linux-kernel - Re: [PATCH net-next v3 4/5] ip_tunnel: constify input argument of ip_tunnel_info

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Date:   Sun, 19 Feb 2023 21:46:00 +0100
From:   Simon Horman <simon.horman@...igine.com>
To:     Gavin Li <gavinl@...dia.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, 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 v3 4/5] ip_tunnel: constify input argument of
 ip_tunnel_info_opts( )

On Sun, Feb 19, 2023 at 09:29:21PM +0100, Simon Horman wrote:
> On Fri, Feb 17, 2023 at 05:39:24AM +0200, Gavin Li wrote:
> > Constify input argument(i.e. struct ip_tunnel_info *info) of
> > ip_tunnel_info_opts( ) so that it wouldn't be needed to W/A it each time
> > in each driver.
> > 
> > Signed-off-by: Gavin Li <gavinl@...dia.com>
> > ---
> >  include/net/ip_tunnels.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
> > index fca357679816..32c77f149c6e 100644
> > --- a/include/net/ip_tunnels.h
> > +++ b/include/net/ip_tunnels.h
> > @@ -485,9 +485,9 @@ 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)
> > +static inline void *ip_tunnel_info_opts(const struct ip_tunnel_info *info)
> >  {
> > -	return info + 1;
> > +	return (void *)(info + 1);
> 
> I'm unclear on what problem this is trying to solve,
> but info being const, and then returning (info +1)
> as non-const feels like it is masking rather than fixing a problem.

I now see that an example of the problem is added by path 5/5.

...
  CC [M]  drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun_vxlan.o
drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun_vxlan.c: In function 'mlx5e_gen_ip_tunnel_header_vxlan':
drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun_vxlan.c:103:43: error: passing argument 1 of 'ip_tunnel_info_opts' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
  103 |                 md = ip_tunnel_info_opts(e->tun_info);
      |                                          ~^~~~~~~~~~
In file included from drivers/net/ethernet/mellanox/mlx5/core/en/tc_tun_vxlan.c:4:
./include/net/ip_tunnels.h:488:64: note: expected 'struct ip_tunnel_info *' but argument is of type 'const struct ip_tunnel_info *'
  488 | static inline void *ip_tunnel_info_opts(struct ip_tunnel_info *info)
      |                                         ~~~~~~~~~~~~~~~~~~~~~~~^~~~
...

But I really do wonder if this patch masks rather than fixes the problem.

> 
> >  }
> >  
> >  static inline void ip_tunnel_info_opts_get(void *to,
> > -- 
> > 2.31.1
> >