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Date: Wed, 18 Apr 2007 10:50:42 +0200 From: Patrick McHardy <kaber@...sh.net> To: Evgeniy Polyakov <johnpol@....mipt.ru> CC: Pavel Emelianov <xemul@...ru>, David Miller <davem@...emloft.net>, Linux Netdev List <netdev@...r.kernel.org>, Andrew Morton <akpm@...l.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, devel@...nvz.org, Kirill Korotaev <dev@...nvz.org> Subject: Re: [NETLINK] Don't attach callback to a going-away netlink socket Evgeniy Polyakov wrote: > On Wed, Apr 18, 2007 at 10:26:31AM +0200, Patrick McHardy (kaber@...sh.net) wrote: > >>>Out of curiosity, why not to fix a netlink_dump_start() to remove >>>callback in error path, since in 'no-error' path it removes it in >>>netlink_dump(). >> >> >>It already does (netlink_destroy_callback), but that doesn't help >>with this race though since without this patch we don't enter the >>error path. > > > I thought that with releasing a socket, which will have a callback > attached only results in a leak of the callback? In that case we can > just free it in dump() just like it is done in no-error path already. > Or do I miss something additional? That would only work if there is nothing to dump (cb->dump returns 0). Otherwise it is not freed. >>The problem is asynchronous processing of the dump request in the >>context of a different process. Process requests a dump, message >>is queued and process returns from sendmsg since some other process >>is already processing the queue. Then the process closes the socket, >>resulting in netlink_release being called. When the dump request >>is finally processed the race Pavel described might happen. This >>can only happen for netlink families that use mutex_try_lock for >>queue processing of course. > > > Doesn't it called from ->sk_data_ready() which is synchronous with > respect to sendmsg, not sure about conntrack though, but it looks so? Yes, but for kernel sockets we end up calling the input function, which when mutex_trylock is used returns immediately when some other process is already processing the queue, so the requesting process might close the socket before the request is processed. - 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
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