Re: [PATCH 2/3] s390/netiucv: Fix return type of netiucv_tx()

[Alexandra Winter]

On 02.11.22 17:32, Nathan Chancellor wrote:
> With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
> indirect call targets are validated against the expected function
> pointer prototype to make sure the call target is valid to help mitigate
> ROP attacks. If they are not identical, there is a failure at run time,
> which manifests as either a kernel panic or thread getting killed. A
> proposed warning in clang aims to catch these at compile time, which
> reveals:
> 
>   drivers/s390/net/netiucv.c:1854:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
>           .ndo_start_xmit         = netiucv_tx,
>                                     ^~~~~~~~~~
> 
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of netiucv_tx() to
> match the prototype's to resolve the warning and potential CFI failure,
> should s390 select ARCH_SUPPORTS_CFI_CLANG in the future.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1750
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> ---
>  drivers/s390/net/netiucv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/s390/net/netiucv.c b/drivers/s390/net/netiucv.c
> index 65aa0a96c21d..1a7f2bc3a87b 100644
> --- a/drivers/s390/net/netiucv.c
> +++ b/drivers/s390/net/netiucv.c
> @@ -1256,7 +1256,7 @@ static int netiucv_close(struct net_device *dev)
>   *         Note: If we return !0, then the packet is free'd by
>   *               the generic network layer.
>   */
> -static int netiucv_tx(struct sk_buff *skb, struct net_device *dev)
> +static netdev_tx_t netiucv_tx(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct netiucv_priv *privptr = netdev_priv(dev);
>  	int rc;

Could you please also remove the corresponding comments:
diff --git a/drivers/s390/net/netiucv.c b/drivers/s390/net/netiucv.c
index 65aa0a96c21d..21e36f63fbc7 100644
--- a/drivers/s390/net/netiucv.c
+++ b/drivers/s390/net/netiucv.c
@@ -1248,13 +1248,6 @@ static int netiucv_close(struct net_device *dev)
 /*
  * Start transmission of a packet.
  * Called from generic network device layer.
- *
- * @param skb Pointer to buffer containing the packet.
- * @param dev Pointer to interface struct.
- *
- * @return 0 if packet consumed, !0 if packet rejected.
- *         Note: If we return !0, then the packet is free'd by
- *               the generic network layer.
  */
 static int netiucv_tx(struct sk_buff *skb, struct net_device *dev)
 {

Reviewed-by: Alexandra Winter <wintera@xxxxxxxxxxxxx>