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

Billy Newsom smartweb at
Tue May 10 20:48:32 PDT 2005

Erik Trulsson wrote:
 > 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.

I felt pretty confident I had tried something of this sort, and of 
course now that you say all of that, it works for me.  I probably didn't 
use the hyphens, nor the equals sign, etc.

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

Which leads me to a test.  I am going to try and take a server down this 
  way and see what resources I can max out or overflow by mounting and 
re-mounting the same thing over and over.  I'll write a shell script 
with an infinite loop, and run it on an old server of mine.  Then maybe 
I'll pull its LAN cable (virtually and actually) and see what happens. 
Perhaps I'll cross-post the results to the hackers list if I get some 
interesting results.


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