[PATCH net] net/packet: Fix a comment about hard_header_len and headroom allocation

[Xie He <xie.he.0141@gmail.com>]

This comment is outdated and no longer reflects the actual implementation
of af_packet.c.

Reasons for the new comment:

1.

In this file, 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 of length (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 this file, 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 | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 2b33e977a905..c808c76efa71 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
+     outside of 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.
 
-- 
2.25.1