Re: [PATCH] ipv4: Correct/silence an endian warning in __ip_do_redirect

[Kunwu Chan]

Hi Eric,
Thank you so much for taking the time to guide me, I'm a rookie who 
really wants to do my part for the kernel, and I can't get started, so I 
thought about eliminating some of the sparse warnings. I've looked at 
some other commits and thought I could resolve the alert this way, sorry 
for the trouble.
Follow your suggestion:
1. I will add a 'Fixes' tag as follows:
'Fixes: 969447f226b4 ("ipv4: use new_gw for redirect neigh lookup")'

2. Refer to the modification method of commit 
3c42b2019863b327caa233072c50739d4144dd16, and modify the patch to:
'n = __ipv4_neigh_lookup(rt->dst.dev, (__force u32)new_gw); '

On 2023/11/18 01:15, Eric Dumazet wrote:
> On Fri, Nov 17, 2023 at 6:07 PM Kunwu Chan <chentao@xxxxxxxxxx> wrote:
> > net/ipv4/route.c:783:46: warning: incorrect type in argument 2 (different base types)
> > net/ipv4/route.c:783:46:    expected unsigned int [usertype] key
> > net/ipv4/route.c:783:46:    got restricted __be32 [usertype] new_gw
> >
> > Signed-off-by: Kunwu Chan <chentao@xxxxxxxxxx>
>
> We need Fixes: tag for networking patches.
>
> > ---
> >   net/ipv4/route.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > index 3290a4442b4a..e8a542c6b031 100644
> > --- a/net/ipv4/route.c
> > +++ b/net/ipv4/route.c
> > @@ -780,7 +780,7 @@ static void __ip_do_redirect(struct rtable *rt, struct sk_buff *skb, struct flow
> >                          goto reject_redirect;
> >          }
> >
> > -       n = __ipv4_neigh_lookup(rt->dst.dev, new_gw);
> > +       n = __ipv4_neigh_lookup(rt->dst.dev, be32_to_cpu(new_gw));
> >          if (!n)
> >                  n = neigh_create(&arp_tbl, &new_gw, rt->dst.dev);
> >          if (!IS_ERR(n)) {
> > --
> > 2.34.1
>
> How was this patch tested ?
>
> You are 'fixing' sparse warnings by replacing them with real bugs.
>
> be32_to_cpu() is going to swap bytes on x86, so the lookup will fail horribly.
>
> Here, if you must silence sparse, you want (__force u32)new_gw
>
> Look at this commit for a template.
>
> commit 3c42b2019863b327caa233072c50739d4144dd16