netdev - Re: [PATCH net-next] tcp: tcp_make

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20140326.151137.875104354732350232.davem@davemloft.net>
Date:	Wed, 26 Mar 2014 15:11:37 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	joe@...ches.com
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: tcp_make_synack() minor changes

From: Joe Perches <joe@...ches.com>
Date: Wed, 26 Mar 2014 11:52:48 -0700

> On Wed, 2014-03-26 at 14:25 -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Wed, 26 Mar 2014 09:57:19 -0700
>> 
>> > From: Eric Dumazet <edumazet@...gle.com>
>> > 
>> > There is no need to allocate 15 bytes in excess for a SYNACK packet,
>> > as it contains no data, only headers.
>> > 
>> > SYNACK are always generated in softirq context, and contain a single
>> > segment, we can use TCP_INC_STATS_BH()
>> > 
>> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>>  ...
>> > -	skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
>> > +	skb = sock_wmalloc(sk, MAX_TCP_HEADER, 1, GFP_ATOMIC);
>> 
>> Do you know where this "+ 15" comes from? :-)
>> It is a kind of Linux networking trivia question.
> 
> It's really old, pre v1 days.
> I presume it was some kind of ethernet minimum packet size padding.

I'm pretty sure it was to accomodate the hard header cache.

HH_DATA_MOD is 16
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html