NFS mount option 'noac' allows same file system to be mounted repeatedly

This document (7010210) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 11 Service Pack 1
SUSE Linux Enterprise Server 11 Service Pack 2

Situation

The same NFS share can be mounted multiple times on the same mount point when using the "noac" mount option.

Normally, the same mount syntax (same share, same options, same mount point) cannot be used to mount something repeatedly . For example, without the 'noac' option, a second (and beyond) mount attempt will result in the expected error:

mount.nfs: /mountpoint is busy or already mounted

However, using the "noac" option unexpectedly allows repeated identical mounts:

host:~ # mount -o noac server:/export /mount_point
host:~ # mount -o noac server:/export /mount_point
host:~ # mount -o noac server:/export /mount_point

host:~ # mount
server:/export on /mount_point type nfs (rw,noac,addr=IP)
server:/export on /mount_point type nfs (rw,noac,addr=IP)
server:/export on /mount_point type nfs (rw,noac,addr=IP)

Resolution

As a temporary workaround, always specify the "sync" option along with "noac". Then a second identical mount attempt will fail as expected. An example mount command would be:

mount -o noac,sync server:/export /mount_point

The SUSE Linux Enterprise 11 SP1 kernel 2.6.32.59 released May 2012 and the SUSE Linux Enterprise 11 SP2 kernel 3.0.26-0.7.6 released in April 2012 include the patch to resolve the problem.

Cause

Additional Information

The reason for the unexpected results is that "noac" implies the "sync" flag. The existing mount is known to be using "sync" but the new mount is not explicitly requesting it. The system incorrectly concludes that the new mount is different and can be allowed.

Disclaimer

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