netfilter: allow hooks to pass error code back up the stack

SELinux would like to pass certain fatal errors back up the stack.  This patch
implements the generic netfilter support for this functionality.

Based-on-patch-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter.h

@@ -33,6 +33,8 @@
 
 #define NF_QUEUE_NR(x) ((((x) << NF_VERDICT_BITS) & NF_VERDICT_QMASK) | NF_QUEUE)
 
+#define NF_DROP_ERR(x) (((-x) << NF_VERDICT_BITS) | NF_DROP)
+
 /* only for userspace compatibility */
 #ifndef __KERNEL__
 /* Generic cache responses from hook functions.

net/netfilter/core.c

@@ -173,9 +173,11 @@ next_hook:
 			     outdev, &elem, okfn, hook_thresh);
 	if (verdict == NF_ACCEPT || verdict == NF_STOP) {
 		ret = 1;
-	} else if (verdict == NF_DROP) {
+	} else if ((verdict & NF_VERDICT_MASK) == NF_DROP) {
 		kfree_skb(skb);
-		ret = -EPERM;
+		ret = -(verdict >> NF_VERDICT_BITS);
+		if (ret == 0)
+			ret = -EPERM;
 	} else if ((verdict & NF_VERDICT_MASK) == NF_QUEUE) {
 		if (!nf_queue(skb, elem, pf, hook, indev, outdev, okfn,
 			      verdict >> NF_VERDICT_BITS))