samba mount fails to write

[NBR]

Ubuntu 17.10 appears to aggressively trigger Samba bug 10541 ( https://bugzilla.samba.org/show_bug.cgi?id=10541 ). Previous versions of Ubuntu worked fine, but 17.10 appears to request quota information on every write, resulting in a complete inability to write to files.

Remote Samba share is mounted as cifs as read-write, and has been working for years and many software releases. Unable to write/create files from bash, from GUI file browser, or even from a Windows VM with access to the mounted location. I haven't seen any scenario where Ubuntu 17.10 can write to the share.

Server is running Samba 4.7.3 with a long-running good configuration.

Every write request from my Ubuntu desktop results in the following server log:

[2017/12/02 16:44:25.306674, 0] ../source3/lib/sysquotas.c:461(sys_get_quota)
  sys_path_to_bdev() failed for path [.]!

There are no other log entries.

The root bug appears to be in Samba, but recent changes to Ubuntu have triggered the bug in a way that breaks write functionality.

[Ubuntu Foundations Team Bug Bot (crichton)]

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[Andreas Hasenack (ahasenack)]

Are you using btrfs, zfs and/or xfs on the server where the exported share is?

[NBR (nbr17)]

Yes, it's on a btrfs partition in line with the samba bug.

[NBR (nbr17)]

I was mistaken about the cause of this bug. It appears that some default setting has changed, and cifs mounts need uid and gid options passed explicitly. In previous Ubuntu versions, I could mount cifs with just credentials provided I set the owners of the mount point and remote files correctly, a one-time detail. In Ubuntu 17.10, if I fail to pass uid,gid to cifs mount options, it's mounted as root:root and the only access is by everyone permissions or if I'm root.

Workaround: when mounting cifs, use options as such: defaults,uid=1001,gid=1001,credentials=[path]

Where the ids can be retrieved using 'id [name]' from the shell.

[Andreas Hasenack (ahasenack)]

When you use the uid/gid options, do all files in the share get owned by that uid/gid?

Can you check the output of the mount command after mounting the share, with and without uid/gid, and see which options ended up being used? That output will include defaults that you did not specify.

For example, here when I mount from a ds216 synology NAS using these options in fstab "username=andreas,vers=3.0,noauto" I get this in the output of mount:
rw,nosuid,nodev,relatime,vers=3.0,cache=strict,username=andreas,domain=,uid=1000,forceuid,gid=1000,forcegid,addr=<ip>,file_mode=0755,dir_mode=0755,nounix,serverino,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,actimeo=1,user=andreas

uid and gid were added automatically in my case. My fstab line is:
//server/downloads /ds216/downloads cifs user,username=andreas,vers=3.0,noauto

This is on artful. cifs-utils 2:6.7-1

[Andreas Hasenack (ahasenack)]

Oh, and I meant to add that this NAS is using btrfs

[Punkadiddle (punkadiddle)]

Same bug on 18.04 samba + btrfs.

[Punkadiddle (punkadiddle)]

There's a hacky workaround to disable quotas that makes the error vanish from the logs. Not sure of sideeffects.

Put a line like this into your smb.conf [global] to point it to the attached script
 get quota command = /opt/bin/samba-btrfs-quota.sh

The script will then tell samba on each request, that quotas are disabled. The way they didn't even quote the parameters suggests, that the option is not well tested.

[Tolga Cakir (tolga9009)]

The "sys_path_to_bdev() failed for path [.]!" error has already been posted upstream 5 years ago (https://bugzilla.samba.org/show_bug.cgi?id=10541), yet this error still shows up. It doesn't do harm in my setup, but it's annoying and clutters the logs. I've done some Google-Fu on this matter and I think I've found the root cause and an easy workaround.

The culprit seems to be within sys_path_to_bdev(): getmntent(). Issue is, if you have a nested BTRFS subvolume as a Samba share, which gets auto-mounted by its parents, there is no fstab entry for this subvolume, but getmntent() looks in fstab for mount points.

Lets say your fstab contains / and /srv as mount points, but you have created a BTRFS subvolume for your share /srv/samba/share. getmntent() tries to find /srv/samba/share in fstab, but fails, thus leading to that error message. Sidenote: there are good reasons to use BTRFS subvolumes for shares e.g. snapper support via "vfs object = snapper", which **requires** the share to be a BTRFS subvolume.

Knowing the root cause, the workaround is pretty simple: add the BTRFS subvolume you're using for Samba to /etc/fstab.

Also: if your Samba share is not a BTRFS subvolume itself, but it's located under an auto-mounted BTRFS subvolume, will still require you to add that subvolue to /etc/fstab. E.g. /srv is in /etc/fstab, /srv/samba is a BTRFS subvolume and your share is a usual folder /srv/samba/share, you will need to add /srv/samba to /etc/fstab.

I've successfully tried this workaround on samba 4.10.10-2, kernel 5.3.11.1-1, btrfs-progs 5.3.1-3 on Arch Linux. Should also work on Ubuntu.

Hope this helps someone.

Cheers,
Tolga