[ncm] Treat a zero divisor as indicating no alignment requirements

A zero divisor will currently lead to a 16-bit integer overflow when
calculating the transmit padding, and a potential division by zero if
assertions are enabled.

Avoid these problems by treating a divisor value of zero as equivalent
to a divisor value of one (i.e. no alignment requirements).

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

diff --git a/src/drivers/net/ncm.c b/src/drivers/net/ncm.c
index 1837291f7df79173ca90b68028a564264198ac0e..cc07a43882d100f65db868f444fe19832056b9de 100644 (file)
--- a/src/drivers/net/ncm.c
+++ b/src/drivers/net/ncm.c
@@ -558,6 +558,8 @@ static int ncm_probe ( struct usb_function *func,
        struct usb_interface_descriptor *comms;
        struct ecm_ethernet_descriptor *ethernet;
        struct ncm_ntb_parameters params;
+       unsigned int remainder;
+       unsigned int divisor;
        int rc;
 
        /* Allocate and initialise structure */
@@ -616,14 +618,15 @@ static int ncm_probe ( struct usb_function *func,
        DBGC2 ( ncm, "NCM %p maximum IN size is %zd bytes\n", ncm, ncm->mtu );
 
        /* Calculate transmit padding */
-       ncm->padding = ( ( le16_to_cpu ( params.out.remainder ) -
-                          sizeof ( struct ncm_ntb_header ) - ETH_HLEN ) &
-                        ( le16_to_cpu ( params.out.divisor ) - 1 ) );
+       divisor = ( params.out.divisor ?
+                   le16_to_cpu ( params.out.divisor ) : 1 );
+       remainder = le16_to_cpu ( params.out.remainder );
+       ncm->padding = ( ( remainder - sizeof ( struct ncm_ntb_header ) -
+                          ETH_HLEN ) & ( divisor - 1 ) );
        DBGC2 ( ncm, "NCM %p using %zd-byte transmit padding\n",
                ncm, ncm->padding );
        assert ( ( ( sizeof ( struct ncm_ntb_header ) + ncm->padding +
-                    ETH_HLEN ) % le16_to_cpu ( params.out.divisor ) ) ==
-                le16_to_cpu ( params.out.remainder ) );
+                    ETH_HLEN ) % divisor ) == remainder );
 
        /* Register network device */
        if ( ( rc = register_netdev ( netdev ) ) != 0 )