[PATCH bpf-next 1/7] bpf: expose is_mptcp flag to bpf_tcp_sock

Mat Martineau
2022-04-20T22:25:18Z

From: Nicolas Rybowski <nicolas.rybowski@tessares.net>

is_mptcp is a field from struct tcp_sock used to indicate that the
current tcp_sock is part of the MPTCP protocol.

In this protocol, a first socket (mptcp_sock) is created with
sk_protocol set to IPPROTO_MPTCP (=262) for control purpose but it
isn't directly on the wire. This is the role of the subflow (kernel)
sockets which are classical tcp_sock with sk_protocol set to
IPPROTO_TCP. The only way to differentiate such sockets from plain TCP
sockets is the is_mptcp field from tcp_sock.

Such an exposure in BPF is thus required to be able to differentiate
plain TCP sockets from MPTCP subflow sockets in BPF_PROG_TYPE_SOCK_OPS
programs.

The choice has been made to silently pass the case when CONFIG_MPTCP is
unset by defaulting is_mptcp to 0 in order to make BPF independent of
the MPTCP configuration. Another solution is to make the verifier fail
in 'bpf_tcp_sock_is_valid_ctx_access' but this will add an additional
'#ifdef CONFIG_MPTCP' in the BPF code and a same injected BPF program
will not run if MPTCP is not set.

An example use-case is provided in
https://github.com/multipath-tcp/mptcp_net-next/tree/scripts/bpf/examples

Suggested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Acked-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Nicolas Rybowski <nicolas.rybowski@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
---
include/uapi/linux/bpf.h | 1 +
net/core/filter.c | 9 ++++++++-
tools/include/uapi/linux/bpf.h | 1 +
3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d14b10b85e51..9ef1f3e1c22f 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -5694,6 +5694,7 @@ struct bpf_tcp_sock {
__u32 delivered; /* Total data packets delivered incl. rexmits */
__u32 delivered_ce; /* Like the above but only ECE marked packets */
__u32 icsk_retransmits; /* Number of unrecovered [RTO] timeouts */
+ __u32 is_mptcp; /* Is MPTCP subflow? */
};

struct bpf_sock_tuple {
diff --git a/net/core/filter.c b/net/core/filter.c
index 143f442a9505..7b1867f1f422 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6754,7 +6754,7 @@ bool bpf_tcp_sock_is_valid_access(int off, int size, enum bpf_access_type type,
struct bpf_insn_access_aux *info)
{
if (off < 0 || off >= offsetofend(struct bpf_tcp_sock,
- icsk_retransmits))
+ is_mptcp))
return false;

if (off % size != 0)
@@ -6888,6 +6888,13 @@ u32 bpf_tcp_sock_convert_ctx_access(enum bpf_access_type type,
case offsetof(struct bpf_tcp_sock, icsk_retransmits):
BPF_INET_SOCK_GET_COMMON(icsk_retransmits);
break;
+ case offsetof(struct bpf_tcp_sock, is_mptcp):
+#ifdef CONFIG_MPTCP
+ BPF_TCP_SOCK_GET_COMMON(is_mptcp);
+#else
+ *insn++ = BPF_MOV32_IMM(si->dst_reg, 0);
+#endif
+ break;
}

return insn - insn_buf;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index d14b10b85e51..9ef1f3e1c22f 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -5694,6 +5694,7 @@ struct bpf_tcp_sock {
__u32 delivered; /* Total data packets delivered incl. rexmits */
__u32 delivered_ce; /* Like the above but only ECE marked packets */
__u32 icsk_retransmits; /* Number of unrecovered [RTO] timeouts */
+ __u32 is_mptcp; /* Is MPTCP subflow? */
};

struct bpf_sock_tuple {
--
2.36.0