netdev - Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAMOZA0K-0LOGMXdFecRUHmyoOmOUabsgvzwA35jB-T=5tzV_TA@mail.gmail.com>
Date:   Thu, 23 Jan 2020 09:25:54 -0800
From:   Luigi Rizzo <lrizzo@...gle.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        netdev@...r.kernel.org, Jesper Dangaard Brouer <hawk@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, sameehj@...zon.com
Subject: Re: [PATCH] net-xdp: netdev attribute to control xdpgeneric skb linearization

On Thu, Jan 23, 2020 at 7:48 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 1/23/20 10:53 AM, Toke Høiland-Jørgensen wrote:
> > Luigi Rizzo <lrizzo@...gle.com> writes:
> >
> >> Add a netdevice flag to control skb linearization in generic xdp mode.
> >> Among the various mechanism to control the flag, the sysfs
> >> interface seems sufficiently simple and self-contained.
> >> The attribute can be modified through
> >>      /sys/class/net/<DEVICE>/xdp_linearize
> >> The default is 1 (on)
>
> Needs documentation in Documentation/ABI/testing/sysfs-class-net.
>
> > Erm, won't turning off linearization break the XDP program's ability to
> > do direct packet access?
>
> Yes, in the worst case you only have eth header pulled into linear section. :/
> In tc/BPF for direct packet access we have bpf_skb_pull_data() helper which can
> pull in up to X bytes into linear section on demand. I guess something like this
> could be done for XDP context as well, e.g. generic XDP would pull when non-linear
> and native XDP would have nothing todo (though in this case you end up writing the
> prog specifically for generic XDP with slowdown when you'd load it on native XDP
> where it's linear anyway, but that could/should be documented if so).

There was some discussion on multi-segment xdp
https://www.spinics.net/lists/netdev/msg620140.html
https://github.com/xdp-project/xdp-project/blob/master/areas/core/xdp-multi-buffer01-design.org

with no clear decision as far as I can tell.

I wanted to point out that linearization might be an issue for native
xdp as well
(specifically with NICs that do header split, LRO, scatter-gather, MTU
> pagesize ...)
and having to unconditionally pay the linearization cost (or disable
the above features)
by just loading an xdp program may be a big performance hit.

cheers
luigi


>
> Thanks,
> Daniel