nf_flow on big-endian (was: Re: linux-next: build warning after merge of the net-next tree)

[Geert Uytterhoeven]

On Thu, Nov 21, 2019 at 8:36 AM Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote:
> After merging the net-next tree, today's linux-next build (powerpc
> allyesconfig) produced this warning:
>
> net/netfilter/nf_flow_table_offload.c: In function 'nf_flow_rule_match':
> net/netfilter/nf_flow_table_offload.c:80:21: warning: unsigned conversion from 'int' to '__be16' {aka 'short unsigned int'} changes value from '327680' to '0' [-Woverflow]
>    80 |   mask->tcp.flags = TCP_FLAG_RST | TCP_FLAG_FIN;
>       |                     ^~~~~~~~~~~~
>
> Introduced by commit
>
>   c29f74e0df7a ("netfilter: nf_flow_table: hardware offload support")

This is now upstream, and must be completely broken on big-endian
platforms.

The other user of the flags field looks buggy, too
(net/core/flow_dissector.c:__skb_flow_dissect_tcp()[*]):

     key_tcp->flags = (*(__be16 *) &tcp_flag_word(th) & htons(0x0FFF));

Disclaimer: I'm not familiar with the code or protocol, so below are just
my gut feelings.

     struct flow_dissector_key_tcp {
            __be16 flags;
    };

Does this have to be __be16, i.e. does it go over the wire?
If not, this should probably be __u16, and set using
"be32_to_cpu(flags) >> 16"?
If yes, "cpu_to_be16(be32_to_cpu(flags) >> 16)"?
(Ugh, needs convenience macros)

[*] ac4bb5de27010e41 ("net: flow_dissector: add support for dissection
of tcp flags")

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds