netdev - [PATCH net-next v2 1/3] net: add sock_rcvbuf_has

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Message-Id: <8ceedaa19fb64010ada1db183f7a6ad5a297835b.1750285100.git.jbaron@akamai.com>
Date: Wed, 18 Jun 2025 19:13:21 -0400
From: Jason Baron <jbaron@...mai.com>
To: netdev@...r.kernel.org
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, horms@...nel.org, kuniyu@...zon.com
Subject: [PATCH net-next v2 1/3] net: add sock_rcvbuf_has_space() helper

Let's add helper function that abstract the sk->sk_rcvbuf limits check
for wraparound that Kuniyuki Iwashima introduce for udp receive path:

commit 5a465a0da13e ("udp: Fix multiple wraparounds of sk->sk_rmem_alloc.")

Note that I copied Kuniyuki Iwashima's comments into
sock_rcvbuf_has_space(). Subsequent patches will use this for udp and
netlink sockets.

Signed-off-by: Jason Baron <jbaron@...mai.com>
---
 include/net/sock.h | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/include/net/sock.h b/include/net/sock.h
index 70c0b570a21f..5f4fdf96bba6 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2484,6 +2484,44 @@ static inline unsigned long sock_wspace(struct sock *sk)
 	return amt;
 }
 
+static inline bool __sock_rcvbuf_has_space(unsigned int rmem,
+					   unsigned int rcvbuf,
+					   unsigned int size)
+{
+	/* Immediately drop when the receive queue is full. */
+	if (rmem + size > rcvbuf) {
+		if (rcvbuf > INT_MAX >> 1)
+			return false;
+
+		/* Always allow at least one packet for small buffer. */
+		if (rmem > rcvbuf)
+			return false;
+	}
+	return true;
+}
+
+/**
+ * sock_rcvbuf_has_space - check if sk->sk_rcvbuf has space
+ * @sk: socket
+ * @skb: socket buffer
+ *
+ * Can skb->truesize bytes be added to the socket receive buffer
+ * while respecting the sk->sk_rcvbuf limit. Note that rcvbuf and
+ * rmem are assigned to unsigned int to avoid wraparound.
+ *
+ * Return: true if there is space, false otherwise
+ */
+static inline bool sock_rcvbuf_has_space(struct sock *sk, struct sk_buff *skb)
+{
+	unsigned int rmem, rcvbuf;
+
+	/* Cast to unsigned int performs the boundary check for INT_MAX. */
+	rmem = atomic_read(&sk->sk_rmem_alloc);
+	rcvbuf = READ_ONCE(sk->sk_rcvbuf);
+
+	return __sock_rcvbuf_has_space(rmem, rcvbuf, skb->truesize);
+}
+
 /* Note:
  *  We use sk->sk_wq_raw, from contexts knowing this
  *  pointer is not NULL and cannot disappear/change.
-- 
2.25.1