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

[Stefano Brivio <sbrivio@redhat.com>]

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On Tue, 13 Sep 2022 16:39:26 +1000
David Gibson <david(a)gibson.dropbear.id.au> wrote:

> 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).

Hmm, if we're talking about msg_flags, it should be ignored on
sendmsg(), and only used for received messages flags (MSG_EOR,
MSG_TRUNC, MSG_CTRUNC, MSG_OOB, MSG_ERRQUEUE) on recvmsg().

But,

> But in any case I'm initializing them all now and it's working.

yes, I guess it's a good idea to avoid sending the kernel random bytes
there, in any case.

-- 
Stefano