netdev - How do I avoid recvmsg races with IP

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Message-ID: <CALCETrW-jkMrEjidwCTraeAaHCHA_+9eExQAz2phvn=OB8RMhQ@mail.gmail.com>
Date:	Tue, 2 Jun 2015 12:40:33 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	Network Development <netdev@...r.kernel.org>
Subject: How do I avoid recvmsg races with IP_RECVERR?

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!
  }

  /* if POLLERR (or maybe unconditionally), recvmsg(..., MSG_ERRQUEUE);
}

The problem is that, if I'm screwing something up (thus causing EINVAL
or something similar), this will just spin forever.

Am I missing something here?  Would it make sense to add
MSG_IGNORE_ERROR to suppress the sock_error check or IP_RECVERR=2 to
stop setting sk_err?

Thanks,
Andy
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