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Date: Thu, 11 Jan 2024 17:41:06 +0100
To: cygwin AT cygwin DOT com
Subject: Re: Cygwin tools to read/write NTFS alternate data streams?
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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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

FTR, I'm generally not a friend of ADS, because they look like a builtin
security problem.  They don't show up in standard directory listings,
and you can't even see that a file has ADS, except you look for them
explicitely.  This is a really user-unfriendly interface.


Corinna

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