Re: [PATCH 6/7] cifs: Fix creating of SFU fifo and socket special files

[Pali Rohár]

Hello Steve,

I have another argument for the empty system file as fifo over the
file with "LnxFIFO" content.

Now I have figured out that even the latest Windows Server 2022 version
provides interoperability of FIFOs in SFU format with Windows NFS 4.1
Server. So if you configure on Windows Server 2022 one share which is
exported over SMB and also NFS at the same time, and over SMB you create
SFU-style fifo, then Windows NFS4.1 server recognize it and properly
reports nfs4type as NFS4FIFO for NFSv4.1 client.

So this SFU FIFO style is not only for old clients and servers, but
still relevant and useful even for latest Windows Server.

And same applies for named sockets.

For testing this scenario it is enough to use just trial version of
latest Windows Server from:
https://go.microsoft.com/fwlink/p/?linkid=2208182
https://go.microsoft.com/fwlink/p/?Linkid=2195280

For setting NFS4.1 and SMB share named "test" I used these cmd commands:

  dism /Online /Enable-Feature /All /FeatureName:ServerForNFS-Infrastructure
  md C:\test
  icacls C:\test /grant Everyone:(OI)(CI)F /T
  nfsshare test=C:\test -o rw unmapped=yes
  net share test=C:\test /grant:Everyone,FULL

(for everyone who is going to reproduce this scenario, beware that new
Windows servers use by default powershell, so first launch cmd.exe then
copy+paste those commands)

Pali

On Saturday 14 September 2024 10:17:42 Pali Rohár wrote:
> On Saturday 14 September 2024 01:21:17 Steve French wrote:
> > On Fri, Sep 13, 2024 at 5:42 PM Pali Rohár <pali@xxxxxxxxxx> wrote:
> > >
> > > On Friday 13 September 2024 17:14:22 Steve French wrote:
> > > > How did you find the format of the FIFO and SOCK file types?  I
> > >
> > > For fifo there are multiple sources on internet, but none of them is
> > > normative. Everything is just what people have tried. For example this
> > > old email on samba list:
> > > https://lists.samba.org/archive/linux-cifs-client/2005-May/000856.html
> > >
> > > Format of the socket I have figured out by creating it in Interix
> > > subsystem and then dumped content of the file from Win32 subsystem.
> > > Then I checked that it has also same format over older MS NFS server.
> > > It was easier than trying to search for some documentation (which I have
> > > not found).
> > >
> > > > couldn't find any references to those so added two new types to allow
> > > > current Linux to be able to create these (especially since they are
> > > > opaque to the server and thus low risk).
> > >
> > > I was searching over internet again and now I have found patent
> > > https://patents.google.com/patent/US20090049459 which describe symlink
> > > content:
> > >
> > > #define NFS_SPECFILE_LNK_V1  0x014b4e4c78746e49 /* “IntxLNK” */
> > >
> > > But does not describe other types.
> > >
> > > > > +     case S_IFSOCK:
> > > > > -             strscpy(pdev.type, "LnxSOCK");
> > > > > +             /* SFU socket is system file with one zero byte */
> > > > > +             pdev_len = 1;
> > > > > +             pdev.type[0] = '\0';
> > > > >               break;
> > > > >       case S_IFIFO:
> > > > > -             strscpy(pdev.type, "LnxFIFO");
> > > > > +             /* SFU fifo is system file which is empty */
> > > > > +             pdev_len = 0;
> > > >
> > > > My worry about the suggested change above is that it is possible that
> > > > we could accidentally match to an empty file.
> > >
> > > I fully understand your concerns, but code in this patch is for creating
> > > new fifos. Not recognizing existing fifos.
> > >
> > > Code for recognizing existing fifos (=empty file with system attribute)
> > > was not changed and is in Linux cifs client for a very long time.
> > <>
> > > > We intentionally added
> > > > the two new dev.type fields for these to avoid collisions with other
> > > > things (and since they are Linux specific).  It seems risky to have an
> > > > empty file with the system attribute marked as a FIFO, and similarly a
> > > > file with one byte null as Socket.   Since this is for only the Linux
> > > > client to recognize, I wanted to do something safe for those file
> > > > types less likely to be confusing (ie strings for Socket and FIFO that
> > > > were similar in length and format to the other special files seemed
> > > > intuitive) and "LnxFIFO" and LnxSOCK" were used as the tags in the
> > > > file to reduce confusion ie the tags for those two start with "Lnx" -
> > > > ie "something used for Linux client" not related to the original
> > > > Interix (those begin with "Intx").
> > >
> > > I see. Now I understand what are those types (as I have not seen them
> > > before). It is somehow misleading if such "LnxFIFO" and LnxSOCK"
> > > functionality is provided by SFU option, but is incompatible with MS SFU
> > > and also with MS NFS server. And is also incompatible with older Linux
> > > cifs clients (as they do not understand those Lnx types).
> > 
> > I am not as worried about FIFO and SOCK type being recognized by
> > older servers (since almost every use case for them would be for them
> > to be seen (only) by the client - e.g. for mounts to servers that
> > don't implement reparse points yet), and since they are less
> > common file types I am willing to let them be unrecognized by
> > old clients (we can tag them for stable if older distros don't have
> > them),
> 
> This is quite pity for old clients, to break existing interoperability.
> At least I see sfu as an compatibility option either for ecosystem with
> old clients, or option where server itself does not support reparse
> points.
> 
> > but I am concerned about allowing "current clients" to
> > create empty files for an unusual purpose which could be
> > confusing/non-intuitive.
> 
> I understand this concern. I thought that this should not be an issue
> because files are created with system attribute which is not common for
> normal/ordinary usage (system attribute could be less confusing) and
> also because this format, at least for fifo is used and understood by
> many SW for about 30 years.
> 
> > And since this change (at least the one to allow FIFOs to be created with "sfu"
> > has been in mainline for a year and also since it uses a more intuitive tag
> > ("LnxFIFO") than the empty one used by very old Windows) the only
> > clients who would have created these would be already using this newer tag
> > (older Linux clients couldn't have created such files - there seems more
> > risk of regression with reverting the change than with continuing with
> > the Linux client specific tag (at least to the one for FIFOs
> > since that has been in much longer than the socket one which is recent)
> 
> This kind of stuff is lot of times used on LTS/stable linux
> distributions and new kernel to these users/admins do not have to be
> delivered yet. Mostly it takes 2-3 years after release. Look for example
> at RHEL cycles.
> 
> I'm looking on this from opposite perspective. I see this an regression
> in -o sfu option that after upgrading from previous LTS version to new,
> -o sfu stopped to be compatible with SFU-style fifos.
> 
> But your point is valid. But maybe it is not an issue because users
> do not have updated yet to new version?
> 
> > Will discuss with others - opinions welcome.
> > 
> > There is an upcoming SMB3.1.1 test event coming up next week (and the annual
> > Storage Developer Conference too) so I can see if others have opinions one
> > way or another on whether to move to empty (or 1 byte) files for
> > creating fifos/sockets
> 
> Ok, perfect, let me know then about the result.
> 
> > > > Note that in the long run we hope to use reparse points by default in
> > > > more servers to store special files like this but there are a few
> > > > cases for unusual workloads that need special file support that would
> > > > have to use sfu still.  The newer reparse tags that Windows uses "WSL"
> > > > have the advantage that they require fewer roundtrips to query (since
> > > > the file type is in the reparse tag).
> > >
> > > Yes, new WSL tags seems to be better. Also SFU mount option is not
> > > activated by default.
> > >
> > > > Also noticed an interesting problem when mounted with "sfu" -
> > > > "smbgetinfo filebasicinfo /mnt/fifo1" will hang (in sys_open).  Is
> > > > that expected for a FIFO?
> > >
> > > Reading from fifo sleep reading process until some other process write
> > > data to fifo. This is how fifos are working. You can try it on local
> > > filesystem (e.g. ext4 or tmpfs).
> > 
> > makes sense - thx