bpf: add skb->tstamp r/w access from tc clsact and cg skb progs

This could be used to rate limit egress traffic in concert with a qdisc
which supports Earliest Departure Time, such as FQ.

Write access from cg skb progs only with CAP_SYS_ADMIN, since the value
will be used by downstream qdiscs. It might make sense to relax this.

Changes v1 -> v2:
  - allow access from cg skb, write only with CAP_SYS_ADMIN

Signed-off-by: Vlad Dumitrescu <vladum@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -2468,6 +2468,7 @@ struct __sk_buff {
 
 	__u32 data_meta;
 	struct bpf_flow_keys *flow_keys;
+	__u64 tstamp;
 };
 
 struct bpf_tunnel_key {

net/core/filter.c

@@ -5573,6 +5573,10 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
 		if (size != sizeof(struct bpf_flow_keys *))
 			return false;
 		break;
+	case bpf_ctx_range(struct __sk_buff, tstamp):
+		if (size != sizeof(__u64))
+			return false;
+		break;
 	default:
 		/* Only narrow read access allowed for now. */
 		if (type == BPF_WRITE) {
@@ -5600,6 +5604,7 @@ static bool sk_filter_is_valid_access(int off, int size,
 	case bpf_ctx_range(struct __sk_buff, data_end):
 	case bpf_ctx_range(struct __sk_buff, flow_keys):
 	case bpf_ctx_range_till(struct __sk_buff, family, local_port):
+	case bpf_ctx_range(struct __sk_buff, tstamp):
 		return false;
 	}
 
@@ -5638,6 +5643,10 @@ static bool cg_skb_is_valid_access(int off, int size,
 		case bpf_ctx_range(struct __sk_buff, priority):
 		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
 			break;
+		case bpf_ctx_range(struct __sk_buff, tstamp):
+			if (!capable(CAP_SYS_ADMIN))
+				return false;
+			break;
 		default:
 			return false;
 		}
@@ -5665,6 +5674,7 @@ static bool lwt_is_valid_access(int off, int size,
 	case bpf_ctx_range_till(struct __sk_buff, family, local_port):
 	case bpf_ctx_range(struct __sk_buff, data_meta):
 	case bpf_ctx_range(struct __sk_buff, flow_keys):
+	case bpf_ctx_range(struct __sk_buff, tstamp):
 		return false;
 	}
 
@@ -5874,6 +5884,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
 		case bpf_ctx_range(struct __sk_buff, priority):
 		case bpf_ctx_range(struct __sk_buff, tc_classid):
 		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
+		case bpf_ctx_range(struct __sk_buff, tstamp):
 			break;
 		default:
 			return false;
@@ -6093,6 +6104,7 @@ static bool sk_skb_is_valid_access(int off, int size,
 	case bpf_ctx_range(struct __sk_buff, tc_classid):
 	case bpf_ctx_range(struct __sk_buff, data_meta):
 	case bpf_ctx_range(struct __sk_buff, flow_keys):
+	case bpf_ctx_range(struct __sk_buff, tstamp):
 		return false;
 	}
 
@@ -6179,6 +6191,7 @@ static bool flow_dissector_is_valid_access(int off, int size,
 	case bpf_ctx_range(struct __sk_buff, tc_classid):
 	case bpf_ctx_range(struct __sk_buff, data_meta):
 	case bpf_ctx_range_till(struct __sk_buff, family, local_port):
+	case bpf_ctx_range(struct __sk_buff, tstamp):
 		return false;
 	}
 
@@ -6488,6 +6501,22 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 		*insn++ = BPF_LDX_MEM(BPF_SIZEOF(void *), si->dst_reg,
 				      si->src_reg, off);
 		break;
+
+	case offsetof(struct __sk_buff, tstamp):
+		BUILD_BUG_ON(FIELD_SIZEOF(struct sk_buff, tstamp) != 8);
+
+		if (type == BPF_WRITE)
+			*insn++ = BPF_STX_MEM(BPF_DW,
+					      si->dst_reg, si->src_reg,
+					      bpf_target_off(struct sk_buff,
+							     tstamp, 8,
+							     target_size));
+		else
+			*insn++ = BPF_LDX_MEM(BPF_DW,
+					      si->dst_reg, si->src_reg,
+					      bpf_target_off(struct sk_buff,
+							     tstamp, 8,
+							     target_size));
 	}
 
 	return insn - insn_buf;

tools/include/uapi/linux/bpf.h

@@ -2468,6 +2468,7 @@ struct __sk_buff {
 
 	__u32 data_meta;
 	struct bpf_flow_keys *flow_keys;
+	__u64 tstamp;
 };
 
 struct bpf_tunnel_key {

tools/testing/selftests/bpf/test_verifier.c

@@ -2446,6 +2446,10 @@ static struct bpf_test tests[] = {
 				    offsetof(struct __sk_buff, tc_index)),
 			BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_0,
 				    offsetof(struct __sk_buff, cb[3])),
+			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, tstamp)),
+			BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_0,
+				    offsetof(struct __sk_buff, tstamp)),
 			BPF_EXIT_INSN(),
 		},
 		.errstr_unpriv = "",
@@ -5297,6 +5301,31 @@ static struct bpf_test tests[] = {
 		.errstr_unpriv = "R2 leaks addr into helper function",
 		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
 	},
+	{
+		"write tstamp from CGROUP_SKB",
+		.insns = {
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_0,
+				    offsetof(struct __sk_buff, tstamp)),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.result_unpriv = REJECT,
+		.errstr_unpriv = "invalid bpf_context access off=152 size=8",
+		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+	},
+	{
+		"read tstamp from CGROUP_SKB",
+		.insns = {
+			BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1,
+				    offsetof(struct __sk_buff, tstamp)),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.result = ACCEPT,
+		.prog_type = BPF_PROG_TYPE_CGROUP_SKB,
+	},
 	{
 		"multiple registers share map_lookup_elem result",
 		.insns = {