netdev - Re: [PATCH next 1/2] sctp: fix the handling of SACK Gap Ack blocks

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Date:   Wed, 21 Sep 2016 08:01:30 -0300
From:   "'Marcelo Ricardo Leitner'" <marcelo.leitner@...il.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        Vlad Yasevich <vyasevich@...il.com>
Subject: Re: [PATCH next 1/2] sctp: fix the handling of SACK Gap Ack blocks

On Wed, Sep 21, 2016 at 10:21:43AM +0000, David Laight wrote:
> From: Marcelo Ricardo Leitner
> > Sent: 20 September 2016 22:19
> > sctp_acked() is using 32bit arithmetics on 16bits vars, via TSN_lte()
> > macros, which is weird and confusing.
> > 
> > Once the offset to ctsn is calculated, all wrapping is already handled
> > and thus to verify the Gap Ack blocks we can just use pure
> > less/big-or-equal than checks.
> > 
> > Also, rename gap variable to tsn_offset, so it's more meaningful, as
> > it doesn't point to any gap at all.
> > 
> > Even so, I don't think this discrepancy resulted in any practical bug.
> 
> I think it might if gab.start/end are greater than 32767

Why? Mind sharing a case that it goes wrong?

  Marcelo

> 
> ...
> > -	gap = tsn - ctsn;
> > -	for (i = 0; i < ntohs(sack->num_gap_ack_blocks); ++i) {
> > -		if (TSN_lte(ntohs(frags[i].gab.start), gap) &&
> > -		    TSN_lte(gap, ntohs(frags[i].gab.end)))
> > +	blocks = ntohs(sack->num_gap_ack_blocks);
> > +	tsn_offset = tsn - ctsn;
> > +	for (i = 0; i < blocks; ++i) {
> > +		if (tsn_offset >= ntohs(frags[i].gab.start) &&
> > +		    tsn_offset <= ntohs(frags[i].gab.end))
> ...
> 
> 	David
> 
> --
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