Date: Mon, 8 Jan 2024 15:09:38 +0100
To: cygwin@cygwin.com
Subject: Re: Cygwin tools to read/write NTFS alternate data streams?
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From: Corinna Vinschen via Cygwin <cygwin@cygwin.com>
Reply-To: cygwin@cygwin.com
Cc: Corinna Vinschen <corinna-cygwin@cygwin.com>
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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:
> >
> > 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.

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.


Corinna

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