netdev - Re: [PATCH net] flow_dissector: fix byteorder of dissected ICMP ID

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Date:   Sun, 14 Mar 2021 20:44:49 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     Alexander Lobakin <alobakin@...me>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Davide Caratti <dcaratti@...hat.com>,
        Guillaume Nault <gnault@...hat.com>, wenxu <wenxu@...oud.cn>,
        Eran Ben Elisha <eranbe@...dia.com>,
        Matteo Croce <mcroce@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] flow_dissector: fix byteorder of dissected ICMP ID

On Sun, Mar 14, 2021 at 09:21:40PM +0100, Jakub Sitnicki wrote:
> On Fri, Mar 12, 2021 at 09:08 PM CET, Alexander Lobakin wrote:
> > flow_dissector_key_icmp::id is of type u16 (CPU byteorder),
> > ICMP header has its ID field in network byteorder obviously.
> > Sparse says:
> >
> > net/core/flow_dissector.c:178:43: warning: restricted __be16 degrades to integer
> >
> > Convert ID value to CPU byteorder when storing it into
> > flow_dissector_key_icmp.
> >
> > Fixes: 5dec597e5cd0 ("flow_dissector: extract more ICMP information")
> > Signed-off-by: Alexander Lobakin <alobakin@...me>
> > ---
> >  net/core/flow_dissector.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> > index 2ef2224b3bff..a96a4f5de0ce 100644
> > --- a/net/core/flow_dissector.c
> > +++ b/net/core/flow_dissector.c
> > @@ -176,7 +176,7 @@ void skb_flow_get_icmp_tci(const struct sk_buff *skb,
> >  	 * avoid confusion with packets without such field
> >  	 */
> >  	if (icmp_has_id(ih->type))
> > -		key_icmp->id = ih->un.echo.id ? : 1;
> > +		key_icmp->id = ih->un.echo.id ? ntohs(ih->un.echo.id) : 1;
> >  	else
> >  		key_icmp->id = 0;
> >  }
> 
> Smells like a breaking change for existing consumers of this value.
> 
> How about we change the type of flow_dissector_key_icmp{}.id to __be16
> instead to make sparse happy?

The struct flow_dissector_key_icmp::id only appears to be used in
bond_xmit_hash, and there, the exact value doesn't seem to matter.

This appears to be a real bug and not just to appease sparse:
ih->un.echo.id has one endianness and "1" has another. Both cannot
be correct.