netdev - Re: How do I avoid recvmsg races with IP

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Date:	Tue, 02 Jun 2015 23:17:02 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Andy Lutomirski <luto@...nel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: How do I avoid recvmsg races with IP_RECVERR?

On Tue, Jun 2, 2015, at 21:40, Andy Lutomirski wrote:
> As far as I can tell, enabling IP_RECVERR causes the presence of a
> queued error to cause recvmsg, etc to return an error (once).  It's
> worse, though: a new error can be queued asynchronously at any time,
> this setting sk_err to a nonzero value.  How do I sensibly distinguish
> recvmsg failures to to genuine errors receiving messages from recvmsg
> failures because there's a queued error?
> 
> The only way I can see to get reliable error handling is to literally
> call recvmsg in a loop:
> 
> while (true /* or while POLLIN is set */) {
>   int ret = recvmsg(..., MSG_ERRQUEUE not set);
>   if (ret < 0 && /* what goes here? */) {
>     whoops!  this might be a harmless asynchronous error!
>     take no action!
>   }

I see either two possibilities:

We export the icmp_err_convert tables along with the udp_lib_err error
conversions to user space and spice them up with flags to mark if they
are transient (icmp_err_convert already has a fatal flag).

Otherwise you should be able to call recvmsg with MSG_ERRQUEUE set after
you got a ret < 0 when calling without MSG_ERRQUEUE and inspect the
sock_extended_err, no?

Bye,
Hannes
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