linux-kernel - Re: [PATCH net-next v6 4/5] ip_tunnel: Preserve pointer const in ip_tunnel_info

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Message-ID: <972f5a5f-7bff-5425-c3fb-d842d89897ef@nvidia.com>
Date:   Mon, 13 Mar 2023 15:17:58 +0800
From:   Gavin Li <gavinl@...dia.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        roopa@...dia.com, eng.alaamohamedsoliman.am@...il.com,
        bigeasy@...utronix.de, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, gavi@...dia.com, roid@...dia.com,
        maord@...dia.com, saeedm@...dia.com
Subject: Re: [PATCH net-next v6 4/5] ip_tunnel: Preserve pointer const in
 ip_tunnel_info_opts


On 3/9/2023 11:05 PM, Eric Dumazet wrote:
> External email: Use caution opening links or attachments
>
>
> On Thu, Mar 9, 2023 at 3:59 PM Eric Dumazet <edumazet@...gle.com> wrote:
>> On Thu, Mar 9, 2023 at 2:48 PM Gavin Li <gavinl@...dia.com> wrote:
>>> Change ip_tunnel_info_opts( ) from static function to macro to cast return
>>> value and preserve the const-ness of the pointer.
>>>
>>> Signed-off-by: Gavin Li <gavinl@...dia.com>
>>> ---
>>>   include/net/ip_tunnels.h | 11 ++++++-----
>>>   1 file changed, 6 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
>>> index fca357679816..3e5c102b841f 100644
>>> --- a/include/net/ip_tunnels.h
>>> +++ b/include/net/ip_tunnels.h
>>> @@ -67,6 +67,12 @@ struct ip_tunnel_key {
>>>          GENMASK((sizeof_field(struct ip_tunnel_info,            \
>>>                                options_len) * BITS_PER_BYTE) - 1, 0)
>>>
>>> +#define ip_tunnel_info_opts(info)                              \
>>> +       _Generic(info,                                          \
>>> +               const typeof(*(info)) * : ((const void *)((info) + 1)),\
>>> +               default : ((void *)((info) + 1))                \
>>> +       )
>>> +
>> Hmm...
>>
>> This looks quite dangerous, we lost type safety with the 'default'
>> case, with all these casts.
>>
>> What about using something cleaner instead ?
>>
>> (Not sure why we do not have an available generic helper for this kind
>> of repetitive things)
>>
> Or more exactly :
>
> diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
> index fca3576798166416982ee6a9100b003810c49830..17fc6c8f7e0b9e5303c1fb9e5dad77c5310e01a9
> 100644
> --- a/include/net/ip_tunnels.h
> +++ b/include/net/ip_tunnels.h
> @@ -485,10 +485,11 @@ static inline void iptunnel_xmit_stats(struct
> net_device *dev, int pkt_len)
>          }
>   }
>
> -static inline void *ip_tunnel_info_opts(struct ip_tunnel_info *info)
> -{
> -       return info + 1;
> -}
> +#define ip_tunnel_info_opts(info)                                      \
> +       (_Generic(info,                                                 \
> +                const struct ip_tunnel_info * : (const void *)((info)
> + 1),    \
> +                struct ip_tunnel_info * : (void *)((info) + 1))        \
> +       )
>
>   static inline void ip_tunnel_info_opts_get(void *to,
>                                             const struct ip_tunnel_info *info)
ACK