[net-next] net/packet: Fix a comment about hard_header_len and add a warning for it

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needed_headroom by fixing an outdated comment and adding a WARN_ON_ONCE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.kernel@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Brian Norris <briannorris@chromium.org>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
---
 net/packet/af_packet.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

On Fri, Sep 11, 2020 at 7:04 AM Xie He <xie.he.0141@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 needed_headroom by fixing an outdated comment and adding a WARN_ON_ONCE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.kernel@gmail.com>
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Brian Norris <briannorris@chromium.org>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Signed-off-by: Xie He <xie.he.0141@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

On Fri, Sep 11, 2020 at 7:32 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> 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

I have recently fixed this problem in the "net" tree in hdlc_fr.c.

Glad to see the number of drivers that have this problem is not very big.

On Sat, Sep 12, 2020 at 1:37 AM Xie He <xie.he.0141@gmail.com> wrote:
>
> On Fri, Sep 11, 2020 at 7:32 AM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > 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
>
> I have recently fixed this problem in the "net" tree in hdlc_fr.c.
>
> Glad to see the number of drivers that have this problem is not very big.

I am concerned about adding a WARN_ON_ONCE that we already expect to
fire on some platforms.

Probably better to add the documentation without the warning.

I know I suggested the check before, sorry for the churn, but I hadn't
checked the existing state yet.

On Sun, Sep 13, 2020 at 1:11 AM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> I am concerned about adding a WARN_ON_ONCE that we already expect to
> fire on some platforms.
>
> Probably better to add the documentation without the warning.
>
> I know I suggested the check before, sorry for the churn, but I hadn't
> checked the existing state yet.

OK. No problem. It's always good to discuss and find out the best way
before we do the changes.

I'll drop the WARN_ON_ONCE part and resubmit the patch.