security: add a secctx_to_secid() hook

Add a secctx_to_secid() LSM hook to go along with the existing
secid_to_secctx() LSM hook.  This patch also includes the SELinux
implementation for this hook.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 233c8b9..0396354 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -4947,6 +4947,11 @@
 	return security_sid_to_context(secid, secdata, seclen);
 }
 
+static int selinux_secctx_to_secid(char *secdata, u32 seclen, u32 *secid)
+{
+	return security_context_to_sid(secdata, seclen, secid);
+}
+
 static void selinux_release_secctx(char *secdata, u32 seclen)
 {
 	kfree(secdata);
@@ -5138,6 +5143,7 @@
 	.setprocattr =                  selinux_setprocattr,
 
 	.secid_to_secctx =		selinux_secid_to_secctx,
+	.secctx_to_secid =		selinux_secctx_to_secid,
 	.release_secctx =		selinux_release_secctx,
 
         .unix_stream_connect =		selinux_socket_unix_stream_connect,