From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Gibson <david@gibson.dropbear.id.au>
To: passt-dev@passt.top
Subject: Re: [PATCH 1/2] udp: Don't drop zero-length outbound UDP packets
Date: Fri, 09 Sep 2022 20:39:44 +1000
Message-ID: <YxsX8IFvOePGi/xE@yekko>
In-Reply-To: <20220909112658.5dccf03a@elisabeth>
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On Fri, Sep 09, 2022 at 11:26:58AM +0200, Stefano Brivio wrote:
> On Fri,  9 Sep 2022 14:27:13 +1000
> David Gibson <david(a)gibson.dropbear.id.au> wrote:
>=20
> > 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.
>=20
> 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").
>=20
> > Adjust this to correctly forward the zero-length packets by using a msghdr
> > with msg_iovlen =3D=3D 0.
> >=20
> > Bugzilla: https://bugs.passt.top/show_bug.cgi?id=3D19
> >=20
> > Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
> > ---
> >  udp.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >=20
> > 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 =3D packet_get(p, i, 0, sizeof(*uh), &len);
> >  		if (!uh_send)
> >  			return p->count;
> > +
> > +		mm[i].msg_hdr.msg_name =3D sa;
> > +		mm[i].msg_hdr.msg_namelen =3D sl;
> > +		count++;
> > +
> >  		if (!len)
> >  			continue;
> > =20
> >  		m[i].iov_base =3D (char *)(uh_send + 1);
> >  		m[i].iov_len =3D len;
>=20
> I haven't tested this yet, but:
>=20
> - shouldn't iov_len be set to 0 (moving also this line before)? Note
>   that I'm not initialising m
>=20
> - shouldn't iov_base point to NULL to avoid noise from valgrind?

No, because with this change m[i] is entirely unreferenced by mm[].

> Also:
>=20
> > =20
> > -		mm[i].msg_hdr.msg_name =3D sa;
> > -		mm[i].msg_hdr.msg_namelen =3D sl;
> > -
> >  		mm[i].msg_hdr.msg_iov =3D m + i;
> >  		mm[i].msg_hdr.msg_iovlen =3D 1;
>=20
> ...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).

Here I'm relying on the fact that mm[] (unlike m[]) *is* initialized,
so if we don't alter it here, msg_iov is NULL and msg_iovlen is 0.

I was looking at removing that initialization, but I haven't gotten
that working yet.

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

My first version actually did that, so it also works, but I think
setting msg_iovlen to 0 is a bit neater.

--=20
David Gibson			| 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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