File system permissions - setgid bit and Netapp NFS volumes

Shaun Johnson shaun at
Fri Mar 23 18:53:00 EET 2018

Greetings Dovecot List,

I have a bit of an edge case I am trying to resolve.  I am currently
using dovecot on Ubuntu 14.04 - Ubuntu package version:


I have attached the output of doveconf -n to this email - but to
describe the configuration in a nutshell:

    my server is configured to use Maildir storage

    I do not use dovecot delivery service (there is a separate program
    in my system delivering messages)

    I am using a 'checkpassword' mechanism for authentication

    My mail storage is mounted via NFSv3 from a Netapp Filer FAS3050
    configured to set anonuid=89, read write access, and root access.

    Dovecot has been configured with:

	mail_fsync = always
        mail_nfs_index = yes
        mail_nfs_storage = yes
        mmap_disable = yes
        lock_method = fcntl

    In order to function with the other components of my system, the
    Mail Storage directories are expected to have permissions:

	drwx--S---     uid 89: gid 89

The oddity I am encountering came up recently when I set up a job to
monitor and correct directory permissions.  I found after observing the
system in action that overtime, the ~/Maildir directories for all
active mailboxes would mysteriously lose the setgid bit.   Now, while
the system still functioned, it was something unexpected so I
investigated further by turning on auditd to monitor what was changing
the permissions on the path.   After tracing through the results, I
found an entry indicating that the 'imap' process of dovecot was
performing a 'chown' call on the directory, with an end result of the
previous mode 2700 being changed to 0700.

Now I understand that on some filesystems, a 'chown' call can end up
losing any setuid or setgid bits on an inode - and apparently Netapp's
WAFL file system is one of them.

Looking at the dovecot source code, I *believe* the source of the chown
call is in src/lib/nfs-workarounds.c function nfs_flush_chown_uid -
where a 'chown' call is being used to flush NFS attribute caches? for
what I believe is the purposes of ascertaining file lock status?

My question for you folks is if anyone else has encountered this on
other file systems (or filers) - and if there is a native method in
dovecot - either the version I am currently running or an updated newer
version - that allows one to either retain the permissions assigned to a
directory or specify the permissions I expect on the Maildir directory

Any feedback would be welcome, thanks!

Shaun Johnson
LinuxMagic Inc.
604-682-0300 Beautiful British Columbia, Canada

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