access control for networked file server


Dear all,

I am writing a user-level networked file system with a user-level
networked file server. My server simply receives requests from its
clients over TCP and performs the necessary requested operations on
the files stored locally at the server. Right now the server is not
run as root, but it can be, if necessary.

I want to add support for POSIX ACL to the server and I am not able to
figure out what would be the best way to do it. Mainly because there
are seem to be two levels of access control that I need to manage a)
file server should verify if the user has the access rights, and b)
the underlying os/fs need to verify of the file server has the
appropriate access rights to perform local file system operations. I
need some advise for this.

I was thinking of using one of the two options:

1) Since the server is running at user-level, let the underlying file
system take care of the access control. I am using reiserfs as the
underlying file system, which supports posix acls. But the problem is
following: while performing file system operations (such as read,
write, open etc) how can the file server supply to the underlying OS/
FS the userid and groupid of the user that is requesting the
operation? Because the userid and groupid of the requester should be
used to check acls and not the effective userid and groupid of my file
server process. Is it possible to tell the underlying fs/os to use a
particular <userid, groupid> rather than the one used by the server?


2) If the file server cannot specify the <userid, groupid> for every
file system call, then I can do the following: I can create a userid
and groupid that will be the effective userid and groupid of the file
server. All files created on the server will always have ACL_USER_OBJ
and ACL_GROUP_OBJ as this userid and groupid. This userid and groupid
will have full permissions on the files, but only the file server will
be able to use these ids. Thus, when the underlying OS/FS checks for
acl the permissions will be always granted.

Now the check/set user's access permissions, my file server will use
the functions provided in acl(5) to manipulate acls. The file server
will check if the user has appropriate permissions, and if yes perform
the read/write/open system calls during which the OS/FS will use the
<userid, groupid> of the file server which has full permissions.

Thus, there will be two levels of acls - one with default userid,
groupid will be used to make sure that the OS grants the file server
all rights for the files *created* at the server and the second level
is the acls maintained by my file server which will be used by the
file server to check of the requester has the necessary permissions.

Can anyone please suggest whether the first approach is feasible and
whether the second approach is sensible or not? Your help will be
greatly appreciated.

Thanks.

-- Fantoosh

.

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