[PATCH] um: net: Fix return type of uml_net_start_xmit()

[Nathan Chancellor]

Ping? I would like to turn this warning on for W=1 and the kbuild test
robot flagged this instance. If the UML folks don't want to carry it,
could I get an ack so that it could go with the warning enablement in
the kbuild tree?

https://lore.kernel.org/20231002-enable-wincompatible-function-pointer-types-strict-w-1-v1-1-808ab955d42d@kernel.org/

On Tue, Oct 03, 2023 at 11:35:53AM -0700, 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
> warning in clang aims to catch these at compile time, which reveals:
> 
>   arch/um/drivers/net_kern.c:353:21: warning: 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 *)' [-Wincompatible-function-pointer-types-strict]
>     353 |         .ndo_start_xmit         = uml_net_start_xmit,
>         |                                   ^~~~~~~~~~~~~~~~~~
>   1 warning generated.
> 
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of uml_net_start_xmit()
> to match the prototype's to resolve the warning. While UML does not
> currently implement support for kCFI, it could in the future, which
> means this warning becomes a fatal CFI failure at run time.
> 
> Reported-by: kernel test robot <lkp at intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202310031340.v1vPh207-lkp@intel.com/
> Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> ---
>  arch/um/drivers/net_kern.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/um/drivers/net_kern.c b/arch/um/drivers/net_kern.c
> index 3d7836c46507..cabcc501b448 100644
> --- a/arch/um/drivers/net_kern.c
> +++ b/arch/um/drivers/net_kern.c
> @@ -204,7 +204,7 @@ static int uml_net_close(struct net_device *dev)
>  	return 0;
>  }
>  
> -static int uml_net_start_xmit(struct sk_buff *skb, struct net_device *dev)
> +static netdev_tx_t uml_net_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct uml_net_private *lp = netdev_priv(dev);
>  	unsigned long flags;
> 
> ---
> base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
> change-id: 20231003-um-net-wifpts-14a7429c7524
> 
> Best regards,
> -- 
> Nathan Chancellor <nathan at kernel.org>
>