Different behavior of POSIX file locks depending on cache mode

Mon Jul 22 20:31:23 UTC 2024

> I was hoping to include the old SMB1 unix extensions in this test also,
but these seem unsupported in current kernels.  When did they go away?

The SMB1 Unix Extensions still work to Samba but ... In investigating this
I found a more serious SMB1 problem.  Reconnect with SMB1 is broken to
Samba - we mount fine with SMB1 (assuming SMB1 is not disabled in the Linux
config) to Samba server and SMB1 Unix Extensions are negotiated by default
(assuming the server hasn't turned them off) but reconnect fails.   With
various rewrites/cleanup of the reconnect code, it is now handled in a
worker thread which skips the call to "reset_cifs_unix_caps()" (to do the
SetFSInfo on the tcon required to enable Unix Extensions for SMB1).   Am
trying to fix that but a little tricky to find best place to call that
function.

On Sun, Jun 9, 2024 at 11:41 PM Andrew Bartlett <abartlet at samba.org> wrote:

> (resend due spam rules on list)
>
> Kia Ora Steve,
>
> I'm working with Kevin on this, and I set up a clean environment with
> the latest software to make sure this is all still an issue on current
> software:
>
> I was hoping to include the old SMB1 unix extensions in this test also,
> but these seem unsupported in current kernels.  When did they go away?
>
> Anyway, here is the data.  It certainly looks like an issue with the
> SMB3 client, as only the client changes with the cache=none
>
> Server is Samba 4.20.1 from Debian Sid.  Kernel is
> Linux debian-sid-cifs-client 6.7.9-amd64 #1 SMP PREEMPT_DYNAMIC Debian
> 6.7.9-2 (2024-03-13) x86_64 GNU/Linux
>
> With SMB1 but not unix extensions (seems unsupported):
>
> root at debian-sid-cifs-client:~# mount.cifs //192.168.122.234/testuser
> mnt -o user=testuser,pass=pass,vers=1.0
> root at debian-sid-cifs-client:~# cd mnt/
> root at debian-sid-cifs-client:~/mnt# ../lock_test foo
> Testing with foo
> Got new file descriptor 3
> Lock set: 1
> Second file descriptor 4
> Read from second fd: x count: 0
> Third file descriptor 5
> Wrote to third fd: 1
>
> root at debian-sid-cifs-client:~# mount.cifs //192.168.122.234/testuser
> mnt -o user=testuser,pass=penguin12#,vers=3.1.1,posix
> root at debian-sid-cifs-client:~# cd mnt/
> root at debian-sid-cifs-client:~/mnt# ../lock_test foo
> Testing with foo
> Got new file descriptor 3
> Lock set: 1
> Second file descriptor 4
> Read from second fd: x count: -1
> Third file descriptor 5
> Wrote to third fd: -1
> root at debian-sid-cifs-client:~# mount.cifs //192.168.122.234/testuser
> mnt -o user=testuser,pass=penguin12#,vers=3.1.1,unix
>
> root at debian-sid-cifs-client:~# mount.cifs //192.168.122.234/testuser
> mnt -o user=testuser,pass=penguin12#,vers=3.1.1,unix,nobrl
> root at debian-sid-cifs-client:~# cd mnt/
> root at debian-sid-cifs-client:~/mnt# ../lock_test foo
> Testing with foo
> Got new file descriptor 3
> Lock set: 1
> Second file descriptor 4
> Read from second fd: o count: 1
> Third file descriptor 5
> Wrote to third fd: 1
>
> And with cache=none
>
> root at debian-sid-cifs-client:~# mount.cifs //192.168.122.234/testuser
> mnt -o user=testuser,pass=penguin12#,vers=3.1.1,posix,cache=none
> root at debian-sid-cifs-client:~# cd mnt/
> root at debian-sid-cifs-client:~/mnt# ../lock_test foo
> Testing with foo
> Got new file descriptor 3
> Lock set: 1
> Second file descriptor 4
> Read from second fd: o count: 1
> Third file descriptor 5
> Wrote to third fd: 1
>
> On Thu, 2024-05-23 at 11:12 -0500, Steve French wrote:
> > What is the behavior with "nobrl" mount option? and what is the
> > behavior when running with the POSIX extensions enabled (e.g. to
> > current Samba or ksmbd adding "posix" to the mount options)
> >
> > On Thu, May 23, 2024 at 11:08 AM Kevin Ottens <
> > kevin.ottens at enioka.com
> > > wrote:
> > > Hello,
> > >
> > > I've been hunting down a bug exhibited by Libreoffice regarding
> > > POSIX file
> > > locks in conjunction with CIFS mounts. In short: just before
> > > saving, it
> > > reopens a file on which it already holds a file lock (via another
> > > file
> > > descriptor in the same process) in order to read from it to create
> > > a backup
> > > copy... but the read call fails.
> > >
> > > I've been in discussion with Andrew Bartlett for a little while
> > > regarding this
> > > issue and, after exploring several venues, he advised me to send an
> > > email to
> > > this list in order to get more opinions about it.
> > >
> > > The latest discovery we did was that the cache option on the
> > > mountpoint seems
> > > to impact the behavior of the POSIX file locks. I made a minimal
> > > test
> > > application (attached to this email) which basically does the
> > > following:
> > >  * open a file for read/write
> > >  * set a POSIX write lock on the whole file
> > >  * open the file a second time and try to read from it
> > >  * open the file a third time and try to write to it
> > >
> > > It assumes there is already some text in the file. Also, as it goes
> > > it outputs
> > > information about the calls.
> > >
> > > The output I get is the following with cache=strict on the mount:
> > > ---
> > > Testing with /mnt/foo
> > > Got new file descriptor 3
> > > Lock set: 1
> > > Second file descriptor 4
> > > Read from second fd: x count: -1
> > > Third file descriptor 5
> > > Wrote to third fd: -1
> > > ---
> > >
> > > If I'm using cache=none:
> > > ---
> > > Testing with /mnt/foo
> > > Got new file descriptor 3
> > > Lock set: 1
> > > Second file descriptor 4
> > > Read from second fd: b count: 1
> > > Third file descriptor 5
> > > Wrote to third fd: 1
> > > ---
> > >
> > > That's the surprising behavior which prompted the email on this
> > > list. Is it
> > > somehow intended that the cache option would impact the semantic of
> > > the file
> > > locks? At least it caught me by surprise and I wouldn't expect such
> > > a
> > > difference in behavior.
> > >
> > > Now, since the POSIX locks are process wide, I would have expected
> > > to have the
> > > output I'm getting for the "cache=none" case to be also the one I'm
> > > getting
> > > for the "cache=strict" case.
> > >
> > > I'm looking forward to feedback on this one. I really wonder if we
> > > missed
> > > something obvious or if there is some kind of bug in the cifs
> > > driver.
> > >
> > > Regards.
> > > --
> > > Kévin Ottens
> > > kevin.ottens at enioka.com
> > >
> > > +33 7 57 08 95 13
> >
> >
>
> --
>
> Andrew Bartlett (he/him)       https://samba.org/~abartlet/
> Samba Team Member (since 2001) https://samba.org
> Samba Team Lead                https://catalyst.net.nz/services/samba
> Catalyst.Net <https://catalyst.net.nz/services/sambaCatalyst.Net> Ltd
>
> Proudly developing Samba for Catalyst.Net Ltd - a Catalyst IT group
> company
>
> Samba Development and Support: https://catalyst.net.nz/services/samba
>
> Catalyst IT - Expert Open Source Solutions
>
> --
> Andrew Bartlett (he/him)       https://samba.org/~abartlet/
> Samba Team Member (since 2001) https://samba.org
> Samba Team Lead                https://catalyst.net.nz/services/samba
> Catalyst.Net <https://catalyst.net.nz/services/sambaCatalyst.Net> Ltd
>
> Proudly developing Samba for Catalyst.Net Ltd - a Catalyst IT group
> company
>
> Samba Development and Support: https://catalyst.net.nz/services/samba
>
> Catalyst IT - Expert Open Source Solutions
>

-- 
Thanks,

Steve