selinux: deprecated fs ocon

The object context type `fs`, not to be confused with the well used
object context type `fscon`, was introduced in the initial git commit
1da177e4 ("Linux-2.6.12-rc2") but never actually used since.

The paper "A Security Policy Configuration for the Security-Enhanced
Linux" [1] mentions it under `7.2 File System Contexts` but also states:

    Currently, this configuration is unused.

The policy statement defining such object contexts is `fscon`, e.g.:

    fscon 2 3 gen_context(system_u:object_r:conA_t,s0) \
        gen_context(system_u:object_r:conB_t,s0)

It is not documented at selinuxproject.org or in the SELinux notebook
and not supported by the Reference Policy buildsystem - the statement is
not properly sorted - and thus not used in the Reference or Fedora
Policy.

Print a warning message at policy load for each such object context:

    SELinux:  void and deprecated fs ocon 02:03

This topic was initially highlighted by Nicolas Iooss [2].

[1]: https://media.defense.gov/2021/Jul/29/2002815735/-1/-1/0/SELINUX-SECURITY-POLICY-CONFIGURATION-REPORT.PDF
[2]: https://lore.kernel.org/selinux/CAJfZ7=mP2eJaq2BfO3y0VnwUJaY2cS2p=HZMN71z1pKjzaT0Eg@mail.gmail.com/Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: tweaked deprecation comment, description line wrapping]
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/ss/policydb.c

@@ -2257,6 +2257,10 @@ static int ocontext_read(struct policydb *p, const struct policydb_compat_info *
 				if (rc)
 					goto out;
 
+				if (i == OCON_FS)
+					pr_warn("SELinux:  void and deprecated fs ocon %s\n",
+						c->u.name);
+
 				rc = context_read_and_validate(&c->context[0], p, fp);
 				if (rc)
 					goto out;

security/selinux/ss/policydb.h

@@ -225,7 +225,7 @@ struct genfs {
 
 /* object context array indices */
 #define OCON_ISID	0 /* initial SIDs */
-#define OCON_FS		1 /* unlabeled file systems */
+#define OCON_FS		1 /* unlabeled file systems (deprecated) */
 #define OCON_PORT	2 /* TCP and UDP port numbers */
 #define OCON_NETIF	3 /* network interfaces */
 #define OCON_NODE	4 /* nodes */