FAT, NTFS, CIFS and DOS attributes

tridge at samba.org tridge at samba.org
Tue Jan 4 00:58:18 GMT 2005

 > Oh geez.  Couldn't you have split out the various data items into 
 > separate xattrs?

fetching and setting this stuff is _really_ common, so I have arranged
to store them in a way to make it as efficient as possible. For
example, when a remote client asks for a directory listing we need to
fetch just one xattr from the kernel per file (directory listings
in the windows world usually return the equivalent of a full stat
structure for every name).

Similarly, when these are set the client tends to set more than one at
a time. The most commonly used set call takes this structure:

	struct {
		enum smb_setfileinfo_level level;
		union setfileinfo_file file;

		struct {
			NTTIME create_time;
			NTTIME access_time;
			NTTIME write_time;
			NTTIME change_time;
			uint32_t attrib;
		} in;
	} basic_info;

Having the items that tend to get read/written together grouped
together allowed me to make it all quite efficient, while still having
a reasonable chance of the EAs all fitting in-inode on filesystems
that support that.

Obviously it is racy when dealt with from user space, but there really
is no way to avoid all these races without a user space accessible
"lock the files meta-data" call and that is why I'm looking forward to
having a Samba LSM module to avoid these races.

 > Samba clearly has other needs than other users, although of course
 > it would be unfortunate if Samba then can't export this
 > information.

I think you'll find that all users of dos attributes on Linux will
have very similar needs to Samba, and will want these things grouped
together. For example:

 - backup/restore apps will want to backup/restore these attributes as
 - wine implements essentially the same APIs as Samba, just in a
   different form, and so tends to get the same groupings of
   attributes get/set calls that Samba does (the SMB protocol is to a
   large degree a on-the-wire version of Win32).

Are there any other significant users of DOS attributes on Linux that
want something different?

Cheers, Tridge

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