dhcpv4: simplify option parsing.

option's codes and lengths were being parsed twice: once in ParseOption
and once in each option type's Parse implementation. Consolidate such
that it only happens once.

Additionally, only pass data to options that they should parse -- we
know the length before the Parse function is called, so the option only
gets to see the data it needs to see.

Also, use uio.Lexer to simplify parsing code in general. Easier to read
and reason about.

1 files changed, 4 insertions, 15 deletions

diff --git a/dhcpv4/option_broadcast_address.go b/dhcpv4/option_broadcast_address.go
index ffa57e3..fdce946 100644
--- a/dhcpv4/option_broadcast_address.go
+++ b/dhcpv4/option_broadcast_address.go
@@ -3,6 +3,8 @@ package dhcpv4
 import (
 	"fmt"
 	"net"
+
+	"github.com/u-root/u-root/pkg/uio"
 )
 
 // This option implements the server identifier option
@@ -16,21 +18,8 @@ type OptBroadcastAddress struct {
 // ParseOptBroadcastAddress returns a new OptBroadcastAddress from a byte
 // stream, or error if any.
 func ParseOptBroadcastAddress(data []byte) (*OptBroadcastAddress, error) {
-	if len(data) < 2 {
-		return nil, ErrShortByteStream
-	}
-	code := OptionCode(data[0])
-	if code != OptionBroadcastAddress {
-		return nil, fmt.Errorf("expected code %v, got %v", OptionBroadcastAddress, code)
-	}
-	length := int(data[1])
-	if length != 4 {
-		return nil, fmt.Errorf("unexepcted length: expected 4, got %v", length)
-	}
-	if len(data) < 6 {
-		return nil, ErrShortByteStream
-	}
-	return &OptBroadcastAddress{BroadcastAddress: net.IP(data[2 : 2+length])}, nil
+	buf := uio.NewBigEndianBuffer(data)
+	return &OptBroadcastAddress{BroadcastAddress: net.IP(buf.CopyN(net.IPv4len))}, buf.FinError()
 }
 
 // Code returns the option code.