net: filter: add vlan tag access

BPF filters lack ability to access skb->vlan_tci

This patch adds two new ancillary accessors :

SKF_AD_VLAN_TAG         (44) mapped to vlan_tx_tag_get(skb)

SKF_AD_VLAN_TAG_PRESENT (48) mapped to vlan_tx_tag_present(skb)

This allows libpcap/tcpdump to use a kernel filter instead of
having to fallback to accept all packets, then filter them in
user space.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Ani Sinha <ani@aristanetworks.com>
Suggested-by: Daniel Borkmann <danborkmann@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/filter.h

@@ -123,6 +123,8 @@ enum {
 	BPF_S_ANC_CPU,
 	BPF_S_ANC_ALU_XOR_X,
 	BPF_S_ANC_SECCOMP_LD_W,
+	BPF_S_ANC_VLAN_TAG,
+	BPF_S_ANC_VLAN_TAG_PRESENT,
 };
 
 #endif /* __LINUX_FILTER_H__ */

include/uapi/linux/filter.h

@@ -127,7 +127,9 @@ struct sock_fprog {	/* Required for SO_ATTACH_FILTER. */
 #define SKF_AD_RXHASH	32
 #define SKF_AD_CPU	36
 #define SKF_AD_ALU_XOR_X	40
-#define SKF_AD_MAX	44
+#define SKF_AD_VLAN_TAG	44
+#define SKF_AD_VLAN_TAG_PRESENT 48
+#define SKF_AD_MAX	52
 #define SKF_NET_OFF   (-0x100000)
 #define SKF_LL_OFF    (-0x200000)
 

net/core/filter.c

@@ -39,6 +39,7 @@
 #include <linux/reciprocal_div.h>
 #include <linux/ratelimit.h>
 #include <linux/seccomp.h>
+#include <linux/if_vlan.h>
 
 /* No hurry in this branch
  *
@@ -341,6 +342,12 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
 		case BPF_S_ANC_CPU:
 			A = raw_smp_processor_id();
 			continue;
+		case BPF_S_ANC_VLAN_TAG:
+			A = vlan_tx_tag_get(skb);
+			continue;
+		case BPF_S_ANC_VLAN_TAG_PRESENT:
+			A = !!vlan_tx_tag_present(skb);
+			continue;
 		case BPF_S_ANC_NLATTR: {
 			struct nlattr *nla;
 
@@ -600,6 +607,8 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
 			ANCILLARY(RXHASH);
 			ANCILLARY(CPU);
 			ANCILLARY(ALU_XOR_X);
+			ANCILLARY(VLAN_TAG);
+			ANCILLARY(VLAN_TAG_PRESENT);
 			}
 		}
 		ftest->code = code;