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udp: drop dead cmsg write

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This commit is contained in:
JackDoan
2026-07-29 11:59:29 -05:00
parent 095a421708
commit 38f11f6e3a
1 changed files with 9 additions and 23 deletions
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udp/udp_linux_writebatch.go
+9 -23
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@@ -83,9 +83,11 @@ func newBatchWriter(fd int, isV4 bool, l *slog.Logger) *batchWriter {
// [0 .. cmsgSegSpace) UDP_SEGMENT (gso_size, uint16)
// [cmsgSegSpace .. cmsgSpace) IP_TOS or IPV6_TCLASS (int32)
//
// Both headers are pre-filled once here; per-call we only rewrite the data
// payload and toggle Hdr.Control / Hdr.Controllen to point at whichever
// subset applies (none / segment-only / ecn-only / both).
// The UDP_SEGMENT header is pre-filled once here and only its data payload
// is rewritten per call. The ECN header is written entirely per entry by
// writeEntryCmsg, since its Level/Type follow the destination's family, not
// the socket's. Per call we toggle Hdr.Control / Hdr.Controllen to point at
// whichever subset applies (none / segment-only / ecn-only / both).
func (w *batchWriter) prepareWriteMessages(n int) {
w.msgs = make([]rawMessage, n)
w.iovs = make([]iovec, n)
@@ -97,28 +99,12 @@ func (w *batchWriter) prepareWriteMessages(n int) {
w.cmsgSpace = w.cmsgSegSpace + w.cmsgEcnSpace
w.cmsg = make([]byte, n*w.cmsgSpace)
// Default the ECN header to the socket's own family. writeEntryCmsg
// finalizes Level/Type per entry from the destination address (a v4-mapped
// dst on a dual-stack v6 socket needs IP_TOS, not IPV6_TCLASS), so this is
// only the value used before the first per-entry rewrite.
ecnLevel := int32(unix.IPPROTO_IP)
ecnType := int32(unix.IP_TOS)
if !w.isV4 {
ecnLevel = unix.IPPROTO_IPV6
ecnType = unix.IPV6_TCLASS
}
for k := 0; k < n; k++ {
base := k * w.cmsgSpace
seg := (*unix.Cmsghdr)(unsafe.Pointer(&w.cmsg[base]))
seg.Level = unix.SOL_UDP
seg.Type = unix.UDP_SEGMENT
setCmsgLen(seg, unix.CmsgLen(2))
ecn := (*unix.Cmsghdr)(unsafe.Pointer(&w.cmsg[base+w.cmsgSegSpace]))
ecn.Level = ecnLevel
ecn.Type = ecnType
setCmsgLen(ecn, unix.CmsgLen(4))
}
for i := range w.msgs {
@@ -427,10 +413,10 @@ func (w *batchWriter) planRun(bufs [][]byte, addrs []netip.AddrPort, ecns []byte
// The outer-ECN cmsg family must match the *destination*, not the socket: on
// the default dual-stack v6 bind, a v4-mapped destination is routed through
// the kernel's IPv4 path, which parses IP_TOS (IPPROTO_IP) and ignores an
// IPV6_TCLASS cmsg. prepareWriteMessages pre-fills a default header; here we
// rewrite its Level/Type (and Len) per entry from dstIsV4 so v4 peers get
// IP_TOS and v6 peers get IPV6_TCLASS. The data payload is a 4-byte int for
// both families, so the pre-computed cmsg space is unchanged.
// IPV6_TCLASS cmsg. The ECN header is written here in full, per entry, from
// dstIsV4 so v4 peers get IP_TOS and v6 peers get IPV6_TCLASS. The data
// payload is a 4-byte int for both families, so the pre-computed cmsg space
// is unchanged.
func (w *batchWriter) writeEntryCmsg(entry, runLen, segSize int, ecn byte, dstIsV4 bool) {
hdr := &w.msgs[entry].Hdr
useSeg := runLen >= 2