When mount.ecrpytfs_private before version 87-0ubuntu1.2 calls setreuid() it doesn't also set the effective group id. So when it creates the new version, mtab.tmp, it's created with the group id of the user running mount.ecryptfs_private.
openSUSE 11.3 openSUSE 11.4 SUSE Linux Enterprise Desktop 11 SP1 SUSE Linux Enterprise Desktop 11 SP2 SUSE Linux Enterprise Server 11 SP1 SUSE Linux Enterprise Server 11 SP1 for Teradata SUSE Linux Enterprise Server 11 SP1 for VMware SUSE Linux Enterprise Server 11 SP1-TERADATA SUSE Linux Enterprise Server 11 SP2 SUSE Linux Enterprise Server 11 SP3 SUSE Linux Enterprise Server 11 SP4 SUSE Linux Enterprise Server for SAP Applications 11 SP1-TERADATA