nfs bug & df: Can I lock up my kernel and overflow this buffer?

Erik Trulsson ertr1013 at
Tue May 10 16:10:44 PDT 2005

On Tue, May 10, 2005 at 05:16:16PM -0500, Billy Newsom wrote:
> Jonathan Noack wrote:
> >> Anyone tried that sort of stuff in fstab?  I'm a little skeptical.
> >
> > I use "that sort of stuff" and have for a long time.  Here's one of my
> > fstab lines:
> >
> > optimator:/usr/home  /usr/home  nfs  rw,-3,-T,-r=32768,-w=32768  0  0
> >
> > It's obvious you don't believe me but why are you unwilling to try it
> > yourself?
> Well, because this fails to work on the commandline:
> #mount -o -s -x 2 -T dell:/nfs /dellbak
> I tried tons of different ways, never could get mount to do that, so I 
> gave up on fstabbing options.

Note that mount(8) (as well as mount_nfs(8)) says about the -o flag
that "Options are specified with a -o flag followed by a comma
separated string of options."

If you try writing it as the manpage says, i.e. like

mount -o -s,-x=2,-T dell:/nfs /dellbak

it should work fine.

> Since the above mount command wouldn't even work, I figured I could 
> forget about putting those same options (which mount calls illegal) in 
> the fstab file.  That's where the man pages only go so far.  Without the 
> examples you give, I was pretty sure that it was pointless to get fstab 
> options to do what mount wouldn't.
> What it boils down to is that mount is fine with these options in fstab, 
> but barfs when doing them on the commandline.  That was so 
> couter-intuitive, I went around it for the sake of getting things done.

It works fine on the commandline, you just got the syntax for the
commandline wrong.

> FreeBSD man pages are nice and all, but without a textbook siting by 
> with some examples, it can be difficult.  I learned Unix pretty much ad 
> hoc, so I find that examples (such as you gave) are worth much more than 
> man pages now that I know most of the basics.

Man pages often have some examples, but learning things only from
manpages can indeed be a bit difficult.

> Thanks.
> But what I did discover is that if I mount the same nfs resource 
> multiple times, I get multiple, identical mounts (using fstab options, 
> or commandline, either one).  I have to umount each one serially.  How 
> is this a feature?

It is a feature in that the system does exactly what you asked it to
do.  This is usually less painful than systems that try to guess what
you actually meant.

>  What good does it do me if I mount the same nfs 
> drive to the same place n times? 

None that I can think of, which doesn't mean that there is no use for

> Won't that eventually cause a deadlock 
> as n increases beyond a few hundred or thousand? -- especially when the 
> NFS server goes down?

Probably not a deadlock, but possible a resource starvation - so don't
do that.

>  Shouldn't the second and subsequent mounts either 
> fail or not be attempted due to a sanity check?
> #mount /usr
> mount: /dev/ad0s1f: Device busy
> Exit 1
> That seems reasonable for /usr.  But as I stated before, NFS resources 
> nevere apparently become "busy", and there is no sanity check to prevent 
> mulitiple simultaneous mounts of identical file systems on identical 
> file trees.

<Insert your favourite quote here.>
Erik Trulsson
ertr1013 at

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