netdev - RE: [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa device

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:   Mon, 20 Dec 2021 03:49:21 +0000
From:   Parav Pandit <parav@...dia.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        David Ahern <dsahern@...il.com>
CC:     "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>
Subject: RE: [iproute2-next v2 4/4] vdpa: Enable user to set mtu of the vdpa
 device



> From: Michael S. Tsirkin <mst@...hat.com>
> Sent: Sunday, December 19, 2021 3:37 AM
> 
> On Sat, Dec 18, 2021 at 01:53:01PM -0700, David Ahern wrote:
> > On 12/17/21 1:08 AM, Parav Pandit wrote:
> > > @@ -204,6 +217,8 @@ static void vdpa_opts_put(struct nlmsghdr *nlh,
> struct vdpa *vdpa)
> > >  	if (opts->present & VDPA_OPT_VDEV_MAC)
> > >  		mnl_attr_put(nlh, VDPA_ATTR_DEV_NET_CFG_MACADDR,
> > >  			     sizeof(opts->mac), opts->mac);
> > > +	if (opts->present & VDPA_OPT_VDEV_MTU)
> > > +		mnl_attr_put_u16(nlh, VDPA_ATTR_DEV_NET_CFG_MTU,
> opts->mtu);
> >
> > Why limit the MTU to a u16? Eric for example is working on "Big TCP"
> > where IPv6 can work with Jumbograms where mtu can be > 64k.
> >
> > https://datatracker.ietf.org/doc/html/rfc2675
> 
> Well it's 16 bit at the virtio level, though we can extend that of course. Making
> it match for now removes need for validation.
> --
As Michael mentioned virtio specification limits the mtu to 64k-1. Hence 16-bit.
First we need to update the virtio spec to support > 64K mtu.
However, when/if (I don't know when) that happens, we need to make this also u32.
So may be we can make it u32 now, but still restrict the mtu value to 64k-1 in kernel, until the spec is updated.

Let me know, if you think that's future proofing is better, I first need to update the kernel to take nla u32.

> MST