sk_is_mptcp() only works with struct tcp_sock *

[mjmartineau]

sk_is_mptcp() looks like it can be called using any struct sock *, but it only gives valid results with struct tcp_sock *:

static inline bool sk_is_mptcp(const struct sock *sk)
{
    return tcp_sk(sk)->is_mptcp;
}

All existing callers use it correctly.

We could rename it to something like tcpsk_is_mptcp(), or keep the existing name and add a sk_is_tcp() check.

[geliangtang]

@mjmartineau Mat, do you mean to change it like this:

static inline bool sk_is_mptcp(const struct sock *sk) { return sk_is_tcp(sk) ? tcp_sk(sk)->is_mptcp : false;

}

[mjmartineau]

As noted on the mailing list (https://lore.kernel.org/mptcp/CABbSpchzSj9jn1-t_nmamRf+oH61jecwDw08mr9U1YP9dnw8Pg@mail.gmail.com/) - instead of changing this function, will depend on extra checks with CONFIG_DEBUG_NET in the mptcp code: https://lore.kernel.org/mptcp/20240201-mptcp-check-protocol-v2-2-1e253ef51990@kernel.org/

This avoids churn and extra execution overhead in the TCP code.