From d62516ed1ee148c58eed60c4fa38c5dc2c3b9622 Mon Sep 17 00:00:00 2001
From: Ophestra <cat@gensokyo.uk>
Date: Wed, 25 Mar 2026 17:11:57 +0900
Subject: [PATCH] internal/netlink: enlarge recvfrom buffer

This also uses an array type for the buffer since its size now uses the hardcoded value found in the kernel.

Signed-off-by: Ophestra <cat@gensokyo.uk>
---
 internal/netlink/netlink.go      | 19 ++++++++++++-------
 internal/netlink/netlink_test.go |  6 +-----
 2 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/internal/netlink/netlink.go b/internal/netlink/netlink.go
index 406f567b..89f692a5 100644
--- a/internal/netlink/netlink.go
+++ b/internal/netlink/netlink.go
@@ -23,6 +23,9 @@ func getpid() uint32 {
 	return nlPid
 }
 
+// net/netlink/af_netlink.c
+const maxRecvmsgLen = 32768
+
 // A conn represents resources associated to a netlink socket.
 type conn struct {
 	// AF_NETLINK socket.
@@ -37,8 +40,8 @@ type conn struct {
 	typ, flags uint16
 	// Outgoing position in buf.
 	pos int
-	// A page holding incoming and outgoing messages.
-	buf []byte
+	// Pages holding incoming and outgoing messages.
+	buf [maxRecvmsgLen]byte
 	// An instant some time after conn was established, but before the first
 	// I/O operation on f through raw. This serves as a cached deadline to
 	// cancel blocking I/O.
@@ -70,17 +73,19 @@ func dial(family int) (*conn, error) {
 	}
 
 	c.pos = syscall.NLMSG_HDRLEN
-	c.buf = make([]byte, os.Getpagesize())
 	c.t = time.Now().UTC()
 	return &c, nil
 }
 
+// ok returns whether conn is still open.
+func (c *conn) ok() bool { return c.family >= 0 }
+
 // Close closes the underlying socket.
 func (c *conn) Close() error {
-	if c.buf == nil {
+	if !c.ok() {
 		return syscall.EINVAL
 	}
-	c.buf = nil
+	c.family = -1
 	return c.f.Close()
 }
 
@@ -245,7 +250,7 @@ type HandlerFunc func(resp []syscall.NetlinkMessage) error
 // returned by f. An error of type [Complete] is returned as nil.
 func (c *conn) receive(ctx context.Context, f HandlerFunc, flags int) error {
 	for {
-		buf := c.buf
+		buf := c.buf[:]
 		if n, _, err := c.recvfrom(ctx, buf, flags); err != nil {
 			return err
 		} else if n < syscall.NLMSG_HDRLEN {
@@ -276,7 +281,7 @@ func (c *conn) receive(ctx context.Context, f HandlerFunc, flags int) error {
 
 // Roundtrip sends the pending message and handles the reply.
 func (c *conn) Roundtrip(ctx context.Context, f HandlerFunc) error {
-	if c.buf == nil {
+	if !c.ok() {
 		return syscall.EINVAL
 	}
 	defer func() { c.seq++ }()
diff --git a/internal/netlink/netlink_test.go b/internal/netlink/netlink_test.go
index 0926e3bd..0b7df0f1 100644
--- a/internal/netlink/netlink_test.go
+++ b/internal/netlink/netlink_test.go
@@ -1,7 +1,6 @@
 package netlink
 
 import (
-	"os"
 	"syscall"
 	"testing"
 )
@@ -23,10 +22,7 @@ func checkPayload(t *testing.T, testCases []payloadTestCase) {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
 
-			c := conn{
-				pos: syscall.NLMSG_HDRLEN,
-				buf: make([]byte, os.Getpagesize()),
-			}
+			c := conn{pos: syscall.NLMSG_HDRLEN}
 			tc.f(&c)
 			if got := c.pending(); string(got) != string(tc.want) {
 				t.Errorf("pending: %#v, want %#v", got, tc.want)