Cygwin 3.7 wishlist: read/write NTFS alternate data streams via openat(O_XATTR)

[Corinna Vinschen]

On Jul 15 19:07, Lionel Cons wrote:
> Corinna?
> 
> Lionel

Well, we're on the cygwin-developers mailing list here.  I expect people
on this list having checked out the Cygwin repository and being able
to check what has been done and what hasn't by themselves.  Git is really
helpful there.


Corinna


> 
> On Sat, 5 Apr 2025 at 00:20, Lionel Cons <lionelcons1972@gmail.com> wrote:
> >
> > On Tue, 1 Apr 2025 at 11:01, Lionel Cons <lionelcons1972@gmail.com> wrote:
> > >
> > > On Wed, 26 Mar 2025 at 12:30, Corinna Vinschen
> > > <corinna-cygwin@cygwin.com> wrote:
> > > >
> > > > On Mar 25 19:21, Lionel Cons wrote:
> > > > > On Tue, 25 Mar 2025 at 17:19, Thomas Wolff <towo@towo.net> wrote:
> > > > > >
> > > > > > [oops, used wrong Reply function]
> > > > > >
> > > > > > Am 24.03.2025 um 21:58 schrieb Corinna Vinschen:
> > > > > > > On Mar 24 21:42, Thomas Wolff wrote:
> > > > > > >>
> > > > > > >> Am 24.03.2025 um 17:08 schrieb Jeffrey Altman:
> > > > > > >>> On 3/24/2025 10:25 AM, Martin Wege via Cygwin wrote:
> > > > > > >>>> Hello,
> > > > > > >>>>
> > > > > > >>>> another wishlist entry (old one):
> > > > > > >>>> read/write NTFS alternate data streams via openat(O_XATTR)
> > > > > > >>>  From my perspective as a filesystem developer, Extended Attributes and
> > > > > > >>> Alternate Data Streams are different concepts which I believe are
> > > > > > >>> inappropriate to mix.
> > > > > > >>>
> > > > > > >>> Filesystems can implement one, the other, both or neither.
> > > > > > >>>
> > > > > > >>> An Extended Attribute is a name:value pair that can be attached to an
> > > > > > >>> inode. The size of individual values and the sum of the value sizes
> > > > > > >>> are often quite constrained because they are stored in the inode
> > > > > > >>> metadata. Copying an inode is guaranteed to copy the Extended Attributes.
> > > > > > >>>
> > > > > > >>> Alternate Data Streams are typically represented as independent
> > > > > > >>> inodes. The Alternate Data Stream namespace can be represented by a
> > > > > > >>> directory inode referenced from the file inode. Each ADS inode might
> > > > > > >>> have their own Extended Attributes to go along with the data stream.
> > > > > > >>> ACLs on ADS inodes can also be independent of the ACL on the primary
> > > > > > >>> inode depending upon the implementation.
> > > > > > >>>
> > > > > > >>> Therefore, I am reluctant to see openat(O_XATTR) be used to access
> > > > > > >>> alternate data streams.
> > > > > > >>>
> > > > > > >>> Jeffrey Altman
> > > > > > >> Why not use the Windows syntax?
> > > > > > >> cat file.txt:ads.txt
> > > > > > >> etc. (Was this discussed before?)
> > > > > > >> Thomas
> > > > > > > The colon is a valid filename character in Cygwin, it will be converted
> > > > > > > to the UNICODE private use area:
> > > > > > > https://cygwin.com/cygwin-ug-net/using-specialnames.html#pathnames-specialchars
> > > > > > >
> > > > > > > Therefore you can't use the colon to specify streams.
> > > > > > >
> > > > > > >
> > > > > > > Corinna
> > > > > > OK, so what about a slash? Handling the base file like a directory for
> > > > > > access and listing...
> > > > >
> > > > > ResierFS tried that, and Linus Torvalds personally crushed John Reiser for that.
> > > > >
> > > > > bash has cd -@ dir and cd -@ file to go into a virtual O_XATTR dir,
> > > > > UNIX has /bin/runat to do the same
> > > >
> > > > In Cygwin, openat is just a frontend to open.  I. e., if we support
> > > > openat(O_XATTR), we have to support open(O_XATTR) and define a way to
> > > > specify a colon in the filename so that it's not converted to the
> > > > UNICODE private use area.
> > > >
> > > > A preliminary change to the path handling code to disable encoding
> > > > the colon is necessary for that.
> > > >
> > > > With this pretty simple change, openat(fd, "bar", O_XATTR) should work
> > > > already,
> > >
> > > That would be cool...
> >
> > ... this is not implemented already, right?
> >
> > Lionel
> 
> 
> 
> -- 
> Lionel