* Christian Schoenebeck (qemu_oss@xxxxxxxxxxxxx) wrote: > On Mittwoch, 12. August 2020 13:18:19 CEST Dr. David Alan Gilbert wrote: > > * Christian Schoenebeck (qemu_oss@xxxxxxxxxxxxx) wrote: > > > On Dienstag, 4. August 2020 13:28:01 CEST Dr. David Alan Gilbert wrote: > > > > > Well, depends on how large you draw the scope here. For instance Samba > > > > > has > > > > > a bunch VFS modules which also uses and hence prohibits certain > > > > > xattrs. > > > > > For instance for supporting (NTFS) alternate data streams (a.k.a. > > > > > resource forks) of Windows clients it uses user.DosStream.*: > > > > > > > > > > https://www.samba.org/samba/docs/current/man-html/vfs_streams_xattr.8. > > > > > html > > > > > > > > > > as well as "user.DOSATTRIB". > > > > > > > > > > And as macOS heavily relies on resource forks (i.e. macOS doesn't work > > > > > without them), there are a bunch of xattr remappings in the dedicated > > > > > Apple VFS module, like "aapl_*": > > > > > > > > > > https://www.samba.org/samba/docs/current/man-html/vfs_fruit.8.html > > > > > https://github.com/samba-team/samba/blob/master/source3/modules/vfs_fr > > > > > uit. > > > > > c > > > > > > > > Thanks; what I've added to virtiofsd at the moment is a generic > > > > remapping thing that lets me add any prefix and block/drop any xattr. > > > > > > Right, makes absolutely sense to make it configurable. There are too many > > > use cases for xattrs, and the precise xattr names are often configurable > > > as well, like with the mentioned Samba VFS modules. > > > > > > > The other samba-ism I found was mvxattr(1) which lets you rename xattr's > > > > ona directory tree; which is quite useful. > > > > > > Haven't seen that before, interesting. > > > > > > BTW, I have plans for adding support for file forks [1] (a.k.a. alternate > > > streams, a.k.a. resource forks) on Linux kernel side, so I will probably > > > come up with an RFC in couple weeks to see whether there would be > > > acceptance for that at all and if yes in which form. > > > > > > That would open a similar problematic to virtiofsd on the long term, as > > > file forks have a namespace on their own. > > > > Yeh I'm sure that'll need wiring into lots of things in weird ways! > > I guess the main difference between an extended attribute and a > > file-fork is that you can access the fork using an fd and it feels more > > like a file? > > Well, that's a very short reduction of its purpose, but it is a common core > feature, yes. > > xattrs are only suitable for very small data (currently <= 64 kiB on Linux), > whereas file forks can be as large as any regular file. And yes, forks > commonly work with fd, so they allow you to do all kinds of I/O operations on > them. Theoretically though you could even allow to use forks with any other > function that accepts an fd. > > The main issue is that file forks are not in POSIX. So every OS currently has > its own concept and API, which probably makes a consensus more difficult for > Linux. > > For instance Solaris allows you to set different ownership and permissions on > forks as well. It does not allow you to create sub-forks though, nor directory > structures for forks. Yeh that's quite a change in semantics. > On macOS there was (or actually still is) even a quite complex API which > separated forks into "resource forks" and "data forks", where resource forks > were typically used as components of an application binary (e.g. menu > structure, icons, executable binary modules, text and translations). So > resource forks not only had names, they also had predefined 16-bit type > identifiers: > https://en.wikipedia.org/wiki/Resource_fork Yeh, lots of different ways. In a way, if you had a way to drop the 64kiB limit on xattr, then you could have one type of object, but then add new ways of accessing them as forks. Dave > Best regards, > Christian Schoenebeck > > -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
