net: WriteMsgUDPAddrPort should accept IPv4 destination addresses on IPv6 UDP sockets

[database64128]

What version of Go are you using (go version)?

$ go version
go version go1.18 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/database64128/.cache/go-build"
GOENV="/home/database64128/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/database64128/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/database64128/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1725863524=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Create an IPv6 UDP socket listening on ::. Call WriteMsgUDPAddrPort with an IPv4 target address.

What did you expect to see?

The write succeeds, just like WriteMsgUDP.

What did you see instead?

write udp [::]:52794->12.34.56.78:12345: address 12.34.56.78: non-IPv6 address

go/src/net/ipsock_posix.go

Lines 215 to 228 in db7183c

	func addrPortToSockaddrInet6(ap netip.AddrPort) (syscall.SockaddrInet6, error) {
	// ipToSockaddrInet6 has special handling here for zero length slices.
	// We do not, because netip has no concept of a generic zero IP address.
	addr := ap.Addr()
	if !addr.Is6() {
	return syscall.SockaddrInet6{}, &AddrError{Err: "non-IPv6 address", Addr: addr.String()}
	}
	sa := syscall.SockaddrInet6{
	Addr: addr.As16(),
	Port: int(ap.Port()),
	ZoneId: uint32(zoneCache.index(addr.Zone())),
	}
	return sa, nil
	}

The check performed here should allow IPv4 as well.

[gopherbot]

Change https://go.dev/cl/399454 mentions this issue: net: fix WriteMsgUDPAddrPort addr handling

[moredure]

@database64128 maybe !addr.IsValid() ?

[dmitshur]

CC @ianlancetaylor, @neild.