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

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Date:	Wed, 03 Jun 2015 02:33:11 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: How do I avoid recvmsg races with IP_RECVERR?

On Wed, Jun 3, 2015, at 02:03, Andy Lutomirski wrote:
> On Tue, Jun 2, 2015 at 2:50 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> >> My proposal would be to make the error conversion lazy:
> >>
> >> Keeping duplicate data is not a good idea in general: So we shouldn't
> >> use sk->sk_err if IP_RECVERR is set at all but let sock_error just use
> >> the sk_error_queue and extract the error code from there.
> >>
> >> Only if IP_RECVERR was not set, we use sk->sk_err logic.
> >>
> >> What do you think?
> >
> > I just noticed that this will probably break existing user space
> > applications which require that icmp errors are transient even with
> > IP_RECVERR. We can mark that with a bit in the sk_error_queue pointer
> > and xchg the pointer, hmmm....
> 
> Do you mean to fix the race like this but to otherwise leave the
> semantics
> alone?  That would be an improvement, but it might be nice to also add
> a non-crappy API for this, too.

Yes, keep current semantics but fix the race you reported.

I currently don't have good proposals for a decent API to handle this
besides adding some ancillary cmsg data to msg_control. This still would
not solve the problem fundamentally, as a -EFAULT/-EINVAL return value
could also mean that msg_control should not be touched, thus we end up
again relying on errno checking. :/ Thus checking error queue after
receiving an error indications is my best hunch so far.

Your proposal with MSG_IGNORE_ERROR seems reasonable so far for ping or
udp, but I haven't fully grasped the TCP semantics of sk->sk_err, yet.

Bye,
Hannes
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