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

On 2017年10月26日 22:11, Eric Dumazet wrote:

On Thu, Oct 26, 2017 at 5:15 AM, Jason Wang [off-list ref] wrote:

quoted

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@google.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Wei Wei <redacted>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reported-by: Wei Wei <redacted>
Signed-off-by: Jason Wang <redacted>
---
- 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