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Petar Penkovborkmann
Petar Penkov
authored and
borkmann
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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 <[email protected]> Signed-off-by: Vlad Dumitrescu <[email protected]> Signed-off-by: Willem de Bruijn <[email protected]> Acked-by: Song Liu <[email protected]> Signed-off-by: Daniel Borkmann <[email protected]>
1 parent de94b65 commit e3da08d

4 files changed

Lines changed: 50 additions & 0 deletions

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‎include/uapi/linux/bpf.h‎

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Original file line numberDiff line numberDiff line change
@@ -2497,6 +2497,7 @@ struct __sk_buff {
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__u32 data_meta;
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struct bpf_flow_keys *flow_keys;
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__u64 tstamp;
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__u32 wire_len;
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};
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struct bpf_tunnel_key {

‎net/core/filter.c‎

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Original file line numberDiff line numberDiff line change
@@ -5773,6 +5773,7 @@ static bool sk_filter_is_valid_access(int off, int size,
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case bpf_ctx_range(struct __sk_buff, flow_keys):
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case bpf_ctx_range_till(struct __sk_buff, family, local_port):
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case bpf_ctx_range(struct __sk_buff, tstamp):
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case bpf_ctx_range(struct __sk_buff, wire_len):
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return false;
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}
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@@ -5797,6 +5798,7 @@ static bool cg_skb_is_valid_access(int off, int size,
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case bpf_ctx_range(struct __sk_buff, tc_classid):
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case bpf_ctx_range(struct __sk_buff, data_meta):
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case bpf_ctx_range(struct __sk_buff, flow_keys):
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case bpf_ctx_range(struct __sk_buff, wire_len):
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return false;
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case bpf_ctx_range(struct __sk_buff, data):
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case bpf_ctx_range(struct __sk_buff, data_end):
@@ -5843,6 +5845,7 @@ static bool lwt_is_valid_access(int off, int size,
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case bpf_ctx_range(struct __sk_buff, data_meta):
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case bpf_ctx_range(struct __sk_buff, flow_keys):
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case bpf_ctx_range(struct __sk_buff, tstamp):
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case bpf_ctx_range(struct __sk_buff, wire_len):
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return false;
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}
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@@ -6273,6 +6276,7 @@ static bool sk_skb_is_valid_access(int off, int size,
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case bpf_ctx_range(struct __sk_buff, data_meta):
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case bpf_ctx_range(struct __sk_buff, flow_keys):
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case bpf_ctx_range(struct __sk_buff, tstamp):
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case bpf_ctx_range(struct __sk_buff, wire_len):
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return false;
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}
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@@ -6360,6 +6364,7 @@ static bool flow_dissector_is_valid_access(int off, int size,
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case bpf_ctx_range(struct __sk_buff, data_meta):
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case bpf_ctx_range_till(struct __sk_buff, family, local_port):
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case bpf_ctx_range(struct __sk_buff, tstamp):
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case bpf_ctx_range(struct __sk_buff, wire_len):
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return false;
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}
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@@ -6685,6 +6690,17 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
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bpf_target_off(struct sk_buff,
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tstamp, 8,
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target_size));
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break;
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case offsetof(struct __sk_buff, wire_len):
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BUILD_BUG_ON(FIELD_SIZEOF(struct qdisc_skb_cb, pkt_len) != 4);
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off = si->off;
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off -= offsetof(struct __sk_buff, wire_len);
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off += offsetof(struct sk_buff, cb);
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off += offsetof(struct qdisc_skb_cb, pkt_len);
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*target_size = 4;
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*insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->src_reg, off);
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}
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return insn - insn_buf;

‎tools/include/uapi/linux/bpf.h‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2497,6 +2497,7 @@ struct __sk_buff {
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__u32 data_meta;
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struct bpf_flow_keys *flow_keys;
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__u64 tstamp;
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__u32 wire_len;
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};
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struct bpf_tunnel_key {

‎tools/testing/selftests/bpf/test_verifier.c‎

Lines changed: 32 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -14033,6 +14033,38 @@ static struct bpf_test tests[] = {
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.result_unpriv = REJECT,
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.result = ACCEPT,
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},
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{
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"check wire_len is not readable by sockets",
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.insns = {
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BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
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offsetof(struct __sk_buff, wire_len)),
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BPF_EXIT_INSN(),
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},
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.errstr = "invalid bpf_context access",
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.result = REJECT,
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},
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{
14047+
"check wire_len is readable by tc classifier",
14048+
.insns = {
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BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1,
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offsetof(struct __sk_buff, wire_len)),
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BPF_EXIT_INSN(),
14052+
},
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.prog_type = BPF_PROG_TYPE_SCHED_CLS,
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.result = ACCEPT,
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},
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{
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"check wire_len is not writable by tc classifier",
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.insns = {
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BPF_STX_MEM(BPF_W, BPF_REG_1, BPF_REG_1,
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offsetof(struct __sk_buff, wire_len)),
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BPF_EXIT_INSN(),
14062+
},
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.prog_type = BPF_PROG_TYPE_SCHED_CLS,
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.errstr = "invalid bpf_context access",
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.errstr_unpriv = "R1 leaks addr",
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.result = REJECT,
14067+
},
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};
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static int probe_filter_length(const struct bpf_insn *fp)

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