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authorXie He <xie.he.0141@gmail.com>2020-09-14 00:41:54 -0700
committerDavid S. Miller <davem@davemloft.net>2020-09-14 13:34:39 -0700
commitb4c5881446b3a5888b6815eaf9cbb5eda0f68363 (patch)
tree029ec3783d5c5a453b4e91ed4fbb06e8f4950d44 /net/packet
parentb91c06c5df511dfed3995c4e1a44729756ef0025 (diff)
net/packet: Fix a comment about hard_header_len and headroom allocation
This comment is outdated and no longer reflects the actual implementation of af_packet.c. Reasons for the new comment: 1.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> Acked-by: Willem de Bruijn <willemb@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/packet')
-rw-r--r--net/packet/af_packet.c15
1 files changed, 9 insertions, 6 deletions
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index af6c93ed9fa0..2d5d5fbb435c 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.