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

On Tue, Sep 13, 2022 at 10:08:46AM +0100, Stefano Brivio wrote:

On Tue, 13 Sep 2022 16:39:26 +1000 David Gibson wrote:

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On Fri, Sep 09, 2022 at 06:06:59PM +0200, Stefano Brivio wrote:

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On Fri, 9 Sep 2022 20:39:44 +1000 David Gibson wrote:

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On Fri, Sep 09, 2022 at 11:26:58AM +0200, Stefano Brivio wrote:

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On Fri, 9 Sep 2022 14:27:13 +1000 David Gibson wrote:

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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").

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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 --- 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[].

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Also:

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- 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.

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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().

Oh, right, I was mixing up msg_flags and the separate flags argument to sendmsg() and sendmmsg().

But,

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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.

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/~dgibson