Re: [PATCH 3/4] tcp: Don't consider FIN flags with mismatching sequence

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

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On Tue, Oct 07, 2025 at 12:32:54AM +0200, Stefano Brivio wrote:
> On Fri, 3 Oct 2025 13:43:32 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Thu, Oct 02, 2025 at 01:51:04PM +0200, Stefano Brivio wrote:
> > > On Thu, 2 Oct 2025 13:02:09 +1000
> > > David Gibson <david@gibson.dropbear.id.au> wrote:  
> > > > On Thu, Oct 02, 2025 at 12:52:31PM +1000, David Gibson wrote:  
> > > > > On Thu, Oct 02, 2025 at 02:06:45AM +0200, Stefano Brivio wrote:  
> > [snip]
> > > > > > diff --git a/tcp.c b/tcp.c
> > > > > > index 3f7dc82..5a7a607 100644
> > > > > > --- a/tcp.c
> > > > > > +++ b/tcp.c
> > > > > > @@ -1769,7 +1769,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> > > > > >  			}
> > > > > >  		}
> > > > > >  
> > > > > > -		if (th->fin)
> > > > > > +		if (th->fin && seq == seq_from_tap)
> > > > > >  			fin = 1;    
> > > > > 
> > > > > Can a FIN segment also contain data?  My quick googling suggests yes.  
> > > 
> > > Yes, absolutely, my slow wiresharking over the years also confirms, and
> > > it's so often the case that (I think) this issue doesn't happen so
> > > frequently as it only occurs if we have a FIN segment without data.  
> > 
> > Makes sense.
> > 
> > > If we have a data segment, with FIN set, that we can't fully transmit,
> > > we already set 'partial_send' and won't set TAP_FIN_RCVD as a result.
> > > 
> > > Another case where we want to ignore a FIN segment with data is if we
> > > have a gap before it, but in that case we'll eventually set 'keep' and
> > > return early.  
> > 
> > Ah, right.  I'd noticed we set fin = 1 in that case, but forgotten
> > about the exit before setting TAP_FIN_RCVD if keep is set.
> > 
> > > > > If so, doesn't this logic need to go after we process the data
> > > > > processing, so that seq_from_tap points to the end of the packet's
> > > > > data, rather than the beginning?  (And the handling of zero-length
> > > > > packets would also need revision to match).    
> > > 
> > > This made sense to me for a moment but now I'm struggling to understand
> > > or remember why. What I want to check here is that a FIN segment
> > > without data (I should have specified in the commit message) is
> > > acceptable because its sequence is as expected.  
> > 
> > Right.  This is correct for zero-data FIN segments, but I think as a
> > side-effect you've made it ignore certain FIN segments _with_ data.
> > It will work in the common case where the data exactly follows on from
> > what we already have.  But in the case where the segment has some data
> > we already have and some new data, the fin = 1 won't trip because seq
> > != seq_from_tap.  There isn't another place that will catch it
> > instead, AFAICT.
> > 
> > I guess it will be fine in the end, because with all the data acked,
> > the guest should retransmit the FIN with no data.
> > 
> > > But going back to FIN segments with data: why should we sum the length
> > > to seq_from_tap before comparing the sequence? I don't understand what
> > > additional check you want to introduce, or what case you want to cover.  
> > 
> > I was thinking about the case above, but I didn't explain it very
> > well.
> > 
> > > > Following on from that, it seems to me like it would make sense for
> > > > FIN segments to also participate in the 'keep' mechanism.  It should
> > > > work eventually, but I expect it would be smoother in the case that we
> > > > get a final burst of packets in a stream out of order.  
> > > 
> > > FIN segments with data already go through that dance.
> > > 
> > > Without data, I guess you're right, we might have in the same batch
> > > (not that I've ever seen it happening in practice) a FIN segment
> > > without data that we process first (and now discard because of the
> > > sequence number), and some data before that we process later, so we
> > > shouldn't throw away the FIN segment because of that. We should,
> > > conceptually, reorder it as well.
> > > 
> > > It probably makes things more complicated for a reason that's not so
> > > critical (ignoring a FIN is fine, we'll get another one), and I wanted
> > > to have the simplest possible fix here.
> > > 
> > > Let me see if I can make this entirely correct without a substantially
> > > bigger change, I haven't really tried.  
> > 
> > How about this:
> > 
> > diff --git a/tcp.c b/tcp.c
> > index 7da41797..42e576b4 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -1774,10 +1774,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> >  			}
> >  		}
> >  
> > -		if (th->fin)
> > -			fin = 1;
> > -
> > -		if (!len)
> > +		if (!len && !th->fin)
> >  			continue;
> >  
> >  		seq_offset = seq_from_tap - seq;
> > @@ -1820,6 +1817,8 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
> >  			break;
> >  		seq_from_tap += size;
> >  		iov_i += count;
> > +		if (th->fin)
> > +			fin = 1;
> >  
> >  		if (keep == i)
> >  			keep = -1;
> > 
> > 
> > We'd need to double check that the "accept data segment" path is safe
> > with len == 0, of course.
> 
> For sure it's not before d2c33f45f7be ("tcp: Convert
> tcp_data_from_tap() to use iov_tail"), because we might add
> zero-length segments to the tcp_iov array, and that would make
> backporting an otherwise simple and critical fix to slightly older
> versions rather complicated.

Kinda.  It's not that complicated to deal with that case, by wrapping
the actual data processing in an `if (len) { ... }`

> After that commit, I'm not sure about the behaviour of
> iov_tail_clone(). I think it will return 0, but it should be tested
> (that assumption, itself, but also that the fix still works).

Right, I think it's safe in that case - that's what I was looking at.

> > But I think that will treat dataless and
> > with-data FINs the same way, and let them use the keep mechanism.
> 
> Given that the only advantage of doing this would be to handle a rare
> (I guess) corner case, that is, an out-of-sequence FIN segment with
> data, which is not critical anyway because the FIN will be
> retransmitted, I would rather keep this (critical) fix as it is.
> 
> I would suggest filing a separate ticket or anyway sending a separate
> patch if you want to fix that other case.

Fair enough.  I'll wait for you to get the basic fix merge, then I
might batch this cleanup/fixup with the reorg to clarify bytes_acked
handling.

-- 
David Gibson (he or they)	| 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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