ebpf: add skb->priority to offset map for usage in {cls, act}_bpf

This adds the ability to read out the skb->priority from an eBPF
program, so that it can be taken into account from a tc filter
or action for the use-case where the priority is not being used
to directly override the filter classification in a qdisc, but
to tag traffic otherwise for the classifier; the priority can be
assigned from various places incl. user space, in future we may
also mangle it from an eBPF program.
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -184,6 +184,7 @@ struct __sk_buff {
 	__u32 vlan_present;
 	__u32 vlan_tci;
 	__u32 vlan_proto;
+	__u32 priority;
 };
 
 #endif /* _UAPI__LINUX_BPF_H__ */

net/core/filter.c

@@ -1304,6 +1304,13 @@ static u32 sk_filter_convert_ctx_access(int dst_reg, int src_reg, int ctx_off,
 				      offsetof(struct sk_buff, vlan_proto));
 		break;
 
+	case offsetof(struct __sk_buff, priority):
+		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, priority) != 4);
+
+		*insn++ = BPF_LDX_MEM(BPF_W, dst_reg, src_reg,
+				      offsetof(struct sk_buff, priority));
+		break;
+
 	case offsetof(struct __sk_buff, mark):
 		return convert_skb_access(SKF_AD_MARK, dst_reg, src_reg, insn);