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, 6 insertions, 17 deletions

diff --git a/dhcpv4/option_server_identifier.go b/dhcpv4/option_server_identifier.go
index 26c21a7..fd0311a 100644
--- a/dhcpv4/option_server_identifier.go
+++ b/dhcpv4/option_server_identifier.go
@@ -3,12 +3,14 @@ package dhcpv4
 import (
 	"fmt"
 	"net"
+
+	"github.com/u-root/u-root/pkg/uio"
 )
 
+// OptServerIdentifier represents an option encapsulating the server identifier.
+//
 // This option implements the server identifier option
 // https://tools.ietf.org/html/rfc2132
-
-// OptServerIdentifier represents an option encapsulating the server identifier.
 type OptServerIdentifier struct {
 	ServerID net.IP
 }
@@ -16,21 +18,8 @@ type OptServerIdentifier struct {
 // ParseOptServerIdentifier returns a new OptServerIdentifier from a byte
 // stream, or error if any.
 func ParseOptServerIdentifier(data []byte) (*OptServerIdentifier, error) {
-	if len(data) < 2 {
-		return nil, ErrShortByteStream
-	}
-	code := OptionCode(data[0])
-	if code != OptionServerIdentifier {
-		return nil, fmt.Errorf("expected code %v, got %v", OptionServerIdentifier, 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 &OptServerIdentifier{ServerID: net.IP(data[2 : 2+length])}, nil
+	buf := uio.NewBigEndianBuffer(data)
+	return &OptServerIdentifier{ServerID: net.IP(buf.CopyN(net.IPv4len))}, buf.FinError()
 }
 
 // Code returns the option code.