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Date:   Thu, 3 Aug 2023 20:53:16 +0300
From:   Andrew Kanner <andrew.kanner@gmail.com>
To:     Jason Wang <jasowang@redhat.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>
Cc:     davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
        pabeni@redhat.com, netdev@vger.kernel.org, dsahern@gmail.com,
        jbrouer@redhat.com, john.fastabend@gmail.com,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
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Subject: Re: [PATCH v4 1/2] drivers: net: prevent tun_build_skb() to exceed
 the packet size limit
References: <20230801220710.464-1-andrew.kanner@gmail.com>
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On Thu, Aug 03, 2023 at 11:19:47AM +0800, Jason Wang wrote:
> > > @@ -1594,7 +1597,13 @@ static bool tun_can_build_skb(struct tun_struct *tun, struct tun_file *tfile,
> > >       if (zerocopy)
> > >               return false;
> > >
> > > -     if (SKB_DATA_ALIGN(len + TUN_RX_PAD) +
> > > +     rcu_read_lock();
> > > +     xdp_prog = rcu_dereference(tun->xdp_prog);
> > > +     if (xdp_prog)
> > > +             pad += XDP_PACKET_HEADROOM;
> > > +     rcu_read_unlock();
> > > +
> >
> > Isolated seen, I guess, this is a correct fix to 7df13219d757.
> 
> I think so.
> 
> Actually, I think we can probably always count XDP_PACKET_HEADROOM
> here. Since there's a window that XDP program might be attached in the
> middle of tun_can_build_skb() and tun_build_skb().


Thanks, that makes sense.
I will do it in v5.


> > Question to Jason Wang:
> > Why fall back (to e.g. tun_alloc_skb()) when size is above PAGE_SIZE?
> >
> > AFAIK tun_build_skb() *can* create get larger packets than PAGE_SIZE
> > from it's page_frag.  Is there a reason for this limitation?
> 
> I couldn't recall but I think we can relax.

Jesper already sent enough info for this idea in v2, I will use it for
the next patch/series.

Jesper, I will add this tag for this next patch/series if you don't
mind:
Suggested-by: Jesper Dangaard Brouer <hawk@kernel.org>

-- 
Andrew Kanner