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Subject: Re: Cygwin tools to read/write NTFS alternate data streams?
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From: Cedric Blancher via Cygwin <cygwin AT cygwin DOT com>
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On Thu, 11 Jan 2024 at 17:41, Corinna Vinschen via Cygwin
<cygwin AT cygwin DOT com> wrote:
>
> On Jan 11 15:00, Martin Wege via Cygwin wrote:
> > On Mon, Jan 8, 2024 at 3:11 PM Corinna Vinschen via Cygwin
> > <cygwin AT cygwin DOT com> wrote:
> > so this is IMO OK.
>
> Yeah, but...
>
> It's not just an open flag, it requires extending functionality of other
> APIs and, for full support, tools, see
> https://docs.oracle.com/cd/E19253-01/816-5175/6mbba7f02/.
>
> It's also rather weird to call alternate data streams "extended
> attributes", because that's just a small part of streams and it's
> already supported via the Linux functions getxattr(2) and friends, see
> https://man7.org/linux/man-pages/man2/getxattr.2.html
>
> > > 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?
>
> IDK.  But there are a few points we have to keep in mind from the
> Solaris man page:
>
> - Given fd is an open file descriptor,
>
>     openat(fd, ".", O_RDONLY|O_XATTR);
>
>   opens the virtual subdir containing the ADS of a file.  It's the
>   only valid way to open the virtual dir.  That's a bit of a relief
>   because it simplifies handling this in openat(2).
>
>     openat(fd, "foo", ...|O_XATTR);
>
>   allows to create or open an ADS for an open file fd.
>
> - unlinkat, renameat, fstatat, fchownat, futimesat and fdopendir need to
>   be made ADS-aware.  They only work on ADS if the fd arg already
>   resolves to an ADS, or if fd is the virtual ADS dir of a file and
>   (except in case of fdopendir) the path argument is the name of an
>   existing ADS.
>
> - pathconf needs to support a new flag XATTR_ENABLED.
>
> This can be made to work given the current fhandler framework in Cygwin,
> but again, https://cygwin.com/acronyms/#SHTDI

bash has cd -@ to enter a virtual dir with XATTRs, if O_XATTR works.

Aside from that, I volunteer with testing of O_XATTR, on demand with 1
or 2 students too.

Still working on testing the cool and new SEEK_HOLE support in Cygwin

Ced
-- 
Cedric Blancher <cedric DOT blancher AT gmail DOT com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur

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