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From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>
Date: Fri, 10 May 2024 13:18:33 +0200
Subject: [PATCH net-next 3/8] mptcp: sockopt: info: stop early if no buffer
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Message-Id: <20240510-upstream-net-next-20240509-misc-improvements-v1-3-4f25579e62ba@kernel.org>
References: <20240510-upstream-net-next-20240509-misc-improvements-v1-0-4f25579e62ba@kernel.org>
In-Reply-To: <20240510-upstream-net-next-20240509-misc-improvements-v1-0-4f25579e62ba@kernel.org>
To: mptcp@lists.linux.dev, Mat Martineau <martineau@kernel.org>, 
 Geliang Tang <geliang@kernel.org>, "David S. Miller" <davem@davemloft.net>, 
 Eric Dumazet <edumazet@google.com>, Jakub Kicinski <kuba@kernel.org>, 
 Paolo Abeni <pabeni@redhat.com>, Florian Westphal <fw@strlen.de>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, 
 "Matthieu Baerts (NGI0)" <matttbe@kernel.org>

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>
---
 net/mptcp/sockopt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 63a3edc75857..cc9ecccf219d 100644
--- 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);

-- 
2.43.0