Re: [PATCH net] tuntap: properly align skb->head before building skb

[Jason Wang]

On 2017å10æ26æ 22:11, Eric Dumazet wrote:
> On Thu, Oct 26, 2017 at 5:15 AM, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > An unaligned alloc_frag->offset caused by previous allocation will
> > result an unaligned skb->head. This will lead unaligned
> > skb_shared_info and then unaligned dataref which requires to be
> > aligned for accessing on some architecture. Fix this by aligning
> > alloc_frag->offset before the frag refilling.
> >
> > Fixes: 0bbd7dad34f8 ("tun: make tun_build_skb() thread safe")
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Willem de Bruijn <willemdebruijn.kernel@xxxxxxxxx>
> > Cc: Wei Wei <dotweiba@xxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Reported-by: Wei Wei <dotweiba@xxxxxxxxx>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> > - The patch is needed for -stable.
> > - Wei, can you try this patch to see if it solves your issue?
> > ---
> >   drivers/net/tun.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index b9973fb..60e44f2 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1286,6 +1286,7 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
> >          buflen += SKB_DATA_ALIGN(len + pad);
> >          rcu_read_unlock();
> >
> > +       alloc_frag->offset = ALIGN((u64)alloc_frag->offset, TUN_RX_PAD);
> You have to align to one cache line (SMP_CACHE_BYTES), or SKB_DATA_ALIGN(1)

Oh right.

> Then eventually use skb_reserve() for NET_IP_ALIGN, but I guess it is
> already done.

Yes.

Thanks