ms-nfs41 mount: Where does Unix_User+0 and Unix_Group+0 come from?

[Martin Wege]

On Fri, Aug 4, 2023 at 1:07 AM Martin Wege <martin.l.wege@gmail.com> wrote:
>
> On Thu, Aug 3, 2023 at 8:46 PM Martin Wege <martin.l.wege@gmail.com> wrote:
> >
> > Hello,
> >
> > We are working to deploy the ms-nfs41-nfs-filesystem on our Windows
> > machines, and encounter some issues with Cygwin (latest stable).
> >
> > For example, if we go to the ms-nfs41-nfs-filesystem (not the NFS
> > version 3 client from Windows!!!) with cd X: ; cd tmp/data9/, and do a
> > ls -l we get 'Unix_User+0' and 'Unix_Group+0' for user and group
> > names.
> > The question is: Where do these names come from - Windows,
> > ms-nfs41-nfs-filesystem or Cygwin?
> > getent passwd and getent group do not list this on Cygwin.
> > NFS version 4 server "lordbatman" has user mlw=uid(3620) and group mlw=gid(3620)
> >
> > Steps to reproduce:
> > 1. Download and install cygwin.
> > 2. Download http://www.citi.umich.edu/projects/nfsv4/windows/120612/
> > and install it.
> > 3. Start NFS version 4 client daemon:
> > ms-nfs41-client-x64/nfsd_debug.exe -d 3 --noldap --gid 3620 --uid 3620
> > 4. Mount NFS version 4 directory:
> > ms-nfs41-client-x64/ms-nfs41-client-x64/nfs_mount.exe -p -o sec=sys X
> > 'lordbatman.bias.dfn.de:/export/home/mlw'
> >
> > Expectation:
> > Cygwin ls -l should list file user/group as mlw/mlw
> >
> > Actual results:
> > Cygwin ls -l lists file user/group as Unix_User+0/Unix_Group+0
>
> Did a cygwin source clone, and found this:
> cygwin$ grep -r -E  'Unix_(User|Group)' .
> ./newlib-cygwin/winsup/utils/mkgroup.c:             "Unix_Group",
> ./newlib-cygwin/winsup/utils/mkgroup.c:                 "Unix_Group",
> ./newlib-cygwin/winsup/utils/mkpasswd.c:                    "Unix_User",
> ./newlib-cygwin/winsup/utils/mkpasswd.c:
>
> But I do not understand this code. In which cases are Unix_User+0 and
> Unix_Group+0 used?

It just got more mysterious. I tried a ls -nl (uid, gid as numbers,
not names from /etc/passwd and /etc/group), and got this:
ls -nl
total 78474
-rwxr-xr-x 1 4278190080 4278190080    33063 Aug  3 12:02 CMakeCache.txt
drwxr-xr-x 1 4278190080 4278190080      374 Aug  3 12:51 CMakeFiles
-rwxr-xr-x 1 4278190080 4278190080     4790 Aug  3 12:02 CPackConfig.cmake
-rwxr-xr-x 1 4278190080 4278190080     5276 Aug  3 12:02 CPackSourceConfig.cmake
-rwxr-xr-x 1 4278190080 4278190080      440 Aug  3 12:03 CTestTestfile.cmake
-rwxr-xr-x 1 4278190080 4278190080    77004 Aug  3 12:03 Makefile

So where does the "4278190080" come from, for uid and gid? Hexadecimal
its 0xff000000, which might be related to this:
./newlib-cygwin/winsup/cygwin/local_includes/security.h
/* For UNIX accounts not mapped to Windows accounts via winbind, Samba returns
  SIDs of the form S-1-22-x-y, with x == 1 for users and x == 2 for groups,
  and y == UNIX uid/gid.  NFS returns no SIDs at all, but the plain UNIX
  uid/gid values.

  UNIX uid/gid values are mapped to Cygwin uid/gid values 0xff000000 +
  unix uid/gid.  This *might* collide with a posix_offset of some trusted
  domain, but it's *very* unlikely.  Define the mapping as macro. */
#define UNIX_POSIX_OFFSET               (0xff000000)
#define UNIX_POSIX_MASK                 (0x00ffffff)
#define MAP_UNIX_TO_CYGWIN_ID(id)       (UNIX_POSIX_OFFSET \
                                        | ((id) & UNIX_POSIX_MASK))

But why is it then Unix_User+0 and not Unix_User+3620 on Windows NFS?
Same for Unix_Group+0 vs Unix_Group+3620?

Martin