linux-kernel - Re: A possible bug in reqsk_queue_hash

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Date:	Tue, 20 Apr 2010 13:06:51 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Li Yu <raise.sail@...il.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: A possible bug in reqsk_queue_hash_req()

Le mardi 20 avril 2010 à 18:35 +0800, Li Yu a écrit :
> Hi,
> 
>      I found out a possible bug in reqsk_queue_hash_req(), it seem
> that we should move "req->dl_next = lopt->syn_table[hash];" statement
> into follow write lock protected scope.
> 
>      As I browsed source code, this function only can be call at rx
> code path which is protected a spin lock over struct sock , but its
> caller (  inet_csk_reqsk_queue_hash_add() ) is a GPL exported symbol,
> so I think that we'd best move this statement into below write lock
> protected scope.
> 
>      Below is the patch to play this change, please do not apply it on
> source code, it's just for show.
> 
>     Thanks.
> 
> Yu
> 
> --- include/net/request_sock.h  2010-04-09 15:27:14.000000000 +0800
> +++ include/net/request_sock.h        2010-04-20 18:11:32.000000000 +0800
> @@ -247,9 +247,9 @@ static inline void reqsk_queue_hash_req(
>         req->expires = jiffies + timeout;
>         req->retrans = 0;
>         req->sk = NULL;
> -       req->dl_next = lopt->syn_table[hash];
> 
>         write_lock(&queue->syn_wait_lock);
> +       req->dl_next = lopt->syn_table[hash];
>         lopt->syn_table[hash] = req;
>         write_unlock(&queue->syn_wait_lock);
>  }

I believe its not really necessary, because we are the only possible
writer at this stage.

The write_lock() ... write_unlock() is there only to enforce a
synchronisation with readers.

All callers of this reqsk_queue_hash_req() must have the socket locked



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