netdev - Re: [PATCH 00/11] XDP unaligned chunk placement support

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]

Date:   Fri, 28 Jun 2019 18:51:37 +0200
From:   Björn Töpel <bjorn.topel@...el.com>
To:     "Laatz, Kevin" <kevin.laatz@...el.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Jonathan Lemon <jonathan.lemon@...il.com>, netdev@...r.kernel.org,
        ast@...nel.org, daniel@...earbox.net, magnus.karlsson@...el.com,
        bpf@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        bruce.richardson@...el.com, ciara.loftus@...el.com
Subject: Re: [PATCH 00/11] XDP unaligned chunk placement support

On 2019-06-28 18:19, Laatz, Kevin wrote:
> On 27/06/2019 22:25, Jakub Kicinski wrote:
>> On Thu, 27 Jun 2019 12:14:50 +0100, Laatz, Kevin wrote:
>>> On the application side (xdpsock), we don't have to worry about the user
>>> defined headroom, since it is 0, so we only need to account for the
>>> XDP_PACKET_HEADROOM when computing the original address (in the default
>>> scenario).
>> That assumes specific layout for the data inside the buffer.  Some NICs
>> will prepend information like timestamp to the packet, meaning the
>> packet would start at offset XDP_PACKET_HEADROOM + metadata len..
> 
> Yes, if NICs prepend extra data to the packet that would be a problem for
> using this feature in isolation. However, if we also add in support for 
> in-order
> RX and TX rings, that would no longer be an issue. However, even for NICs
> which do prepend data, this patchset should not break anything that is 
> currently
> working.

(Late on the ball. I'm in vacation mode.)

In your example Jakub, how would this look in XDP? Wouldn't the
timestamp be part of the metadata (xdp_md.data_meta)? Isn't
data-data_meta (if valid) <= XDP_PACKET_HEADROOM? That was my assumption.

There were some discussion on having meta data length in the struct
xdp_desc, before AF_XDP was merged, but the conclusion was that this was
*not* needed, because AF_XDP and the XDP program had an implicit
contract. If you're running AF_XDP, you also have an XDP program running
and you can determine the meta data length (and also getting back the
original buffer).

So, today in AF_XDP if XDP metadata is added, the userland application
can look it up before the xdp_desc.addr (just like regular XDP), and how
the XDP/AF_XDP application determines length/layout of the metadata i
out-of-band/not specified.

This is a bit messy/handwavy TBH, so maybe adding the length to the
descriptor *is* a good idea (extending the options part of the
xdp_desc)? Less clean though. OTOH the layout of the meta data still
need to be determined.

Björn