netdev - Re: __sk_buff.data

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Message-ID: <20170420001218.GA38173@ast-mbp.thefacebook.com>
Date:   Wed, 19 Apr 2017 17:12:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Johannes Berg <johannes@...solutions.net>,
        Alexei Starovoitov <ast@...nel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: __sk_buff.data_end

On Thu, Apr 20, 2017 at 02:01:49AM +0200, Daniel Borkmann wrote:
> On 04/20/2017 12:20 AM, Johannes Berg wrote:
> >On Wed, 2017-04-19 at 23:31 +0200, Johannes Berg wrote:
> >>Hi Alexei, Daniel,
> >>
> >>I'm looking at adding the __wifi_sk_buff I talked about, and I notice
> >>that it uses CB space to store data_end. Unfortunately, in a lot of
> >>cases, we don't have any CB space to spare in wifi.
> >
> >I guess I can work around this, would this seem reasonable?
> >
> >  struct bpf_skb_data_end {
> >         struct qdisc_skb_cb qdisc_cb;
> >-       void *data_end;
> >+       /*
> >+        * The alignment here is for mac80211, since that doesn't use
> >+        * a pointer but a u64 value and needs to save/restore that
> >+        * across running its BPF programs.
> >+        */
> >+       void *data_end __aligned(sizeof(u64));
> >  };
> 
> Yeah, should work as well for the 32 bit archs, on 64 bit we
> have this effectively already:
> 
> struct bpf_skb_data_end {
>         struct qdisc_skb_cb        qdisc_cb;             /*     0    28 */
> 
>         /* XXX 4 bytes hole, try to pack */
> 
>         void *                     data_end;             /*    32     8 */
> 
>         /* size: 40, cachelines: 1, members: 2 */
>         /* sum members: 36, holes: 1, sum holes: 4 */
>         /* last cacheline: 40 bytes */
> };
> 
> Can you elaborate on why this works for mac80211? It uses cb
> only up to that point from where you invoke the prog?

+1

also didn't we discuss that wifi has crazy non-linear skb?
this data/data_end is used by cls_bpf with headlen only
for direct packet access where performance matters.
Since wifi skbs have only eth in headlen, there is not much
pointing adding support for data/data_end to wifi.
Just use ld_abs/ld_ind instructions and load_bytes() helper.