On Tue, Sep 13, 2022 at 10:08:46AM +0100, Stefano Brivio wrote:On Tue, 13 Sep 2022 16:39:26 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote:Oh, right, I was mixing up msg_flags and the separate flags argument to sendmsg() and sendmmsg().On Fri, Sep 09, 2022 at 06:06:59PM +0200, Stefano Brivio wrote: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().On Fri, 9 Sep 2022 20:39:44 +1000 David Gibson <david(a)gibson.dropbear.id.au> wrote: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).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.But,Right. -- 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/~dgibsonBut 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.