Re: [PATCH 0/7] cifs: Improve client SFU support for special files

[Steve French]

tentatively merged into cifs-2.6.git for-next pending review and more testing

Do you remember if there is a reference to  sections of the protocol
documentation that can help confirm some of the changes?

On Thu, Sep 12, 2024 at 7:06 AM Pali Rohár <pali@xxxxxxxxxx> wrote:
>
> This patch series implement full support for SFU-style of special files
> (fifo, socket, block, char, symlink) and makes client cifs's -o sfu
> mount option to be fully compatible with SFU.
>
> Before this patch series, kernel cifs client was able to:
> * create new char and block devices in SFU-style, and detect them
> * detect existing SFU fifo, but no create/mkfifo SFU functionality
> * detect symlink SFU symlink, but no readlink() functionality and
>   neither no create SFU symlink functionality
>
> And it was able to create some SFU-incompatible sockets and fifos (when
> -o sfu was specified) which were not recognized by neither original MS
> SFU implementation and neither by others.
>
> This patch series implements missing functionality, which is:
> * detect existing SFU sockets
> * create new SFU sockets
> * create new SFU fifos
> * create new SFU symlink
> * readlink() support for existing SFU symlinks
>
> In following pragraphs are some details about these SFU special files
> which usage on Linux has to be activated by -o sfu mount option.
>
> SFU-style fifo is empty file with system attribute set. This format is
> used by old Microsoft POSIX subsystem and later also by OpenNT/Interix
> subsystem (which replaced Microsoft POSIX subsystem and is part of
> Microsoft SFU). OpenNT is previous name of Interix versions 1.x/2.x.
> Microsoft POSIX subsystem is available since the first Windows NT
> version 3.1, and it was replaced by Interix since Windows XP. Interix
> continue to use this format up to the its last released version for
> Windows 8 / Server 2012 (part of Subsystem for UNIX-based Applications).
> Hence these SFU-style fifos are for very long time unified and widely
> supported.
>
> SFU-style socket is file which has system attribute set and file content
> is one zero byte.
>
> SFU-style symlink is file which has system attribute set and file content
> is buffer "IntxLNK\1" (8th byte is 0x01) followed by the target location
> encoded in little endian UCS-2/UTF-16. There is no trailing nul.
>
> SFU-style char or block device is file which has system attribute set
> and file content is buffer "IntxBLK\0" or "IntxCHR\0" (8th byte is 0x00)
> followed by major and minor numbers encoded in twos 64-bit little endian.
>
> Format of SFU-style sockets, symlinks, char and block devices was
> introduced in Interix 3.0 subsystem, as part of the Microsoft SFU 3.0
> and is used also by all later versions, up to the Windows 8 / Server 2012
> (part of Subsystem for UNIX-based Applications) where it was deprecated.
> Previous OpenNT/Interix versions had no support for UNIX domain sockets
> (and socket files), symlinks or possibility to create character or block
> devices (but they had block/char devices in-memory, returned by stat()).
>
> Microsoft NFS server up to the version included in Windows Server 2008 R2
> also uses this SFU-style format of special files when storing them on
> local filesystem. Later Microsoft NFS server versions (starting in
> Windows Server 2012) use new NFS reparse format, which Interix subsystem
> (included in SFU or SUA) does not understand.
>
> Even SFU-style of special files is old format, it has one big advantage,
> this format does not require any support on SMB/CIFS server of special
> files, as everything is stored only in the file content. The only
> requirement from server is support for system attribute. So this allows
> to store special files also on FAT filesystem.
>
> This patch series makes cifs -o sfu mount option compatible with
> SFU-style of special files, and so compatible with the latest SFU/SUA.
>
> Note that -o sfu is by default turned off, so these changes should have
> no effect on default cifs mounts.
>
> Manually tested with MS SFU 3.5 (for Windows XP) and MS SUA 6.2 (latest
> released version of Interix) that interop works correctly, special files
> created by POSIX/Interix application can be recognized by Linux cifs
> client (exported over MS SMB) with these patches (and vice-versa setup,
> created by Linux cifs client and recognized in POSIX/Interix subsystem).
>
> Manually tested that old Linux 4.19 cifs client version can recognize
> SFU-style of special files created by Linux cifs client this patch
> series (except socket, which is unsupported in this Linux cifs version).
>
> Patch series is based on the latest upstream tag v6.11-rc7.
>
> Pali Rohár (7):
>   cifs: Fix recognizing SFU symlinks
>   cifs: Add support for reading SFU symlink location
>   cifs: Put explicit zero byte into SFU block/char types
>   cifs: Show debug message when SFU Fifo type was detected
>   cifs: Recognize SFU socket type
>   cifs: Fix creating of SFU fifo and socket special files
>   cifs: Add support for creating SFU symlinks
>
>  fs/smb/client/cifspdu.h    |  6 ---
>  fs/smb/client/cifsproto.h  |  4 ++
>  fs/smb/client/cifssmb.c    |  8 ++--
>  fs/smb/client/fs_context.c | 13 ++++---
>  fs/smb/client/inode.c      | 42 ++++++++++++++++++--
>  fs/smb/client/link.c       |  3 ++
>  fs/smb/client/smb1ops.c    |  2 +-
>  fs/smb/client/smb2ops.c    | 78 ++++++++++++++++++++++++++++----------
>  8 files changed, 117 insertions(+), 39 deletions(-)
>
> --
> 2.20.1
>
>


-- 
Thanks,

Steve