Re: fs_use_trans

[Stephen Smalley <sds@tycho.nsa.gov>]

On 10/14/2014 11:00 AM, William Roberts wrote:
> Yeah looking at this statement doesn't really just allow for the use of
> type_transition statements on that filesystem? It doesn't actually generate
> labels, you still need the typetrans rule. It appears that the definition
> is overreaching for its actual function and probably inferring something
> from refpolicy.

Each of the fs_use_* statements specifies how to determine the label for
existing inodes in the filesystem.  fs_use_xattr tells SELinux to fetch
the inode label via ->getxattr().  fs_use_task tells SELinux to assign
the inode the label of its creator.  fs_use_trans tells SELinux to
compute the inode label based on the result of security_transition_sid()
on the creating process SID and the filesystem SID.  What
security_transition_sid() returns depends on whether or not you have a
transition rule in policy.  So fs_use_trans doesn't guarantee that you
have a transition rule in place; it just allows you to use transition
rules if you wish to label the inodes based on some combination of the
creating process domain and the filesystem type.