netdev - Re: seeing strange values for tcp sk_rmem

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: Windows password security audit tool. GUI, reports in PDF.

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

Date:	Tue, 01 Dec 2009 18:52:12 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Chris Friesen <cfriesen@...tel.com>
CC:	netdev@...r.kernel.org, Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: seeing strange values for tcp sk_rmem_alloc

Chris Friesen a écrit :

> I realize this.  I sent the data from a socket to itself.  It could just
> as easily be done with two tcp sockets.  The important thing is that I
> control both the tx and rx sides, so I know how much data should be
> present in the rx queue at any point in time.
> 
> The part that surprised me was that I could send multiple chunks of data
> without sk_rmem_alloc changing on the socket to which the data was being
> sent.  Then it would jump up by a large amount (up to 20K) all at once.
> 
> I'm starting to suspect that the discrepency might have something to do
> with the skb_copy_datagram_iovec() call in tcp_data_queue(), and how
> skb_set_owner_r() is only called if "eaten" is <= 0.  This could be
> totally off-base though.
> 

If you dont read() your socket, then skb_copy_datagram_iovec() is not called

But be careful of sender tcp stack : It might be delayed a bit,
because it waits for receiver to open its window (slow start)

You probably need something like

while (1) {
	send(fd1, buffer, 2Kbytes);
	sleep(2); // let tcp stack flush its write buffers
	display_sk_rmem_alloc(fd2);
}

--
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