On Fri, Sep 11, 2020 at 7:04 AM Xie He <xie.he.0...@gmail.com> wrote:
>
> This patch tries to clarify the difference between hard_header_len and
> needed_headroom by fixing an outdated comment and adding a WARN_ON_ONCE
> warning for hard_header_len.
>
> The difference between hard_header_len and needed_headroom as understood
> by this patch is based on the following reasons:
>
> 1.
>
> In af_packet.c, the function packet_snd first reserves a headroom of
> length (dev->hard_header_len + dev->needed_headroom).
> Then if the socket is a SOCK_DGRAM socket, it calls dev_hard_header,
> which calls dev->header_ops->create, to create the link layer header.
> If the socket is a SOCK_RAW socket, it "un-reserves" a headroom of
> length (dev->hard_header_len), and checks if the user has provided a
> header sized between (dev->min_header_len) and (dev->hard_header_len)
> (in dev_validate_header).
> This shows the developers of af_packet.c expect hard_header_len to
> be consistent with header_ops.
>
> 2.
>
> In af_packet.c, the function packet_sendmsg_spkt has a FIXME comment.
> That comment states that prepending an LL header internally in a driver
> is considered a bug. I believe this bug can be fixed by setting
> hard_header_len to 0, making the internal header completely invisible
> to af_packet.c (and requesting the headroom in needed_headroom instead).
>
> 3.
>
> There is a commit for a WiFi driver:
> commit 9454f7a895b8 ("mwifiex: set needed_headroom, not hard_header_len")
> According to the discussion about it at:
> https://patchwork.kernel.org/patch/11407493/
> The author tried to set the WiFi driver's hard_header_len to the Ethernet
> header length, and request additional header space internally needed by
> setting needed_headroom.
> This means this usage is already adopted by driver developers.
>
> Cc: Willem de Bruijn <willemdebruijn.ker...@gmail.com>
> Cc: Eric Dumazet <eric.duma...@gmail.com>
> Cc: Brian Norris <briannor...@chromium.org>
> Cc: Cong Wang <xiyou.wangc...@gmail.com>
> Signed-off-by: Xie He <xie.he.0...@gmail.com>
> ---
> net/packet/af_packet.c | 17 +++++++++++------
> 1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index af6c93ed9fa0..93c89d3b2511 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -93,12 +93,15 @@
>
> /*
> Assumptions:
> - - if device has no dev->hard_header routine, it adds and removes ll header
> - inside itself. In this case ll header is invisible outside of device,
> - but higher levels still should reserve dev->hard_header_len.
> - Some devices are enough clever to reallocate skb, when header
> - will not fit to reserved space (tunnel), another ones are silly
> - (PPP).
> + - If the device has no dev->header_ops, there is no LL header visible
> + above the device. In this case, its hard_header_len should be 0.
> + The device may prepend its own header internally. In this case, its
> + needed_headroom should be set to the space needed for it to add its
> + internal header.
> + For example, a WiFi driver pretending to be an Ethernet driver should
> + set its hard_header_len to be the Ethernet header length, and set its
> + needed_headroom to be (the real WiFi header length - the fake Ethernet
> + header length).
> - packet socket receives packets with pulled ll header,
> so that SOCK_RAW should push it back.
>
> @@ -2936,6 +2939,8 @@ static int packet_snd(struct socket *sock, struct
> msghdr *msg, size_t len)
> skb_reset_network_header(skb);
>
> err = -EINVAL;
> + if (!dev->header_ops)
> + WARN_ON_ONCE(dev->hard_header_len != 0);
> if (sock->type == SOCK_DGRAM) {
> offset = dev_hard_header(skb, dev, ntohs(proto), addr, NULL,
> len);
> if (unlikely(offset < 0))
>From a quick scan, a few device types that might trigger this
net/atm/clip.c
drivers/net/wan/hdlc_fr.c
drivers/net/appletalk/ipddp.c
drivers/net/ppp/ppp_generic.c
drivers/net/net_failover.c
