On Sat, Feb 28, 2015 at 9:21 PM, David Miller [off-list ref] wrote:

From: Eyal Birger <redacted>
Date: Thu, 26 Feb 2015 21:07:01 +0200

quoted

As part of an effort to move skb->dropcount to skb->cb[], 4 bytes
of additional room are needed in skb->cb[] in packet sockets.

Store the skb original length in skb->dev instead of skb->cb[] for
this purpose.

Signed-off-by: Eyal Birger <redacted>

I'm a little confused, why is this even needed?

packet_skb_cb is 24 bytes by my calculations, which is much
smaller than the cb[] size which is 48 bytes.

Note the BUILD_BUG_ON in packet_rcv().

packet_skb_cb may contain an address as large as MAX_ADDR_LEN (32)
Therefore the required space is sizeof(packet_skb_cb) + MAX_ADDR_LEN - 8
which is 48 bytes before this change.

Regards,
Eyal.