Re: [PATCH v2] tap: Drop frames if no client connected

On Fri, Sep 12, 2025 at 04:17:05PM +0800, Yumei Huang wrote:

If no client is attached, discard outgoing frames and report them as sent. 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.

There are several different approaches we can take here. Here's some reasoning as David explained:

* 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.

Suggested-by: David Gibson Signed-off-by: Yumei Huang

Reviewed-by: David Gibson

--- tap.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tap.c b/tap.c index 7ba6399..2e371b3 100644 --- a/tap.c +++ b/tap.c @@ -507,13 +507,16 @@ 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

I don't think it's worth a respin, but this might be better phrased as: "number of frames actually sent, or silently dropped"

*/ 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) + return nframes; + if (!nframes) return 0;

-- 2.47.0

-- 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