netdev - Re: [PATCH 3/3] s390/lcs: Fix return type of lcs_start

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Date:   Thu, 3 Nov 2022 16:27:26 +0100
From:   Alexandra Winter <wintera@...ux.ibm.com>
To:     Nathan Chancellor <nathan@...nel.org>,
        Wenjia Zhang <wenjia@...ux.ibm.com>
Cc:     Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH 3/3] s390/lcs: Fix return type of lcs_start_xmit()



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/lcs.c:2090:21: 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         = lcs_start_xmit,
>                                     ^~~~~~~~~~~~~~
>   drivers/s390/net/lcs.c:2097:21: 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         = lcs_start_xmit,
>                                     ^~~~~~~~~~~~~~
> 
> ->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
> 'netdev_tx_t', not 'int'. Adjust the return type of lcs_start_xmit() 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@...nel.org>
> ---
>  drivers/s390/net/lcs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/s390/net/lcs.c b/drivers/s390/net/lcs.c
> index 84c8981317b4..4cbb9802bf22 100644
> --- a/drivers/s390/net/lcs.c
> +++ b/drivers/s390/net/lcs.c
> @@ -1519,7 +1519,7 @@ lcs_txbuffer_cb(struct lcs_channel *channel, struct lcs_buffer *buffer)
>  /*
>   * Packet transmit function called by network stack
>   */
> -static int
> +static netdev_tx_t
>  __lcs_start_xmit(struct lcs_card *card, struct sk_buff *skb,
>  		 struct net_device *dev)
>  {
> @@ -1582,7 +1582,7 @@ __lcs_start_xmit(struct lcs_card *card, struct sk_buff *skb,
>  	return rc;
>  }
>  
> -static int
> +static netdev_tx_t
>  lcs_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct lcs_card *card;

Thanks a lot for the fix.
Could you please also fix the indentation of these lines?
With that:
Reviewed-by: Alexandra Winter <wintera@...ux.ibm.com>