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Subject: Re: Cygwin tools to read/write NTFS alternate data streams?
To: cygwin AT cygwin DOT com
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From: Martin Wege via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Martin Wege <martin DOT l DOT wege AT gmail DOT com>
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On Mon, Jan 8, 2024 at 3:11 PM Corinna Vinschen via Cygwin
<cygwin AT cygwin DOT 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 AT cygwin DOT com> wrote:
> > >
> > > On Nov 30 04:55, Martin Wege via Cygwin wrote:
> > > > Hello,
> > > >
> > > > Does Cygwin have tools (modified /usr/bin/dd ?) to read/write NTFS
> > > > alternate data streams?
> > >
> > > No.  As you know, the colon is translated to a normal filename
> > > character, and there's no POSIX-like API to expose ADS raw to user
> > > space.
> > >
> > > There is, however, an old function we still expose to user space
> > > for backward compat:
> > >
> > >   #include <sys/cygwin.h>
> > >
> > >   int cygwin_attach_handle_to_fd (char *name,
> > >                                   int fd,
> > >                                   HANDLE handle,
> > >                                   mode_t bin,
> > >                                   DWORD myaccess);
> > >
> > > This allows to sneak in a HANDLE into a Cygwin file descriptor
> > > representation, kind of like this:
> > >
> > >   HANDLE h;
> > >   int fd;
> > >
> > >   h = CreateFile ("foo:bar", GENERIC_READ, FILE_SHARE_VALID_FLAGS,
> > >                   NULL, OPEN_EXISTING, 0, NULL);
> > >   if (h != INVALID_HANDLE_VALUE)
> > >     {
> > >       fd = cygwin_attach_handle_to_fd ("foo", -1, h, 0, GENERIC_READ);
> > >       if (fd < 0)
> > >         bail_out;
> > >     }
> > >
> > > For the bin parameter, only 0, O_BINARY or O_TEXT are acceptable,
> > > for myaccess, only GENERIC_READ and/or GENERIC_WRITE are acceptable.
> >
> > Could this be abstracted into O_XATTR support, i.e. openat() file with
> > O_XATTR, and then have access to the alternate data streams in a
> > (virtual) subdir?
>
> I'm not too hot on adding more open(2) flags, the reason being,
> that we have room left for only 7 more flags in the 32 bit mode.
> Nobody knows what comes along officially in future.

Right, but Solaris, Illumos, and Opengroup draft already have O_XATTR,
so this is IMO OK.

Also, it would provide a good abstraction for alternate data stream
support, which is compatible with Windows, OSX, Solaris, Illumos, and
NTFS, ZFS, SMB, NFSv4.

>
> 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?

Thanks,
Martin

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