1 files changed, 25 insertions, 12 deletions

diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 7e32b31b4..91fe7b6a5 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -89,7 +89,17 @@ type IPv4Fields struct {
 	// DstAddr is the "destination ip address" of an IPv4 packet.
 	DstAddr tcpip.Address
 
-	// Options is between 0 and 40 bytes or nil if empty.
+	// Options must be 40 bytes or less as they must fit along with the
+	// rest of the IPv4 header into the maximum size describable in the
+	// IHL field. RFC 791 section 3.1 says:
+	//    IHL:  4 bits
+	//
+	//    Internet Header Length is the length of the internet header in 32
+	//    bit words, and thus points to the beginning of the data.  Note that
+	//    the minimum value for a correct header is 5.
+	//
+	// That leaves ten 32 bit (4 byte) fields for options. An attempt to encode
+	// more will fail.
 	Options IPv4Options
 }
 
@@ -275,22 +285,19 @@ func (b IPv4) DestinationAddress() tcpip.Address {
 // IPv4Options is a buffer that holds all the raw IP options.
 type IPv4Options []byte
 
-// AllocationSize implements stack.NetOptions.
+// SizeWithPadding implements stack.NetOptions.
 // It reports the size to allocate for the Options. RFC 791 page 23 (end of
 // section 3.1) says of the padding at the end of the options:
 //    The internet header padding is used to ensure that the internet
 //    header ends on a 32 bit boundary.
-func (o IPv4Options) AllocationSize() int {
+func (o IPv4Options) SizeWithPadding() int {
 	return (len(o) + IPv4IHLStride - 1) & ^(IPv4IHLStride - 1)
 }
 
-// Options returns a buffer holding the options or nil.
+// Options returns a buffer holding the options.
 func (b IPv4) Options() IPv4Options {
 	hdrLen := b.HeaderLength()
-	if hdrLen > IPv4MinimumSize {
-		return IPv4Options(b[options:hdrLen:hdrLen])
-	}
-	return nil
+	return IPv4Options(b[options:hdrLen:hdrLen])
 }
 
 // TransportProtocol implements Network.TransportProtocol.
@@ -368,14 +375,20 @@ func (b IPv4) Encode(i *IPv4Fields) {
 	// worth a bit of optimisation here to keep the copy out of the fast path.
 	hdrLen := IPv4MinimumSize
 	if len(i.Options) != 0 {
-		// AllocationSize is always >= len(i.Options).
-		aLen := i.Options.AllocationSize()
+		// SizeWithPadding is always >= len(i.Options).
+		aLen := i.Options.SizeWithPadding()
 		hdrLen += aLen
 		if hdrLen > len(b) {
 			panic(fmt.Sprintf("encode received %d bytes, wanted >= %d", len(b), hdrLen))
 		}
-		if aLen != copy(b[options:], i.Options) {
-			_ = copy(b[options+len(i.Options):options+aLen], []byte{0, 0, 0, 0})
+		opts := b[options:]
+		// This avoids bounds checks on the next line(s) which would happen even
+		// if there's no work to do.
+		if n := copy(opts, i.Options); n != aLen {
+			padding := opts[n:][:aLen-n]
+			for i := range padding {
+				padding[i] = 0
+			}
 		}
 	}
 	b.SetHeaderLength(uint8(hdrLen))