netdev - Re: tcp md5: one more crypto-sg-on-the-stack instance

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Message-ID: <1466827910.6850.123.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Sat, 25 Jun 2016 06:11:50 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: tcp md5: one more crypto-sg-on-the-stack instance

On Fri, 2016-06-24 at 18:51 -0700, Andy Lutomirski wrote:
> Hi all-
> 
> tcp_md5_hash_header does crypto using an sg that points to the stack.
> This will break with virtually mapped stacks.  It also looks like it's
> probably much slower than it deserves to be (it's trying to compute
> the MD5 hash of a few tens of bytes -- going through a scatterlist is
> a lot of overhead for an otherwise very fast operation).

I guess nobody cares about TCP MD5 speed really.

> 
> I don't suppose one of you could fix it or at least advise as to how
> it should be fixed.

Simply extend tcp_md5sig_pool to contain a copy of the TCP headers ?

At most 40 bytes of extra per cpu storage is not a big problem.