On Tue, Feb 18, 2025 at 1:23 AM Marcus Wichelmann
[off-list ref] wrote:
Enable the support for the bpf_xdp_adjust_meta helper function for XDP
buffers initialized by the tun driver. This allows to reserve a metadata
area that is useful to pass any information from one XDP program to
another one, for example when using tail-calls.
Whether this helper function can be used in an XDP program depends on
how the xdp_buff was initialized. Most net drivers initialize the
xdp_buff in a way, that allows bpf_xdp_adjust_meta to be used. In case
of the tun driver, this is currently not the case.
There are two code paths in the tun driver that lead to a
bpf_prog_run_xdp and where metadata support should be enabled:
1. tun_build_skb, which is called by tun_get_user and is used when
writing packets from userspace into the device. In this case, the
xdp_buff created in tun_build_skb has no support for
bpf_xdp_adjust_meta and calls of that helper function result in
ENOTSUPP.
For this code path, it's sufficient to set the meta_valid argument of
the xdp_prepare_buff call. The reserved headroom is large enough
already.
2. tun_xdp_one, which is called by tun_sendmsg which again is called by
other drivers (e.g. vhost_net). When the TUN_MSG_PTR mode is used,
another driver may pass a batch of xdp_buffs to the tun driver. In
this case, that other driver is the one initializing the xdp_buff.
See commit 043d222f93ab ("tuntap: accept an array of XDP buffs
through sendmsg()") for details.
For now, the vhost_net driver is the only one using TUN_MSG_PTR and
it already initializes the xdp_buffs with metadata support and
sufficient headroom. But the tun driver disables it again, so the
xdp_set_data_meta_invalid call has to be removed.
Signed-off-by: Marcus Wichelmann <redacted>
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Acked-by: Jason Wang <redacted>
Thanks