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

[Anton Ivanov]

On 05/12/2023 17:53, Nathan Chancellor wrote:
> 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>
>>
> 
> 

Looks ok to me.
Acked-By: Anton Ivanov <anton.ivanov at cambridgegreys.com>
-- 
Anton R. Ivanov
Cambridgegreys Limited. Registered in England. Company Number 10273661
https://www.cambridgegreys.com/