Digging into BSD's choice of Unix group for new directories and files
I have to eat some humble pie here. In comments on my entry on
chmod failure, Greg A.
Woods pointed out that FreeBSD's behavior of creating everything
inside a directory with the group of the directory is actually
traditional BSD behavior (it dates all the way back to the 1980s),
not some odd new invention by FreeBSD. As traditional behavior it
makes sense that it's explicitly allowed by the standards, but
I've also come to think that it makes sense in context and in
general. To see this, we need some background about the problem
In the beginning, two things were true in Unix: there was no
mkdir() system call, and processes
could only be in one group at a time. With processes being in only
one group, the choice of the group for a newly created filesystem
object was easy; it was your current group. This was felt to be
sufficiently obvious behavior that the V7
doesn't even mention it.
(The actual behavior is implemented in the kernel in
Now things get interesting. 4.1c BSD seems to be where
is introduced and where
creat() stops being a system call
and becomes an option to
It's also where processes can be in multiple groups for the first
time. The 4.1c BSD
open(2) manpage is silent about the group of
newly created files, while the
mkdir(2) manpage specifically
claims that new directories will have your effective group (ie, the
V7 behavior). This is actually wrong. In both
maknode() in ufs_syscalls.c,
the group of the newly created object is set to the group of the
parent directory. Then finally in the 4.2 BSD
the group of the new directory is correctly documented (the 4.2
continues to say nothing about this). So BSD's traditional behavior
was introduced at the same time as processes being in multiple groups,
and we can guess that it was introduced as part of that change.
When your process can only be in a single group, as in V7, it makes perfect sense to create new filesystem objects with that as their group. It's basically the same case as making new filesystem objects be owned by you; just as they get your UID, they also get your GID. When your process can be in multiple groups, things get less clear. A filesystem object can only be in one group, so which of your several groups should a new filesystem object be owned by, and how can you most conveniently change that choice?
One option is to have some notion of a 'primary group' and then
provide ways to shuffle around which of your groups is the primary
group. One problem with this is that it's awkward and error-prone
to work in different areas of the filesystem where you want your
new files and directories to be in different groups; every time you
cd around, you may have to remember to change your primary group.
If you move into a collaborative directory, better shift (in your
shell) to that group;
cd back to
$HOME, or simply want to write
a new file in
$HOME, and you'd better remember to change back.
Another option is the BSD choice of inheriting the group from
context. By far the most common case is that you want your new files
and directories to be created in the 'context', ie the group, of
the surrounding directory. If you're working in
$HOME, this is
your primary login group; if you're working in a collaborative area,
this is the group being used for collaboration. Arguably it's a
feature that you don't even have to be in that group (if directory
permissions allow you to make new files). Since you can
directories that you own, this option also gives you a relatively
easy and persistent way to change which group is chosen for any
If you fully embrace the idea of Unix processes being in multiple groups, not just having one primary group and then some number of secondary groups, then the BSD choice makes a lot of sense. And for all of its faults, BSD tended to relatively fully embrace its changes (not totally, perhaps partly because it had backwards compatibility issues to consider). While it leads to some odd issues, such as the one I ran into, pretty much any choice here is going to have some oddities. It's also probably the more usable choice in general if you expect much collaboration between different people (well, different Unix logins), partly because it mostly doesn't require people to remember to do things.
(I know that on our systems, a lot of directories intended for collaborative work tend to end up being setgid specifically to get this behavior.)
chmod can fail for interesting reasons
I'll start by presenting this rather interesting and puzzling failure in illustrated form:
; mkdir /tmp/newdir ; chmod g+s /tmp/newdir chmod: /tmp/newdir: Operation not permitted
How can I not be able to make this chmod change when I just made the directory and I own it? For extra fun, some people on this particular system won't experience this problem, and in fact many of them are the people you might report this problem to, namely the sysadmins.
At first I wondered if this particular
/tmp filesystem disallowed
setuid and setgid entirely, but it turned out to be not that
; ls -ld /tmp/newdir drwxr-xr-x 2 cks wheel 512 May 3 00:35 /tmp/newdir
This at least explains why my chmod attempt failed. I'm not in group
wheel, and for good reasons you can't make a file setgid to a
group that you're not a member of. But how on earth did my newly created
/tmp wind up in group wheel, a group I'm not a member
of? Well, perhaps someone made
/tmp setgid, so all directories created
in it inherited its group (presumably group
wheel). Let's see:
; ld -ld /tmp drwxrwxrwt 157 root wheel 11776 May 3 00:41 /tmp
/tmp is indeed group
wheel, it has perfectly ordinary
permissions (mode 777 and sticky ('
t'), so you can only delete or
rename your own files). There's no setgid to be seen.
The answer to this mystery is that this is a FreeBSD machine, and
on FreeBSD, well, let's quote the
The directory's owner ID is set to the process's effective user ID. The directory's group ID is set to that of the parent directory in which it is created.
And also the section of the
open(2) manpage that deals
with creation of new files:
When a new file is created it is given the group of the directory which contains it.
In other words, on FreeBSD all directories have an implicit setgid
bit. Everything created inside them (whether directories or files)
inherits the directory's group. Normally this is not a problem and
you'll probably never notice, but
/var/tmp) are special
because they allow everyone to create files and directories in them,
and so there are a lot of people making things there who are not a
member of the directory's group.
(The sysadmins usually are members of group
wheel, though, so things
will work for them. This should add extra fun if a user reports the
general chmod issue as a problem, since sysadmins can't reproduce it as
You might think that this is an obscure issue that no one will ever care about, but actually it caused a Go build failure on FreeBSD for a while. Tracking down the problem took me a while and a bunch of head scratching.
PS: arguably GID 0 should not be group
wheel but instead something
else that only
root is a member of and
wheel should be a
completely separate group. To have group
wheel used for group
ownership as well as
su access to
root is at least confusing.