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

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

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On Fri, Sep 09, 2022 at 06:06:59PM +0200, Stefano Brivio wrote:
> On Fri, 9 Sep 2022 20:39:44 +1000
> David Gibson <david(a)gibson.dropbear.id.au> wrote:
> 
> > 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.
> 
> Oops, I see now.
> 
> So, I suppose that if you want to drop that initialisation, you might
> need to zero msg_hdr.controllen as well.

Duh.  I completely failed to consider the other fields.  I actually
suspect msg_hdr.flags is the most vital one (without flags I don't
know if it will examine control or controllen).  But in any case I'm
initializing them all now and it's working.

> And msg_hdr.control too: other than keeping valgrind happy, not leaking
> random stuff to the kernel might make this marginally more secure.
> 
> That should be better than the huge memset() at the beginning, because
> we're already writing to msg_iovlen anyway.
> 
> If you already tried that, though, I don't have any other quick idea.
> 
> By the way, I had a mechanism in place, just for TCP though, to avoid
> reassigning those pointers and also length descriptors.
> 
> I got rid of it in commit 38fbfdbcb95d ("tcp: Get rid of iov with
> cached MSS, drop sendmmsg(), add deferred flush") because it didn't
> really help with throughput. I don't see any significant "userspace"
> overhead on guest-to-host TCP paths with perf(1).
> 
> ...maybe for UDP that's different, I haven't focused that much on UDP
> performance.
> 
> > > 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.
> 
> Right. Maybe it was just me being thick, or perhaps that could use a
> comment:
> 
> 		/* Zero-length packet: don't use any buffer, msg_iovlen is 0 */
> 		if (!len)
> 			continue;
> 

-- 
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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