cp --reflink and target file open flags

[David Disseldorp]

On Wed, 16 Oct 2013 18:36:19 -0500
Steve French <smfrench at gmail.com> wrote:

> cp --reflink opens the target file for O_WRONLY before invoking the
> (BTRFS) ioctl for clone file, but for copy offload over the network
> the SMB2 specification requires that the target file be open O_RDWR.
> 
> I may be able to upgrade the target file handle on the fly by
> reopening it in cifs.ko, and of course I can write an SMB2/SMB3
> specific copy command, but it would be preferable to allow use of cp
> --reflink since so many people are familiar with it.
> 
> There is quite a bit of flexibility in server side copy offload  -
> more than cp an offer, especially when using SMB3 or later dialects
> (e.g. in number of chunks sent at one time, chunk size, attributes
> copied, and even whether to use T10 style offload), but still it would
> be nice to support "cp --reflink" over the network.  Any ideas on
> this?
> 
> After looking at copy.c in coreutils for cp - I couldn't think of any
> trivial way to force cp to open the target RW.
> 
> Ideas?

You should be able to avoid this by using FSCTL_SRV_COPYCHUNK_WRITE
instead of FSCTL_SRV_COPYCHUNK on the wire. The former doesn't require
read access on the target, while the latter does. See [MS-SMB2] 2.2.31
and smbtorture's copy_chunk_bad_access test.

Samba only supported FSCTL_SRV_COPYCHUNK until now, as that's what
Windows Server 2k12 uses for copies initiated by Explorer. I've just
sent out the trivial patches adding FSCTL_SRV_COPYCHUNK_WRITE support.

Cheers, David