netdev - Re: [PATCH net 2/2] packet: round up linear to header len

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Date:   Wed, 8 Feb 2017 08:37:19 -0800
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net 2/2] packet: round up linear to header len

On Wed, Feb 8, 2017 at 7:34 AM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> On (02/07/17 15:57), Willem de Bruijn wrote:
>> @@ -2816,8 +2816,9 @@ static int packet_snd(struct socket *sock, struct msghdr *msg, size_t len)
>>       err = -ENOBUFS;
>>       hlen = LL_RESERVED_SPACE(dev);
>>       tlen = dev->needed_tailroom;
>> -     skb = packet_alloc_skb(sk, hlen + tlen, hlen, len,
>> -                            __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len),
>> +     linear = __virtio16_to_cpu(vio_le(), vnet_hdr.hdr_len);
>> +     linear = max(linear, min_t(int, len, dev->hard_header_len));
>> +     skb = packet_alloc_skb(sk, hlen + tlen, hlen, len, linear,
>>                              msg->msg_flags & MSG_DONTWAIT, &err);
>
> do we need a similar check in packet_sendsmg_spkt (even if it
> is deprecated, would be better to get it to align with packet_snd?)

That path does not need this check, because it does not support
large packets and always allocates the entire packet as linear.

> Also tpacket_fill_skb should ensure that copylen is set up like
> the code above?

Do you mean the difference that it unconditionally pulls
hard_header_len, optionally with zero padding, whereas this new
path can check against new min_header_len and thus allows
packets shorter than hard_header_len?