Re: Cannot mount DFS shares after upgrade to 10.9

[Salvatore Bonaccorso <carnil@debian.org>]

Hi,

On Sat, Apr 03, 2021 at 06:32:50PM +0000, Joel Davies wrote:
> We have some autofs mounts that stopped working after the 10.9
> updates were applied. Seems that the kernel update is the cause - I
> applied just the kernel update and same thing happens. The mounts
> that stopped working were targeting Windows domain DFS shares. The
> DFS root can still be mounted (//domain.local/DFS) which we may have
> to end up using but then will have to reconfigure stuff. Subfolders
> of normal shares also can still be mounted
> (//Server.domain.local/Share/Subfolder). It is just the DFS shares
> that don't work currently (//domain.local/DFS/Share).
> 
> Here is what happens mounting the shares manually:
> 
> Under kernel 4.19.0-14-amd64 (previous to 10.9 updates):
> 
> $sudo mount -t cifs -o username=user,vers=3.0 //domain.local/DFS/Share /mnt --verbose
> Password for user@//domain.local/DFS/Share:  *********
> mount.cifs kernel mount options: ip=10.x.x.x,unc=\\domain.local\DFS,vers=3.0,user=user,prefixpath=Share,pass=********
> 
> Normally no messages in syslog and share is successfully mounted.
> 
> After updating kernel to 4.19.0-16-amd64:
> 
> $sudo mount -t cifs -o username=user, //domain.local/DFS/Share /mnt --verbose
> Password for user@//domain.local/DFS/Share:  *********
> mount.cifs kernel mount options: ip=10.x.x.x,unc=\\domain.local\DFS,vers=3.0,user=user,prefixpath=Share,pass=********
> mount error(2): No such file or directory
> Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)
> 
> Syslog shows:
> CIFS VFS: cifs_read_super: get root inode failed
> 
> I didn't find much of use online to troubleshoot that message. I did
> try different values of vers option as well as each of serverino and
> noserverino and several combinations of both - no difference. Any
> suggestions?
> 
> (Occasionally syslog also shows "CIFS VFS: error -2 on ioctl to get
> interface list". This happened about one in five mount attempts both
> before and after the kernel update, regardless of whether or not the
> share mounted. Since it did appear before the update when mounting
> always worked, I am assuming it is not relevant.)

This is probably a regression between 4.19.171 and 4.19.181.

There were cifs related changes. a738c93fb1c1 ("cifs: Set
CIFS_MOUNT_USE_PREFIX_PATH flag on setting cifs_sb->prepath.") might
be a candidate.

Can you please report a bug against the kernel?

If you can test a build, please do test with that commit reverted and
see if the issue disappers.

Regards,
Salvatore