[Samba] permissions, and maybe a violation of the least surprise principle

Nick Couchman nick.e.couchman at gmail.com
Tue Nov 9 18:44:15 UTC 2021

On Tue, Nov 9, 2021 at 12:49 PM Patrick Goetz via samba <
samba at lists.samba.org> wrote:

> OK, I think I'm starting to absorb the idiomatic usage of permissions in
> Windows. The group always just defaults to Domain Users and then you
> control access through filesystem permissions.
> However, (see below)
> On 11/8/21 16:50, L.P.H. van Belle via samba wrote:
> > I'll add my view on it.
> >
> > Windows can only hold 1 primary group for a user, which is by "Domain
> Users".
> > So,yes, every file holds the "domain users" by default. Lets say GID
> 10000 is assigned.
> >
> > By example.
> > We have 2 users, bugger and bogger.
> > Bugger is member of "domain users" (GID 10000) and SomeUseless group.
> (GID 10001)
> > Bogger is member of "domain users" (GID 10000) and Staff group. (GID
> 10002)
> >
> >  From a windows machine, default rights are set as you see in you output.
> > Which is all correct as far is see.
> >
> > Now, let remove the windows thoughts and just use POSIX.
> > You change the default group in windows for both users to its group with
> GID.
> >
> > Bogger places a file in the SomeUseless group, so bugger can open it.
> > But the file owner now is bogger:staff, bugger isnt a member,
> > so to bad he cant open/change it, even if its in the right folder.
> >
> > This is why, i use in a  "linux with mixed windows" rights setup the
> windows defaults
> >
> > So, all "file rights" are "domain users" as group and every member kan
> open/change it.
> > The fixes the above rights problem.
> >
> > On the "folder part".
> > The acl is obeyed from windows and linux users cant enter it.
> > You use a group as security group to allow access only.
> >
> > Only one important part, or you need to change rights later on.
> > Set the UID/GIDS first thing in the objectes, before you create folders,
> or the GID doesnt show/is set.
> > Still need to look better into that, only so little time currently.
> >
> > Use from windows to posix are key "Creator Owner" and "Creator Group"
> (mainly creator group)
> >
> > Windows               : Posix
> > ( Creator owner ) : 1770 (through sticky bit) ( normaly chmod 4770)
> > ( creator group ) : chmod 2770
> > ( creator owner and group ) : chmod 3770
> >
> > https://chmodcommand.com/ has a nice explantion on the i at sticky bit
> and SetGid.
> >
> I'm not seeing what the setuid/setgid bits are doing for Windows
> permissions.  In particular, on linux at least the setuid bit is just
> ignored when set on a directory (see the link just above).
> I can see setting the setgid bit, as this means group permissions behave
> property when set from linux, but this doesn't seem to do anything on
> Windows permissions.  And when you save a file to the folder, the
> default group set on the file is Domain Users.
> And, as far as I can tell Creator Owner and Creator Group are set on
> every folder regardless of whether or not the setuid/setgid is set.
A couple of notes, here - forgive me if I'm headed off in a direction that
doesn't make sense or that you've already covered. In your original post,
you posted the following share configuration:

   comment = Share Directory
   path = /data/share
   guest ok = no
   browseable = yes
   writeable = yes
   create mask = 0770
   directory mask = 0770
   # inherit permissions = yes
   follow symlinks = yes

The manual page for smb.conf says of "create mask" (directory mask is the

           When a file is created, the necessary permissions are calculated
           according to the mapping from DOS modes to UNIX permissions, and
           the resulting UNIX mode is then bit-wise 'AND'ed with this
           parameter. This parameter may be thought of as a bit-wise MASK
           the UNIX modes of a file. Any bit not set here will be removed
           the modes set on a file when it is created.

Since you've omitted the setuid, setgid, and other bits, here (the "0" on
the left-hand side of the mask), those bits will be removed when
Samba writes files, which means your files/directories will not retain
their setuid/setgid status. From the "chmod" man page:

       A numeric mode is from one to  four  octal  digits  (0-7),  derived
       adding up the bits with values 4, 2, and 1.  Omitted digits are
       to be leading zeros.  The first digit selects the set user ID  (4)
       set group ID (2) and restricted deletion or sticky (1) attributes.
       second digit selects permissions for the user who owns the  file:
       (4),  write  (2),  and  execute  (1); the third selects permissions
       other users in the file's group, with the same values; and  the
       for other users not in the file's group, with the same values.

So, if you wanted to allow Samba to retain the setuid and setgid bits on
both files and directories, you'd need to set:

create mask = 6770
directory mask = 6770

If you only wanted these kept on directories, keep "create mask" as 0770
and only alter "directory mask". If you only want setgid, then you'd set:

create mask = 2770
directory mask = 2770

etc. Also, if you're still looking to have files owned by a group other
than domain users, you could set the "force group" option to something:

force group = +"DOMAIN\Special Group"

The + in front causes Samba to only force that group if the user is already
a member of that group.


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