netdev - Re: [PATCH v2 1/4] net: tun: fix tun_xdp_one() for IFF

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Date:   Wed, 23 Jun 2021 09:30:32 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org
Cc:     Eugenio Pérez <eperezma@...hat.com>
Subject: Re: [PATCH v2 1/4] net: tun: fix tun_xdp_one() for IFF_TUN mode

On Wed, 2021-06-23 at 11:45 +0800, Jason Wang wrote:
> As replied in previous version, it would be better if we can unify 
> similar logic in tun_get_user().

Ah sorry, I missed that the first time.

Yes, that was my initial inclination too. But in the tun_get_user()
case we already *have* "pi", having read it separately into a local
variable; it never made it to the skb. So the cases are subtly
different enough that abstracting it out didn't seem to make sense.

If I try harder to unify it, I suppose it looks something like this and
*might* just make the cut for "simple enough to be backported to stable
kernels in a bug fix".

I'll add the PI mode to my test cases and try it (as well as *actually*
unifying the offending code, of course).

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2332,10 +2332,12 @@ static int tun_xdp_one(struct tun_struct *tun,
        unsigned int datasize = xdp->data_end - xdp->data;
        struct tun_xdp_hdr *hdr = xdp->data_hard_start;
        struct virtio_net_hdr *gso = NULL;
+       struct tun_pi *pi = NULL;
        struct bpf_prog *xdp_prog;
        struct sk_buff *skb = NULL;
        u32 rxhash = 0, act;
        int buflen = hdr->buflen;
+       int reservelen = xdp->data - xdp->data_hard_start;
        int err = 0;
        bool skb_xdp = false;
        struct page *page;
@@ -2343,6 +2345,11 @@ static int tun_xdp_one(struct tun_struct *tun,
        if (tun->flags & IFF_VNET_HDR)
                gso = &hdr->gso;
 
+       if (!(tun->flags & IFF_NO_PI)) {
+               pi = xdp->data;
+               reservelen += sizeof(*pi);
+       }
+
        xdp_prog = rcu_dereference(tun->xdp_prog);
        if (xdp_prog) {
                if (gso && gso->gso_type) {
@@ -2388,7 +2395,7 @@ static int tun_xdp_one(struct tun_struct *tun,
                goto out;
        }
 
-       skb_reserve(skb, xdp->data - xdp->data_hard_start);
+       skb_reserve(skb, reservelen);
        skb_put(skb, xdp->data_end - xdp->data);
 
        if (gso && virtio_net_hdr_to_skb(skb, gso, tun_is_little_endian(tun))) {



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