[cifs-protocol] Clarification on length limit in SMB2_FILE_RENAME_INFORMATION filename

[Jeremy Allison]

Hi Dochelp,

A Samba user discovered a strange issue running the
smbclient code against a Windows 10 server.

Trying to rename a file over SMB2 at the top
level fails with NT_STATUS_INFO_LENGTH_MISMATCH
if the target name is a single character.

E.g. renaming AAAA -> Z

returns NT_STATUS_INFO_LENGTH_MISMATCH.

The Samba SMB2 server accepts this rename
request without complaint, as does the
Windows 10 SMB1 server.

I've fixed this on the Samba client side
by checking if the SMB2_FILE_RENAME_INFORMATION
buffer size is less than 24 bytes, and
padding out with zeros if this is the
case.

The definition of SMB2_FILE_RENAME_INFORMATION
shows the filename should be a variable-length
non-null terminated UCS2 field, but it seems
that a length of 2 is too short for the Windows
server.

Any target name longer than this ends up with
the SMB2_FILE_RENAME_INFORMATION being 24-bytes
or greater, and the rename succeeds (so long
as the name is a valid one).

It's not a 4-byte alignment issue, as I can
vary the target name by adding characters and
seeing the target buffer expand by 2 bytes
per additional character.

I think Windows just has a hard check internally
that the rename buffer must be >= 24 bytes, and
the Windows client always pads to that.

It would be good to have a Windows behavior
note confirming this.

Thanks !

Jeremy.