:33:43PM +0900, Mark Fletcher wrote:

>>> Unless I've misunderstood the question, you can tell if
>>> something is mounted at a mount point by checking if anything is
>>> present under the mount point [...]
>> 
>> Not if the file system is really empty (and no, lost+found doesn't 
>> really count, since not all file systems have that).
> 
> But one use-case is unattended synchronisation, possibly over a
> network. If the directory is mounted, but with some kind of problem,
> then a check for mounting could succeed without any data being
> visible to the synchronising application. In the worst case, the
> backup directory might be synchronised to an apparently empty
> one
> 
> I prefer to test for the existence of a known lower directory in
> this case, which tests not only for mounting but for a successful
> read.

That test can be spoofed, however, by the creation of a directory with
the same name (and/or other characteristics) under the mount point while
the mount is not active.

Even if you don't think anything malicious is ever going to try to spoof
this in whatever case is at hand, can you be sure no script (or, for
that matter, user) will ever attempt to create that directory under the
mistaken impression that the mount is active?

-- 
   The Wanderer

The reasonable man adapts himself to the world; the unreasonable one
persists in trying to adapt the world to himself. Therefore all
progress depends on the unreasonable man. -- George Bernard Shaw



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Re: quick scripting 'is /P/Q mounted'

[Joe]

On Tue, 13 Mar 2018 14:49:56 +0100
 wrote:

> -BEGIN PGP SIGNED MESSAGE-
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> 
> On Tue, Mar 13, 2018 at 10:33:43PM +0900, Mark Fletcher wrote:

> > >   
> > Unless I've misunderstood the question, you can tell if something
> > is mounted at a mount point by checking if anything is present
> > under the mount point [...]  
> 
> Not if the file system is really empty (and no, lost+found doesn't
> really count, since not all file systems have that).
> 

But one use-case is unattended synchronisation, possibly over a network.
If the directory is mounted, but with some kind of problem, then a check
for mounting could succeed without any data being visible to the
synchronising application. In the worst case, the backup directory
might be synchronised to an apparently empty one

I prefer to test for the existence of a known lower directory in this
case, which tests not only for mounting but for a successful read.

-- 
Joe

Re: quick scripting 'is /P/Q mounted'

[Greg Wooledge]

On Tue, Mar 13, 2018 at 07:51:32AM -0700, Mike McClain wrote:
> Thank you Richard.
> I suspect $(grep /south40/docs/ /proc/mounts) would be faster than

But that would be wrong, because it would incorrectly return "true"
if you have something mounted at /south40/docs/subdir or /media/south40/docs/
(or if there is a device that contains the string "/south40/docs/"
somewhere in its name, etc.).

Use the mountpoint command instead.  This is why it exists.  (Use the
-q option in a shell script to suppress the human-readable output.)

Re: quick scripting 'is /P/Q mounted'

[Mike McClain]

Thank you Richard.
I suspect $(grep /south40/docs/ /proc/mounts) would be faster than
$( mount | grep 'south40/docs').
And I'm sure [ -f /south40/docs/.flag ] would be.
Much obliged.
Mike

On Tue, Mar 13, 2018 at 05:37:07PM +1300, Richard Hector wrote:
> On 13/03/18 16:40, Mike McClain wrote:
> > If my other computer is South40 and I want to mount South40's /docs
> > on my /south40/docs/ directory I can do that. As one script calls
> > another I want to know if I need to mount South40 without
> > $( mount | grep 'south40/docs').
>
> You could look at /proc/mounts, but that's similar to the output of mount.
>
> Or if it's for a specific directory, and you can put something in it,
> you can test for the existence of a special file in the directory.
>
> Something like
>
> [ -f /south40/docs/.flag ]
>
> Richard
>
--
"Any intelligent fool can make things bigger and more complex...
It takes a touch of genius - and a lot of courage to move in the
opposite direction."   - Albert Einstein

Re: quick scripting 'is /P/Q mounted'

[Mike McClain]

Thank you David.
As it happens I have util-linux installed but as with most of Gnu/Linux
there are hundreds of programs I've never used and don't know what do.
Appreciate the heads-up.
Mike

On Tue, Mar 13, 2018 at 08:49:58PM +1100, David wrote:
> On 13 March 2018 at 14:40, Mike McClain  wrote:
> >
> > If my other computer is South40 and I want to mount South40's /docs
> > on my /south40/docs/ directory I can do that. As one script calls
> > another I want to know if I need to mount South40 without
> > $( mount | grep 'south40/docs').
> >
> > Suggestions?
>
> Installing the package util-linux will provide the mountpoint command
> which exits true=0 if its argument is in use as a mountpoint. Example:
>
> $ if mountpoint / ; then echo "exit status is $?" ; fi
> / is a mountpoint
> exit status is 0

--
"Any intelligent fool can make things bigger and more complex...
It takes a touch of genius - and a lot of courage to move in the
opposite direction."   - Albert Einstein

Re: quick scripting 'is /P/Q mounted'

[tomas]

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On Tue, Mar 13, 2018 at 10:33:43PM +0900, Mark Fletcher wrote:
> On Tue, Mar 13, 2018 at 08:49:58PM +1100, David wrote:
> > On 13 March 2018 at 14:40, Mike McClain  wrote:
> > >
> > > If my other computer is South40 and I want to mount South40's /docs
> > > on my /south40/docs/ directory I can do that. As one script calls
> > > another I want to know if I need to mount South40 without
> > > $( mount | grep 'south40/docs').
> > >
> > > Suggestions?
> > 
> > Installing the package util-linux will provide the mountpoint command
> > which exits true=0 if its argument is in use as a mountpoint. Example:
> > 
> > $ if mountpoint / ; then echo "exit status is $?" ; fi
> > / is a mountpoint
> > exit status is 0
> > 
> Unless I've misunderstood the question, you can tell if something is 
> mounted at a mount point by checking if anything is present under the 
> mount point [...]

Not if the file system is really empty (and no, lost+found doesn't really
count, since not all file systems have that). Watch this:

  tomas@trotzki:~$ dd if=/dev/zero of=fatty bs=1024 count=1024
  1024+0 records in
  1024+0 records out
  1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00582808 s, 180 MB/s
  tomas@trotzki:~$ mkfs.vfat fatty
  bash: mkfs.vfat: command not found
  tomas@trotzki:~$ /sbin/mkfs.vfat fatty
  mkfs.fat 4.1 (2017-01-24)
  tomas@trotzki:~$ sudo mount -ouid=tomas,gid=tomas fatty /mnt
  [sudo] password for tomas: 
  tomas@trotzki:~$ ls -al /mnt
  total 20
  drwxr-xr-x  2 tomas tomas 16384 Jan  1  1970 .
  drwxr-xr-x 23 root  root   4096 Feb 23 12:42 ..

Totally empty...

Conversely, the directory you mount things on doesn't have to be empty:
its contents is just shadowed while the mount lasts.

Cheers
- -- tomás
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Re: quick scripting 'is /P/Q mounted'

[Mark Fletcher]

On Tue, Mar 13, 2018 at 08:49:58PM +1100, David wrote:
> On 13 March 2018 at 14:40, Mike McClain  wrote:
> >
> > If my other computer is South40 and I want to mount South40's /docs
> > on my /south40/docs/ directory I can do that. As one script calls
> > another I want to know if I need to mount South40 without
> > $( mount | grep 'south40/docs').
> >
> > Suggestions?
> 
> Installing the package util-linux will provide the mountpoint command
> which exits true=0 if its argument is in use as a mountpoint. Example:
> 
> $ if mountpoint / ; then echo "exit status is $?" ; fi
> / is a mountpoint
> exit status is 0
> 
Unless I've misunderstood the question, you can tell if something is 
mounted at a mount point by checking if anything is present under the 
mount point, eg if you know there is a directory /Y that gets mounted 
under mount point /X, you can can make sure /X/Y doesn't exist under the 
mount poiunt and then check for the existence of /X/Y -- it will be 
there if the mount point is in use and not if not.

Mark

Re: quick scripting 'is /P/Q mounted'

[David]

On 13 March 2018 at 14:40, Mike McClain  wrote:
>
> If my other computer is South40 and I want to mount South40's /docs
> on my /south40/docs/ directory I can do that. As one script calls
> another I want to know if I need to mount South40 without
> $( mount | grep 'south40/docs').
>
> Suggestions?

Installing the package util-linux will provide the mountpoint command
which exits true=0 if its argument is in use as a mountpoint. Example:

$ if mountpoint / ; then echo "exit status is $?" ; fi
/ is a mountpoint
exit status is 0

Re: quick scripting 'is /P/Q mounted'

[Richard Hector]

On 13/03/18 16:40, Mike McClain wrote:
> A while back, Pierre Gaston posted this little tidbit to quickly
> determine if my network is up:
> [ "$( 
> Now I wonder if there is a similar file in /sys that would tell if
> anything is mounted on a particular directory. I've browsed /sys but
> not found what I'm looking for.
> 
> If my other computer is South40 and I want to mount South40's /docs
> on my /south40/docs/ directory I can do that. As one script calls
> another I want to know if I need to mount South40 without
> $( mount | grep 'south40/docs').

You could look at /proc/mounts, but that's similar to the output of mount.

Or if it's for a specific directory, and you can put something in it,
you can test for the existence of a special file in the directory.

Something like

[ -f /south40/docs/.flag ]

Richard




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quick scripting 'is /P/Q mounted'

[Mike McClain]

A while back, Pierre Gaston posted this little tidbit to quickly
determine if my network is up:
[ "$(