[crypto] Allow for parsing of partial ASN.1 cursors

Allow code to create a partial ASN.1 cursor containing only the type
and length bytes, so that asn1_start() may be used to determine the
length of a large ASN.1 blob without first allocating memory to hold
the entire blob.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/asn1.c

  *
  * @v cursor		ASN.1 object cursor
  * @v type		Expected type, or ASN1_ANY
+ * @v extra		Additional length not present within partial cursor
  * @ret len		Length of object body, or negative error
  *
  * The object cursor will be updated to point to the start of the
  * the length of the object body (i.e. the number of bytes until the
  * following object tag, if any) is returned.
  */
-static int asn1_start ( struct asn1_cursor *cursor, unsigned int type ) {
+int asn1_start ( struct asn1_cursor *cursor, unsigned int type, size_t extra ) {
 	unsigned int len_len;
 	unsigned int len;
 
 		cursor->data++;
 		cursor->len--;
 	}
-	if ( cursor->len < len ) {
+	if ( ( cursor->len + extra ) < len ) {
 		DBGC ( cursor, "ASN1 %p bad length %d (max %zd)\n",
-		       cursor, len, cursor->len );
+		       cursor, len, ( cursor->len + extra ) );
 		return -EINVAL_ASN1_LEN;
 	}
 
 int asn1_enter ( struct asn1_cursor *cursor, unsigned int type ) {
 	int len;
 
-	len = asn1_start ( cursor, type );
+	len = asn1_start ( cursor, type, 0 );
 	if ( len < 0 ) {
 		asn1_invalidate_cursor ( cursor );
 		return len;
 int asn1_skip_if_exists ( struct asn1_cursor *cursor, unsigned int type ) {
 	int len;
 
-	len = asn1_start ( cursor, type );
+	len = asn1_start ( cursor, type, 0 );
 	if ( len < 0 )
 		return len;
 
 
 	/* Find end of object */
 	memcpy ( &temp, cursor, sizeof ( temp ) );
-	len = asn1_start ( &temp, type );
+	len = asn1_start ( &temp, type, 0 );
 	if ( len < 0 ) {
 		asn1_invalidate_cursor ( cursor );
 		return len;

src/include/ipxe/asn1.h

 	return ( ( cursor->len >= sizeof ( *type ) ) ? *type : ASN1_END );
 }
 
+extern int asn1_start ( struct asn1_cursor *cursor, unsigned int type,
+			size_t extra );
 extern int asn1_enter ( struct asn1_cursor *cursor, unsigned int type );
 extern int asn1_skip_if_exists ( struct asn1_cursor *cursor,
 				 unsigned int type );