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

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

[-- Attachment #1: Type: text/plain, Size: 2767 bytes --]

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:
> 
> > 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?

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

> 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).

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.

-- 
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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]