[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 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>

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