unix: add Ifreq.Inet4Addr methods for manipulating IPv4 addresses

ioctls such as SIOCGIFADDR deal with AF_INET sockaddr addresses, but none of
the fields aside from the embedded IPv4 address are used. To keep the interface
more simple, we directly expose Inet4Addr get and set methods which enable use
of these ioctls with the Ifreq wrapper.

Change-Id: Ia8b6ab9730f852cb99f4152e334a59d395476d2f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/343250
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/ifreq_linux.go

@@ -41,7 +41,7 @@ func NewIfreq(name string) (*Ifreq, error) {
 	return &Ifreq{raw: ifr}, nil
 }
 
-// TODO(mdlayher): get/set methods for sockaddr, char array, etc.
+// TODO(mdlayher): get/set methods for hardware address sockaddr, char array, etc.
 
 // Name returns the interface name associated with the Ifreq.
 func (ifr *Ifreq) Name() string {
@@ -54,6 +54,46 @@ func (ifr *Ifreq) Name() string {
 	return BytePtrToString(&ifr.raw.Ifrn[0])
 }
 
+// According to netdevice(7), only AF_INET addresses are returned for numerous
+// sockaddr ioctls. For convenience, we expose these as Inet4Addr since the Port
+// field and other data is always empty.
+
+// Inet4Addr returns the Ifreq union data from an embedded sockaddr as a C
+// in_addr/Go []byte (4-byte IPv4 address) value. If the sockaddr family is not
+// AF_INET, an error is returned.
+func (ifr *Ifreq) Inet4Addr() ([]byte, error) {
+	raw := *(*RawSockaddrInet4)(unsafe.Pointer(&ifr.raw.Ifru[:SizeofSockaddrInet4][0]))
+	if raw.Family != AF_INET {
+		// Cannot safely interpret raw.Addr bytes as an IPv4 address.
+		return nil, EINVAL
+	}
+
+	return raw.Addr[:], nil
+}
+
+// SetInet4Addr sets a C in_addr/Go []byte (4-byte IPv4 address) value in an
+// embedded sockaddr within the Ifreq's union data. v must be 4 bytes in length
+// or an error will be returned.
+func (ifr *Ifreq) SetInet4Addr(v []byte) error {
+	if len(v) != 4 {
+		return EINVAL
+	}
+
+	var addr [4]byte
+	copy(addr[:], v)
+
+	ifr.clear()
+	*(*RawSockaddrInet4)(
+		unsafe.Pointer(&ifr.raw.Ifru[:SizeofSockaddrInet4][0]),
+	) = RawSockaddrInet4{
+		// Always set IP family as ioctls would require it anyway.
+		Family: AF_INET,
+		Addr:   addr,
+	}
+
+	return nil
+}
+
 // Uint16 returns the Ifreq union data as a C short/Go uint16 value.
 func (ifr *Ifreq) Uint16() uint16 {
 	return *(*uint16)(unsafe.Pointer(&ifr.raw.Ifru[:2][0]))