libselinux: is_selinux_enabled(): drop no-policy-loaded test.

upstream commit 685f4aeeadc0b60f3770404d4f149610d656e3c8.

SELinux can be disabled via the selinux=0 kernel parameter or via
/sys/fs/selinux/disable (triggered by setting SELINUX=disabled in
/etc/selinux/config).  In either case, selinuxfs will be unmounted
and unregistered and therefore it is sufficient to check for the
selinuxfs mount.  We do not need to check for no-policy-loaded and
treat that as SELinux-disabled anymore; that is a relic of Fedora Core 2
days.  Drop the no-policy-loaded test, which was a bit of a hack anyway
(checking whether getcon_raw() returned "kernel" as that can only happen
if no policy is yet loaded and therefore security_sid_to_context() only
has the initial SID name available to return as the context).

May possibly fix https://bugzilla.redhat.com/show_bug.cgi?id=1195074


by virtue of removing the call to getcon_raw() and therefore avoiding
use of tls on is_selinux_enabled() calls.  Regardless, it will make
is_selinux_enabled() faster and simpler.

[sds:  Adapted for the Android libselinux port.  Also drops the
fallback to scanning /proc/filesystems for selinuxfs as this was
already done upstream; init mounts selinuxfs via libselinux prior to any
is_selinux_enabled() checks.  The tls bug is not relevant in Android
since the Android libselinux port does not use tls, but this change
is nonetheless useful to optimize is_selinux_enabled().]

Change-Id: Ia8b484a3a2fe7f604b0bfb8f5b109ad7674c1152
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>

src/enabled.c

@@ -10,56 +10,10 @@
 
 int is_selinux_enabled(void)
 {
-	char buf[BUFSIZ];
-	FILE *fp;
-	char *bufp;
-	int enabled = 0;
-	char * con;
-
 	/* init_selinuxmnt() gets called before this function. We
  	 * will assume that if a selinux file system is mounted, then
  	 * selinux is enabled. */
-	if (selinux_mnt) {
-
-		/* Since a file system is mounted, we consider selinux
-		 * enabled. If getcon fails, selinux is still enabled.
-		 * We only consider it disabled if no policy is loaded. */
-		enabled = 1;
-		if (getcon(&con) == 0) {
-			if (!strcmp(con, "kernel"))
-				enabled = 0;
-			freecon(con);
-		}
-		return enabled;
-        }
-
-	/* Drop back to detecting it the long way. */
-	fp = fopen("/proc/filesystems", "r");
-	if (!fp)
-		return -1;
-
-	while ((bufp = fgets(buf, sizeof buf - 1, fp)) != NULL) {
-		if (strstr(buf, "selinuxfs")) {
-			enabled = 1;
-			break;
-		}
-	}
-
-	if (!bufp)
-		goto out;
-
-	/* Since an selinux file system is available, we consider
-	 * selinux enabled. If getcon fails, selinux is still
-	 * enabled. We only consider it disabled if no policy is loaded. */
-	if (getcon(&con) == 0) {
-		if (!strcmp(con, "kernel"))
-			enabled = 0;
-		freecon(con);
-	}
-
-      out:
-	fclose(fp);
-	return enabled;
+	return (selinux_mnt ? 1 : 0);
 }
 
 hidden_def(is_selinux_enabled)