Re: What's the correct new API to request pcap_linux to not open an eventfd

[Bill Fenner via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Bill Fenner <fenner () gmail com>
>
> Date: Wed, 1 Jun 2022 11:11:52 -0400
>
> On Fri, May 20, 2022 at 6:10 PM Bill Fenner <fenner () gmail com> wrote:
>
> > On Fri, May 20, 2022 at 12:36 PM Guy Harris <gharris () sonic net> wrote:
> >
> > > If it's putting them in non-blocking mode, and using some
> > > select/poll/epoll/etc. mechanism in a single event loop, then the right
> > > name for the API is pcap_setnonblock().  There's no need for an eventfd to
> > > wake up the blocking poll() if there *is* no blocking poll(), so:
> > >
> > >         if non-blocking mode is on before pcap_activate() is called, no
> > > eventfd should be opened, and poll_breakloop_fd should be set to -1;
> > >
> > >         if non-blocking mode is turned on after pcap_activate() is
> > > called, the eventfd should be closed, and poll_breakloop_fd should be set
> > > to -1;
> > >
> > >         if non-blocking mode is turned *off* afterwards, an eventfd
> > > should be opened, and poll_breakloop_fd should be set to it;
> > >
> > >         if poll_breakloop_fd is -1, the poll() should only wait on the
> > > socket FD;
> > >
> > > so this can be handled without API changes.
> >
> > Thank you for the excellent observation, Guy.  It is indeed setting
> > non-block before pcap_activate().  I'll work on this plan.
>
> Actually, I confused myself.  It turns out that pcap_linux is buggy when
> you set non-block before pcap_activate() -- it uses the handlep->timeout
> value to remember whether or not non-block was set, but, pcap_activate()
> unconditionally overwrites handlep->timeout.  So it just comes down to, for
> now, we always open the eventfd and then can close it when non-blocking
> mode is turned on.  This just means the first item on your list is not
> done, but the last 3 are enough.
>
> (I think that the right fix for setting nonblock before activate could be
> to add a bool to the handlep, to separate the nonblock status from the
> timeout, but that can be a separate fix.)
>
>   Bill
>
> --- End Message ---
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