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

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