Re: [PATCH net 1/2] veth: fix peer NETDEV_XDP_ACT_NDO_XMIT after GRO is toggled while down

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Tianyi Gao <tianyi@cloudflare.com> writes:

> A veth device advertises NETDEV_XDP_ACT_NDO_XMIT only if its peer has an
> XDP program attached or GRO enabled, that is, only if the peer will have
> NAPI to receive the frames.
>
> veth_set_features() updates the peer's flag when GRO is toggled, but
> returns early if the device is down, and veth_open() only refreshes the
> flags of the device being opened. Toggling GRO while the device is down
> therefore leaves the peer's flag stale after the device comes up.
>
> If GRO was enabled while down, the device comes up with NAPI but the
> peer does not advertise NDO_XMIT, and devmap rejects redirects to the
> peer with -EOPNOTSUPP. If GRO was disabled while down, the device comes
> up without NAPI but the peer still advertises NDO_XMIT, so redirects are
> accepted and then dropped in veth_xdp_xmit() with -ENXIO.
>
> Commit 7a6102aa6df0 ("veth: Update XDP feature set when bringing up
> device") made veth_open() refresh the device's own flags. Refresh the
> peer's flags there too. The peer's flag depends on this device's XDP
> program and GRO setting, not on whether the peer is up, so it is
> correct to set it even if the peer is down.
>
> Fixes: 8267fc71abb2 ("veth: take into account peer device for NETDEV_XDP_ACT_NDO_XMIT xdp_features flag")
> Signed-off-by: Tianyi Gao <tianyi@cloudflare.com>

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>