Cygwin tools to read/write NTFS alternate data streams?

[Roland Mainz]

On Thu, Jan 11, 2024 at 3:00 PM Martin Wege via Cygwin
<cygwin@cygwin.com> wrote:
> On Mon, Jan 8, 2024 at 3:11 PM Corinna Vinschen via Cygwin
> <cygwin@cygwin.com> wrote:
> > On Dec 18 18:47, Martin Wege via Cygwin wrote:
> > > On Fri, Dec 1, 2023 at 10:52 AM Corinna Vinschen via Cygwin
> > > <cygwin@cygwin.com> wrote:
[snip]
> > Apart from that, this sounds like a nice idea for Cygwin 3.6,
> > provided somebody implements it, https://cygwin.com/acronyms/#SHTDI
> >
> > Assuming we can live without actually having a subdir and just
> > allowing to open and create a file with the O_XATTR flag, it might be
> > pretty simple to implement.  The path handling code would just have to
> > drop the colon from the list of characters converted to the private-use
> > Unicode area.
> >
> > Implementing the subdir is a bit more complicated, especially when
> > taking opendir/readdir of that virtual subdir into account, but it
> > would certainly be doable.
>
> How do other OSes implement the O_XATTR subdir?

Basically you open a random file or dir with |openat(filename,
O_XATTR)| and get a fd to that (virtual) attribute directory.
The "." in that dir is virtual, but the ".." refers to the underlying
file, so a |open("..", ...)| will return a fd to a file/dir to which
the attributes belong to.

https://illumos.org/man/7/fsattr has the whole API description.

A Windows implementation should be therefore quite easy, as alternate
data streams show up as "foo:stream1", "foo:stream2" etc., so just the
directory lookup needs to be modified to skip the files with ':' in
general, and only show the streams for an O_XATTR subdir, e.g.
|openat("foo", O_XATTR)| would give a dir which lists only Windows
file names which match "foo:*"

I can support such an effort with testing, and a cleanroom
implementation of https://illumos.org/man/1/runat (see
https://github.com/illumos/illumos-gate/blob/master/usr/src/cmd/runat/runat.c
for the Solaris/Illumos code)

----

Bye,
Roland
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