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

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

[Nathan Chancellor]

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@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202310031340.v1vPh207-lkp@intel.com/
Signed-off-by: Nathan Chancellor <nathan@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@kernel.org>


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Re: [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@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202310031340.v1vPh207-lkp@intel.com/
> Signed-off-by: Nathan Chancellor <nathan@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@kernel.org>
> 

Re: [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@intel.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202310031340.v1vPh207-lkp@intel.com/
>> Signed-off-by: Nathan Chancellor <nathan@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@kernel.org>
>>
> 
> 

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

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

[Nathan Chancellor]

On Wed, Dec 06, 2023 at 07:19:35AM +0000, Anton Ivanov wrote:
> 
> 
> 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@intel.com>
> > > Closes: https://lore.kernel.org/oe-kbuild-all/202310031340.v1vPh207-lkp@intel.com/
> > > Signed-off-by: Nathan Chancellor <nathan@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@kernel.org>
> > > 
> > 
> > 
> 
> Looks ok to me.
> Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>

Thanks a lot, I have included this change in v2 of the series to turn
this warning on for W=1, so Masahiro should be able to carry it:

https://lore.kernel.org/20231206-enable-wincompatible-function-pointer-types-strict-w-1-v2-1-91311b4c37b0@kernel.org

Cheers,
Nathan