[PATCH bpf-next v3 1/6] net: tun: enable XDP metadata support

[Marcus Wichelmann <marcus.wichelmann@hetzner-cloud.de>]

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 acf96f262488..4ec8fbd93c8d 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1709,7 +1709,7 @@ 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) {
@@ -2467,7 +2467,6 @@ 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);
-- 
2.43.0