Commit b79a80bd authored Sep 16, 2020 by Xie He Committed by David S. Miller Sep 17, 2020

net/packet: Fix a comment about mac_header



1. Change all "dev->hard_header" to "dev->header_ops"

2. On receiving incoming frames when header_ops == NULL:

The comment only says what is wrong, but doesn't say what is right.
This patch changes the comment to make it clear what is right.

3. On transmitting and receiving outgoing frames when header_ops == NULL:

The comment explains that the LL header will be later added by the driver.

However, I think it's better to simply say that the LL header is invisible
to us. This phrasing is better from a software engineering perspective,
because this makes it clear that what happens in the driver should be
hidden from us and we should not care about what happens internally in the
driver.

4. On resuming the LL header (for RAW frames) when header_ops == NULL:

The comment says we are "unlikely" to restore the LL header.

However, we should say that we are "unable" to restore it.
It's not possible (rather than not likely) to restore it, because:

1) There is no way for us to restore because the LL header internally
processed by the driver should be invisible to us.

2) In function packet_rcv and tpacket_rcv, the code only tries to restore
the LL header when header_ops != NULL.

Cc: Willem de Bruijn <willemdebruijn.kernel@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>

parent 31660a97

net/packet/af_packet.c

+12 −11

Original line number	Diff line number	Diff line
		@@ -108,36 +108,37 @@
		On receive:
		-----------

		Incoming, dev->hard_header!=NULL
		Incoming, dev->header_ops != NULL
		mac_header -> ll header
		data -> data

		Outgoing, dev->hard_header!=NULL
		Outgoing, dev->header_ops != NULL
		mac_header -> ll header
		data -> ll header

		Incoming, dev->hard_header==NULL
		mac_header -> UNKNOWN position. It is very likely, that it points to ll
		header. PPP makes it, that is wrong, because introduce
		assymetry between rx and tx paths.
		Incoming, dev->header_ops == NULL
		mac_header -> data
		However drivers often make it point to the ll header.
		This is incorrect because the ll header should be invisible to us.
		data -> data

		Outgoing, dev->hard_header==NULL
		mac_header -> data. ll header is still not built!
		Outgoing, dev->header_ops == NULL
		mac_header -> data. ll header is invisible to us.
		data -> data

		Resume
		If dev->hard_header==NULL we are unlikely to restore sensible ll header.
		If dev->header_ops == NULL we are unable to restore the ll header,
		because it is invisible to us.


		On transmit:
		------------

		dev->hard_header != NULL
		dev->header_ops != NULL
		mac_header -> ll header
		data -> ll header

		dev->hard_header == NULL (ll header is added by device, we cannot control it)
		dev->header_ops == NULL (ll header is invisible to us)
		mac_header -> data
		data -> data

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