filter: introduce SKF_AD_VLAN_TPID BPF extension

If vlan offloading takes place then vlan header is removed from frame
and its contents, both vlan_tci and vlan_proto, is available to user
space via TPACKET interface. However, only vlan_tci can be used in BPF
filters.

This commit introduces a new BPF extension. It makes possible to load
the value of vlan_proto (vlan TPID) to register A. Support for classic
BPF and eBPF is being added, analogous to skb->protocol.

Cc: Daniel Borkmann <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Jiri Pirko <[e-mail]>

Signed-off-by: Michal Sekletar <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Reviewed-by: Jiri Pirko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/filter.txt

···
   rxhash                                skb->hash
   cpu                                   raw_smp_processor_id()
   vlan_tci                              skb_vlan_tag_get(skb)
-  vlan_pr                               skb_vlan_tag_present(skb)
+  vlan_avail                            skb_vlan_tag_present(skb)
+  vlan_tpid                             skb->vlan_proto
   rand                                  prandom_u32()
 
 These extensions can also be prefixed with '#'.

include/linux/filter.h

···
 		BPF_ANCILLARY(VLAN_TAG_PRESENT);
 		BPF_ANCILLARY(PAY_OFFSET);
 		BPF_ANCILLARY(RANDOM);
+		BPF_ANCILLARY(VLAN_TPID);
 		}
 		/* Fallthrough. */
 	default:

include/uapi/linux/bpf.h

···
 	__u32 protocol;
 	__u32 vlan_present;
 	__u32 vlan_tci;
+	__u32 vlan_proto;
 };
 
 #endif /* _UAPI__LINUX_BPF_H__ */

include/uapi/linux/filter.h

···
 #define SKF_AD_VLAN_TAG_PRESENT 48
 #define SKF_AD_PAY_OFFSET	52
 #define SKF_AD_RANDOM	56
-#define SKF_AD_MAX	60
+#define SKF_AD_VLAN_TPID	60
+#define SKF_AD_MAX	64
 #define SKF_NET_OFF   (-0x100000)
 #define SKF_LL_OFF    (-0x200000)
 

net/core/filter.c

···
 		insn += cnt - 1;
 		break;
 
+	case SKF_AD_OFF + SKF_AD_VLAN_TPID:
+		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, vlan_proto) != 2);
+
+		/* A = *(u16 *) (CTX + offsetof(vlan_proto)) */
+		*insn++ = BPF_LDX_MEM(BPF_H, BPF_REG_A, BPF_REG_CTX,
+				      offsetof(struct sk_buff, vlan_proto));
+		/* A = ntohs(A) [emitting a nop or swap16] */
+		*insn = BPF_ENDIAN(BPF_FROM_BE, BPF_REG_A, 16);
+		break;
+
 	case SKF_AD_OFF + SKF_AD_PAY_OFFSET:
 	case SKF_AD_OFF + SKF_AD_NLATTR:
 	case SKF_AD_OFF + SKF_AD_NLATTR_NEST:
···
 
 		*insn++ = BPF_LDX_MEM(BPF_H, dst_reg, src_reg,
 				      offsetof(struct sk_buff, protocol));
+		break;
+
+	case offsetof(struct __sk_buff, vlan_proto):
+		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, vlan_proto) != 2);
+
+		*insn++ = BPF_LDX_MEM(BPF_H, dst_reg, src_reg,
+				      offsetof(struct sk_buff, vlan_proto));
 		break;
 
 	case offsetof(struct __sk_buff, mark):

tools/net/bpf_exp.l

···
 "#"?("cpu")	{ return K_CPU; }
 "#"?("vlan_tci") { return K_VLANT; }
 "#"?("vlan_pr")	{ return K_VLANP; }
+"#"?("vlan_avail")	{ return K_VLANP; }
+"#"?("vlan_tpid")	{ return K_VLANTPID; }
 "#"?("rand")	{ return K_RAND; }
 
 ":"		{ return ':'; }

tools/net/bpf_exp.y

···
 %token OP_LDXI
 
 %token K_PKT_LEN K_PROTO K_TYPE K_NLATTR K_NLATTR_NEST K_MARK K_QUEUE K_HATYPE
-%token K_RXHASH K_CPU K_IFIDX K_VLANT K_VLANP K_POFF K_RAND
+%token K_RXHASH K_CPU K_IFIDX K_VLANT K_VLANP K_VLANTPID K_POFF K_RAND
 
 %token ':' ',' '[' ']' '(' ')' 'x' 'a' '+' 'M' '*' '&' '#' '%'
 
···
 	| OP_LDB K_RAND {
 		bpf_set_curr_instr(BPF_LD | BPF_B | BPF_ABS, 0, 0,
 				   SKF_AD_OFF + SKF_AD_RANDOM); }
+	| OP_LDB K_VLANTPID {
+		bpf_set_curr_instr(BPF_LD | BPF_B | BPF_ABS, 0, 0,
+				   SKF_AD_OFF + SKF_AD_VLAN_TPID); }
 	;
 
 ldh
···
 	| OP_LDH K_RAND {
 		bpf_set_curr_instr(BPF_LD | BPF_H | BPF_ABS, 0, 0,
 				   SKF_AD_OFF + SKF_AD_RANDOM); }
+	| OP_LDH K_VLANTPID {
+		bpf_set_curr_instr(BPF_LD | BPF_H | BPF_ABS, 0, 0,
+				   SKF_AD_OFF + SKF_AD_VLAN_TPID); }
 	;
 
 ldi
···
 	| OP_LD K_RAND {
 		bpf_set_curr_instr(BPF_LD | BPF_W | BPF_ABS, 0, 0,
 				   SKF_AD_OFF + SKF_AD_RANDOM); }
+	| OP_LD K_VLANTPID {
+		bpf_set_curr_instr(BPF_LD | BPF_W | BPF_ABS, 0, 0,
+				   SKF_AD_OFF + SKF_AD_VLAN_TPID); }
 	| OP_LD 'M' '[' number ']' {
 		bpf_set_curr_instr(BPF_LD | BPF_MEM, 0, 0, $4); }
 	| OP_LD '[' 'x' '+' number ']' {