netfilter: reduce NF_VERDICT_MASK to 0xff

NF_VERDICT_MASK is currently 0xffff. This is because the upper
16 bits are used to store errno (for NF_DROP) or the queue number
(NF_QUEUE verdict).

As there are up to 0xffff different queues available, there is no more
room to store additional flags.

At the moment there are only 6 different verdicts, i.e. we can reduce
NF_VERDICT_MASK to 0xff to allow storing additional flags in the 0xff00 space.

NF_VERDICT_BITS would then be reduced to 8, but because the value is
exported to userspace, this might cause breakage; e.g.:

e.g. 'queuenr = (1 << NF_VERDICT_BITS) | NF_QUEUE' would now break.

Thus, remove NF_VERDICT_BITS usage in the kernel and move the old value
to the 'userspace compat' section.

Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>

include/linux/netfilter.h

···
 #define NF_MAX_VERDICT NF_STOP
 
 /* we overload the higher bits for encoding auxiliary data such as the queue
- * number. Not nice, but better than additional function arguments. */
-#define NF_VERDICT_MASK 0x0000ffff
-#define NF_VERDICT_BITS 16
+ * number or errno values. Not nice, but better than additional function
+ * arguments. */
+#define NF_VERDICT_MASK 0x000000ff
 
+/* extra verdict flags have mask 0x0000ff00 */
+
+/* queue number (NF_QUEUE) or errno (NF_DROP) */
 #define NF_VERDICT_QMASK 0xffff0000
 #define NF_VERDICT_QBITS 16
 
-#define NF_QUEUE_NR(x) ((((x) << NF_VERDICT_BITS) & NF_VERDICT_QMASK) | NF_QUEUE)
+#define NF_QUEUE_NR(x) ((((x) << 16) & NF_VERDICT_QMASK) | NF_QUEUE)
 
-#define NF_DROP_ERR(x) (((-x) << NF_VERDICT_BITS) | NF_DROP)
+#define NF_DROP_ERR(x) (((-x) << 16) | NF_DROP)
 
 /* only for userspace compatibility */
 #ifndef __KERNEL__
···
    <= 0x2000 is used for protocol-flags. */
 #define NFC_UNKNOWN 0x4000
 #define NFC_ALTERED 0x8000
+
+/* NF_VERDICT_BITS should be 8 now, but userspace might break if this changes */
+#define NF_VERDICT_BITS 16
 #endif
 
 enum nf_inet_hooks {
···
 
 #ifdef __KERNEL__
 #ifdef CONFIG_NETFILTER
+static inline int NF_DROP_GETERR(int verdict)
+{
+	return -(verdict >> NF_VERDICT_QBITS);
+}
 
 static inline int nf_inet_addr_cmp(const union nf_inet_addr *a1,
 				   const union nf_inet_addr *a2)

net/netfilter/core.c

···
 		ret = 1;
 	} else if ((verdict & NF_VERDICT_MASK) == NF_DROP) {
 		kfree_skb(skb);
-		ret = -(verdict >> NF_VERDICT_BITS);
+		ret = NF_DROP_GETERR(verdict);
 		if (ret == 0)
 			ret = -EPERM;
 	} else if ((verdict & NF_VERDICT_MASK) == NF_QUEUE) {
 		ret = nf_queue(skb, elem, pf, hook, indev, outdev, okfn,
-			       verdict >> NF_VERDICT_BITS);
+			       verdict >> NF_VERDICT_QBITS);
 		if (ret < 0) {
 			if (ret == -ECANCELED)
 				goto next_hook;

net/netfilter/nf_queue.c

···
 	case NF_QUEUE:
 		err = __nf_queue(skb, elem, entry->pf, entry->hook,
 				 entry->indev, entry->outdev, entry->okfn,
-				 verdict >> NF_VERDICT_BITS);
+				 verdict >> NF_VERDICT_QBITS);
 		if (err < 0) {
 			if (err == -ECANCELED)
 				goto next_hook;