mptcp: sockopt: info: stop early if no buffer

Up to recently, it has been recommended to use getsockopt(MPTCP_INFO) to
check if a fallback to TCP happened, or if the client requested to use
MPTCP.

In this case, the userspace app is only interested by the returned value
of the getsocktop() call, and can then give 0 for the option length, and
NULL for the buffer address. An easy optimisation is then to stop early,
and avoid filling a local buffer -- which now requires two different
locks -- if it is not needed.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20240514011335.176158-4-martineau@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index fcca9433c85839dee4a613b655c812c17be19c66..a77b33488176289a997826619fe12f94ca1c0f43 100644 (file)
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -999,6 +999,10 @@ static int mptcp_getsockopt_info(struct mptcp_sock *msk, char __user *optval, in
        if (get_user(len, optlen))
                return -EFAULT;
 
+       /* When used only to check if a fallback to TCP happened. */
+       if (len == 0)
+               return 0;
+
        len = min_t(unsigned int, len, sizeof(struct mptcp_info));
 
        mptcp_diag_fill_info(msk, &m_info);