Re: [PATCH] tap: Drop frames if no client connected

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Sep 11, 2025 at 11:54:25AM +0200, Stefano Brivio wrote:
> On Thu, 11 Sep 2025 16:55:19 +0800
> Yumei Huang <yuhuang@redhat.com> wrote:
> 
> > If no client is attached, discard outgoing frames and report them as
> > sent. This mimics the behavior of a physical host with its network
> > cable unplugged.
> > 
> > Suggested-by: David Gibson <david@gibson.dropbear.id.au>
> > Signed-off-by: Yumei Huang <yuhuang@redhat.com>
> 
> Thanks, the fix itself obviously makes sense, but I have a few questions
> and comments:
> 
> - first off, what happens if we don't return early in tap_send_frames()?
>   Commit messages for fixes (assuming this is a fix) should always say
>   what concrete problem we had, what is going to be fixed, or if we're
>   not aware of any real issue but things are just fragile / wrong

Without this we will get an EBADF in either writev() (pasta) or
sendmsg() (passt).  That's basically harmless, but a bit ugly.
Explicitly catching this case results in behaviour that's probably a
bit clearer to debug if we hit it.

Putting that context in the commit message would be useful.

> - until a while ago, this couldn't happen at all. We were just blocking
>   the whole execution as long as the tap / guest / container interface
>   wasn't up and running.
> 
>   I wonder when this changed and if it makes sense to go back to the
>   previous behaviour. I had just a quick look and I wonder if I
>   accidentally broke this in c9b241346569 ("conf, passt, tap: Open
>   socket and PID files before switching UID/GID").
> 
>   Before that, main() would call tap_sock_init(), which would call
>   tap_sock_unix_open(), a blocking function.
> 
>   Should we make the whole thing blocking again? If not, is there
>   anything else that's breaking with that? Timers, other inputs, etc.

I don't think we can quite do that.  I'm not sure if it's the only
reason, but for vhost-user I believe we need the epoll loop up and
running before we have the tap connection fully set up, because we
need it to process the vhost-user control messages.  Laurent, can you
verify?

There are several different approaches we can take here.  I discussed
some with Yumei and suggested she take this one.  Here's some
reasoning (maybe this would also be useful in the commit message,
though it's rather bulky)

# Don't listen() until the tap connection is ready

 - It's not clear that the host rejecting the connection is better
   than the host accepting, then the connection stalling until the 
   guest is ready.
 - Would require substantial rework because we currently listen() as
   we parse the command line and don't store the information we'd need
   to do it later.

# Don't accept() until the tap connection is ready

 - To the peer, will behave basically the same as this patch - the
   host will complete the TCP handshake, then the connection will stall
   until the guest is ready.
 - More work to implement, because essentially every sock-side handler
   has to check fd_tap and abort early

# Drop packets in tap_send_frames(), but return 0

 - To the peer, would behave basically the same
 - Would make the TCP code do a bunch of busy work attempting to
   resend, probably to no avail
 - Handling of errors returned by tap_send_frames() is on the basis
   that it's probably a transient fault (buffer full) and we want to
   resend very soon.  That approach doesn't make sense for a missing
   guest.

>   I didn't really have time to investigate until now, I can try to
>   have another look soon though, unless you find out more meanwhile.
> 
> > ---
> >  tap.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tap.c b/tap.c
> > index 7ba6399..e01219d 100644
> > --- a/tap.c
> > +++ b/tap.c
> > @@ -507,13 +507,17 @@ static size_t tap_send_frames_passt(const struct ctx *c,
> >   * @iov must have total length @bufs_per_frame * @nframes, with each set of
> >   * @bufs_per_frame contiguous buffers representing a single frame.
> >   *
> > - * Return: number of frames actually sent
> > + * Return: number of frames actually sent, or accounted as sent
> >   */
> >  size_t tap_send_frames(const struct ctx *c, const struct iovec *iov,
> >  		       size_t bufs_per_frame, size_t nframes)
> >  {
> >  	size_t m;
> >  
> > +	if (c->fd_tap == -1)
> > +		/* If no client connected, account the frames have been sent */
> 
> I think the comment is redundant because, well, if c->fd_tap is -1
> (obvious, documented), we return 'nframes' (also documented).
> 
> If it's not redundant, for any reason, "to account" in this sense
> isn't transitive. You could say: "consider that the frames have been
> sent" but not "account that the frames have been sent".
> 
> You can pick a different meaning of "to account" and say "account the
> frames as sent", though.

It's an amusing truth of the passt project that you'll get more
English usage notes from the Italian living in Germany than the native
English speaker living in an English speaking country :).

Fwiw, I agree that the comment can probably just be dropped.  If kept,
I'd suggest:
	If no client is connected, silently drop the frames

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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