On Sat, Feb 28, 2015 at 10:08 PM, David Miller <davem@xxxxxxxxxxxxx> wrote: > From: Eyal Birger <eyal.birger@xxxxxxxxx> > Date: Sat, 28 Feb 2015 21:39:34 +0200 > >> On Sat, Feb 28, 2015 at 9:21 PM, David Miller <davem@xxxxxxxxxxxxx> wrote: >>> From: Eyal Birger <eyal.birger@xxxxxxxxx> >>> Date: Thu, 26 Feb 2015 21:07:01 +0200 >>> >>>> 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 <eyal.birger@xxxxxxxxx> >>> >>> 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. > > So let's take a step back. > > What link layer we support has 32-byte hardware addresses? The largest one I've been hinted of is INFINIBAND_ALEN which is 20 bytes. > If the answer is lower than 32, we should use that value instead. Won't that value become the 'real' MAX_ADDR_LEN in that case? My concern is that any value I pick based on the existing implementations would need to be adjusted come a protocol with a larger address length. Also, I would have to assert the available amount of space at run-time as the address length is provided by a call to dev_parse_header() instead of using a build-time assert. Regards, Eyal. -- To unsubscribe from this list: send the line "unsubscribe linux-bluetooth" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
