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 marcus.wichelmann@hetzner-cloud.de --- drivers/net/tun.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/drivers/net/tun.c b/drivers/net/tun.c index 28624cca91f8..c95ab9c46bd2 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c @@ -1713,7 +1713,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun, u32 act;
xdp_init_buff(&xdp, buflen, &tfile->xdp_rxq); - xdp_prepare_buff(&xdp, buf, pad, len, false); + xdp_prepare_buff(&xdp, buf, pad, len, true);
act = bpf_prog_run_xdp(xdp_prog, &xdp); if (act == XDP_REDIRECT || act == XDP_TX) { @@ -2471,7 +2471,6 @@ static int tun_xdp_one(struct tun_struct *tun, }
xdp_init_buff(xdp, buflen, &tfile->xdp_rxq); - xdp_set_data_meta_invalid(xdp);
act = bpf_prog_run_xdp(xdp_prog, xdp); ret = tun_xdp_act(tun, xdp_prog, xdp, act);