bpf: allow BPF read access to qdisc pkt_len

The pkt_len field in qdisc_skb_cb stores the skb length as it will
appear on the wire after segmentation. For byte accounting, this value
is more accurate than skb->len. It is computed on entry to the TC
layer, so only valid there.

Allow read access to this field from BPF tc classifier and action
programs. The implementation is analogous to tc_classid, aside from
restricting to read access.

To distinguish it from skb->len and self-describe export as wire_len.

Changes v1->v2
  - Rename pkt_len to wire_len

Signed-off-by: Petar Penkov <ppenkov@google.com>
Signed-off-by: Vlad Dumitrescu <vladum@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 16 insertions, 0 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index bd0df75dc7b6..3d54af4c363d 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -5773,6 +5773,7 @@ static bool sk_filter_is_valid_access(int off, int size,
 	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):
+	case bpf_ctx_range(struct __sk_buff, wire_len):
 		return false;
 	}
 
@@ -5797,6 +5798,7 @@ static bool cg_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, wire_len):
 		return false;
 	case bpf_ctx_range(struct __sk_buff, data):
 	case bpf_ctx_range(struct __sk_buff, data_end):
@@ -5843,6 +5845,7 @@ static bool lwt_is_valid_access(int off, int size,
 	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):
+	case bpf_ctx_range(struct __sk_buff, wire_len):
 		return false;
 	}
 
@@ -6273,6 +6276,7 @@ static bool sk_skb_is_valid_access(int off, int size,
 	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):
+	case bpf_ctx_range(struct __sk_buff, wire_len):
 		return false;
 	}
 
@@ -6360,6 +6364,7 @@ static bool flow_dissector_is_valid_access(int off, int size,
 	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):
+	case bpf_ctx_range(struct __sk_buff, wire_len):
 		return false;
 	}
 
@@ -6685,6 +6690,17 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
 					      bpf_target_off(struct sk_buff,
 							     tstamp, 8,
 							     target_size));
+		break;
+
+	case offsetof(struct __sk_buff, wire_len):
+		BUILD_BUG_ON(FIELD_SIZEOF(struct qdisc_skb_cb, pkt_len) != 4);
+
+		off = si->off;
+		off -= offsetof(struct __sk_buff, wire_len);
+		off += offsetof(struct sk_buff, cb);
+		off += offsetof(struct qdisc_skb_cb, pkt_len);
+		*target_size = 4;
+		*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg, off);
 	}
 
 	return insn - insn_buf;