conn: store IP_PKTINFO cmsg in StdNetendpoint src

Replace the src storage inside StdNetEndpoint with a copy of the raw
control message buffer, to reduce allocation and perform less work on a
per-packet basis.

Signed-off-by: James Tucker <james@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

1 files changed, 49 insertions, 56 deletions

diff --git a/conn/sticky_linux.go b/conn/sticky_linux.go
index 274fa38..a30ccc7 100644
--- a/conn/sticky_linux.go
+++ b/conn/sticky_linux.go
@@ -14,6 +14,37 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+func (e *StdNetEndpoint) SrcIP() netip.Addr {
+	switch len(e.src) {
+	case unix.CmsgSpace(unix.SizeofInet4Pktinfo):
+		info := (*unix.Inet4Pktinfo)(unsafe.Pointer(&e.src[unix.CmsgLen(0)]))
+		return netip.AddrFrom4(info.Spec_dst)
+	case unix.CmsgSpace(unix.SizeofInet6Pktinfo):
+		info := (*unix.Inet6Pktinfo)(unsafe.Pointer(&e.src[unix.CmsgLen(0)]))
+		// TODO: set zone. in order to do so we need to check if the address is
+		// link local, and if it is perform a syscall to turn the ifindex into a
+		// zone string because netip uses string zones.
+		return netip.AddrFrom16(info.Addr)
+	}
+	return netip.Addr{}
+}
+
+func (e *StdNetEndpoint) SrcIfidx() int32 {
+	switch len(e.src) {
+	case unix.CmsgSpace(unix.SizeofInet4Pktinfo):
+		info := (*unix.Inet4Pktinfo)(unsafe.Pointer(&e.src[unix.CmsgLen(0)]))
+		return info.Ifindex
+	case unix.CmsgSpace(unix.SizeofInet6Pktinfo):
+		info := (*unix.Inet6Pktinfo)(unsafe.Pointer(&e.src[unix.CmsgLen(0)]))
+		return int32(info.Ifindex)
+	}
+	return 0
+}
+
+func (e *StdNetEndpoint) SrcToString() string {
+	return e.SrcIP().String()
+}
+
 // getSrcFromControl parses the control for PKTINFO and if found updates ep with
 // the source information found.
 func getSrcFromControl(control []byte, ep *StdNetEndpoint) {
@@ -35,81 +66,43 @@ func getSrcFromControl(control []byte, ep *StdNetEndpoint) {
 		if hdr.Level == unix.IPPROTO_IP &&
 			hdr.Type == unix.IP_PKTINFO {
 
-			info := pktInfoFromBuf[unix.Inet4Pktinfo](data)
-			ep.src.Addr = netip.AddrFrom4(info.Spec_dst)
-			ep.src.ifidx = info.Ifindex
+			if ep.src == nil || cap(ep.src) < unix.CmsgSpace(unix.SizeofInet4Pktinfo) {
+				ep.src = make([]byte, 0, unix.CmsgSpace(unix.SizeofInet4Pktinfo))
+			}
+			ep.src = ep.src[:unix.CmsgSpace(unix.SizeofInet4Pktinfo)]
 
+			hdrBuf := unsafe.Slice((*byte)(unsafe.Pointer(&hdr)), unix.SizeofCmsghdr)
+			copy(ep.src, hdrBuf)
+			copy(ep.src[unix.CmsgLen(0):], data)
 			return
 		}
 
 		if hdr.Level == unix.IPPROTO_IPV6 &&
 			hdr.Type == unix.IPV6_PKTINFO {
 
-			info := pktInfoFromBuf[unix.Inet6Pktinfo](data)
-			ep.src.Addr = netip.AddrFrom16(info.Addr)
-			ep.src.ifidx = int32(info.Ifindex)
+			if ep.src == nil || cap(ep.src) < unix.CmsgSpace(unix.SizeofInet6Pktinfo) {
+				ep.src = make([]byte, 0, unix.CmsgSpace(unix.SizeofInet6Pktinfo))
+			}
+
+			ep.src = ep.src[:unix.CmsgSpace(unix.SizeofInet6Pktinfo)]
 
+			hdrBuf := unsafe.Slice((*byte)(unsafe.Pointer(&hdr)), unix.SizeofCmsghdr)
+			copy(ep.src, hdrBuf)
+			copy(ep.src[unix.CmsgLen(0):], data)
 			return
 		}
 	}
 }
 
-// pktInfoFromBuf returns type T populated from the provided buf via copy(). It
-// panics if buf is of insufficient size.
-func pktInfoFromBuf[T unix.Inet4Pktinfo | unix.Inet6Pktinfo](buf []byte) (t T) {
-	size := int(unsafe.Sizeof(t))
-	if len(buf) < size {
-		panic("pktInfoFromBuf: buffer too small")
-	}
-	copy(unsafe.Slice((*byte)(unsafe.Pointer(&t)), size), buf)
-	return t
-}
-
 // setSrcControl sets an IP{V6}_PKTINFO in control based on the source address
 // and source ifindex found in ep. control's len will be set to 0 in the event
 // that ep is a default value.
 func setSrcControl(control *[]byte, ep *StdNetEndpoint) {
-	*control = (*control)[:cap(*control)]
-	if len(*control) < int(unsafe.Sizeof(unix.Cmsghdr{})) {
-		*control = (*control)[:0]
+	if cap(*control) < len(ep.src) {
 		return
 	}
-
-	if ep.src.ifidx == 0 && !ep.SrcIP().IsValid() {
-		*control = (*control)[:0]
-		return
-	}
-
-	if len(*control) < srcControlSize {
-		*control = (*control)[:0]
-		return
-	}
-
-	hdr := (*unix.Cmsghdr)(unsafe.Pointer(&(*control)[0]))
-	if ep.SrcIP().Is4() {
-		hdr.Level = unix.IPPROTO_IP
-		hdr.Type = unix.IP_PKTINFO
-		hdr.SetLen(unix.CmsgLen(unix.SizeofInet4Pktinfo))
-
-		info := (*unix.Inet4Pktinfo)(unsafe.Pointer(&(*control)[unix.SizeofCmsghdr]))
-		info.Ifindex = ep.src.ifidx
-		if ep.SrcIP().IsValid() {
-			info.Spec_dst = ep.SrcIP().As4()
-		}
-		*control = (*control)[:unix.CmsgSpace(unix.SizeofInet4Pktinfo)]
-	} else {
-		hdr.Level = unix.IPPROTO_IPV6
-		hdr.Type = unix.IPV6_PKTINFO
-		hdr.SetLen(unix.CmsgLen(unix.SizeofInet6Pktinfo))
-
-		info := (*unix.Inet6Pktinfo)(unsafe.Pointer(&(*control)[unix.SizeofCmsghdr]))
-		info.Ifindex = uint32(ep.src.ifidx)
-		if ep.SrcIP().IsValid() {
-			info.Addr = ep.SrcIP().As16()
-		}
-		*control = (*control)[:unix.CmsgSpace(unix.SizeofInet6Pktinfo)]
-	}
-
+	*control = (*control)[:0]
+	*control = append(*control, ep.src...)
 }
 
 var srcControlSize = unix.CmsgSpace(unix.SizeofInet6Pktinfo)