netdev - Re: Fix bugs in "Whether sock accept queue is full" checking

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Date:	Thu, 22 Feb 2007 03:13:10 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	weid@...css.fujitsu.com
Cc:	netdev@...r.kernel.org
Subject: Re: Fix bugs in "Whether sock accept queue is full" checking

From: weidong <weid@...css.fujitsu.com>
Date: Wed, 14 Feb 2007 11:30:57 -0500

> 	when I use linux TCP socket, and find there is a bug in function
> sk_acceptq_is_full(). 
 ...
> Consider an example:
> 
>  After listen(sockfd, 1) system call, sk->sk_max_ack_backlog is set to
> 1. As we know, sk->sk_ack_backlog is initialized to 0. Assuming accept()
> system call is not invoked now.
> 
> 1. 1st connection comes. invoke sk_acceptq_is_full(). sk-
> >sk_ack_backlog=0 sk->sk_max_ack_backlog=1, function return 0 accept
> this connection. Increase the sk->sk_ack_backlog
> 2. 2nd connection comes. invoke sk_acceptq_is_full(). sk-
> >sk_ack_backlog=1 sk->sk_max_ack_backlog=1, function return 0 accept
> this connection. Increase the sk->sk_ack_backlog
> 3. 3rd connection comes. invoke sk_acceptq_is_full(). sk-
> >sk_ack_backlog=2 sk->sk_max_ack_backlog=1, function return 1. Refuse
> this connection.
> 
> I think it has bugs. after listen system call. sk->sk_max_ack_backlog=1
> but now it can accept 2 connections. The following patch can fix this
> problem.
> 
> signed-off-by: Wei Dong <weid@...css.fujitsu.com>

Thank you very much for the detailed analysis and fix.

Actually, this bug exists all over the tree I believe, not just
in TCP.  I would like to audit all uses of sk_ack_backlog and
sk_max_ack_backlog before applying your patch, so please give
me some time to perform a quick audit.

Thank you.

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