[xferbuf] Add xfer_buffer() to provide direct access to underlying buffer

Allow data transfer buffer users to provide direct access to their
underlying data transfer buffer.

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

src/core/xferbuf.c

 	.write = xferbuf_umalloc_write,
 	.read = xferbuf_umalloc_read,
 };
+
+/**
+ * Get underlying data transfer buffer
+ *
+ * @v interface		Data transfer interface
+ * @ret xferbuf		Data transfer buffer, or NULL on error
+ *
+ * This call will check that the xfer_buffer() handler belongs to the
+ * destination interface which also provides xfer_deliver() for this
+ * interface.
+ *
+ * This is done to prevent accidental accesses to a data transfer
+ * buffer which may be located behind a non-transparent datapath via a
+ * series of pass-through interfaces.
+ */
+struct xfer_buffer * xfer_buffer ( struct interface *intf ) {
+	struct interface *dest;
+	xfer_buffer_TYPE ( void * ) *op =
+		intf_get_dest_op ( intf, xfer_buffer, &dest );
+	void *object = intf_object ( dest );
+	struct interface *xfer_deliver_dest;
+	struct xfer_buffer *xferbuf;
+
+	/* Check that this operation is provided by the same interface
+	 * which handles xfer_deliver().
+	 */
+	intf_get_dest_op ( intf, xfer_deliver, &xfer_deliver_dest );
+
+	if ( op && ( dest == xfer_deliver_dest ) ) {
+		xferbuf = op ( object );
+	} else {
+		/* Default is to not have a data transfer buffer */
+		xferbuf = NULL;
+	}
+
+	intf_put ( xfer_deliver_dest );
+	intf_put ( dest );
+	return xferbuf;
+}

src/include/ipxe/xferbuf.h

 #include <stdint.h>
 #include <ipxe/iobuf.h>
 #include <ipxe/uaccess.h>
+#include <ipxe/interface.h>
 #include <ipxe/xfer.h>
 
 /** A data transfer buffer */
 			     struct io_buffer *iobuf,
 			     struct xfer_metadata *meta );
 
+extern struct xfer_buffer * xfer_buffer ( struct interface *intf );
+#define xfer_buffer_TYPE( object_type ) \
+	typeof ( struct xfer_buffer * ( object_type ) )
+
 #endif /* _IPXE_XFERBUF_H */