Re: [PATCH 1/2] udp: Don't drop zero-length outbound UDP packets

[Stefano Brivio <sbrivio@redhat.com>]

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On Fri,  9 Sep 2022 14:27:13 +1000
David Gibson <david(a)gibson.dropbear.id.au> wrote:

> udp_tap_handler() currently skips outbound packets if they have a payload
> length of zero.  This is not correct, since in a datagram protocol zero
> length packets still have meaning.

Right, nice catch. As far as I can tell it's an issue I added with
commit bb708111833e ("treewide: Packet abstraction with mandatory
boundary checks").

> Adjust this to correctly forward the zero-length packets by using a msghdr
> with msg_iovlen == 0.
> 
> Bugzilla: https://bugs.passt.top/show_bug.cgi?id=19
> 
> Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
> ---
>  udp.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index c4ebecc..caa852a 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -1075,19 +1075,19 @@ int udp_tap_handler(struct ctx *c, int af, const void *addr,
>  		uh_send = packet_get(p, i, 0, sizeof(*uh), &len);
>  		if (!uh_send)
>  			return p->count;
> +
> +		mm[i].msg_hdr.msg_name = sa;
> +		mm[i].msg_hdr.msg_namelen = sl;
> +		count++;
> +
>  		if (!len)
>  			continue;
>  
>  		m[i].iov_base = (char *)(uh_send + 1);
>  		m[i].iov_len = len;

I haven't tested this yet, but:

- shouldn't iov_len be set to 0 (moving also this line before)? Note
  that I'm not initialising m

- shouldn't iov_base point to NULL to avoid noise from valgrind?

Also:

>  
> -		mm[i].msg_hdr.msg_name = sa;
> -		mm[i].msg_hdr.msg_namelen = sl;
> -
>  		mm[i].msg_hdr.msg_iov = m + i;
>  		mm[i].msg_hdr.msg_iovlen = 1;

...I guess we should still go through those even if the size is zero,
because we're appending a message. If we don't, I would expect some
subsequent messages in the batch to be dropped (as many as zero sized
packets we have).

That is, I suppose we could just drop the continue statement on if
(!len) above -- but, again, I haven't tested it.

-- 
Stefano