netdev - RE: [PATCHv3 net-next 4/4] sctp: implement sender-side procedures for Add Incoming/Outgoing Streams Request Parameter

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Date:   Tue, 24 Jan 2017 12:34:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>
CC:     'Xin Long' <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        "linux-sctp@...r.kernel.org" <linux-sctp@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        Vlad Yasevich <vyasevich@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCHv3 net-next 4/4] sctp: implement sender-side procedures
 for Add Incoming/Outgoing Streams Request Parameter

From: Marcelo Ricardo Leitner
> Sent: 23 January 2017 18:48
> On Mon, Jan 23, 2017 at 11:25:56AM +0000, David Laight wrote:
> > From: Xin Long
> > > Sent: 19 January 2017 17:19
> > > This patch is to implement Sender-Side Procedures for the Add
> > > Outgoing and Incoming Streams Request Parameter described in
> > > rfc6525 section 5.1.5-5.1.6.
> > >
> > > It is also to add sockopt SCTP_ADD_STREAMS in rfc6525 section
> > > 6.3.4 for users.
> > ...
> > > +	out = params->sas_outstrms;
> > > +	in  = params->sas_instrms;
> > > +
> > > +	if (!out && !in)
> > > +		goto out;
> > > +
> > > +	if (out) {
> > > +		__u16 nums = stream->outcnt + out;
> >
> > Make nums 'unsigned int', the code will be smaller and you can
> > use the value for the overflow check.
> 
> Smaller as in to avoid the sum below?
> 
> >
> > > +		/* Check for overflow, can't use nums here */
> > > +		if (stream->outcnt + out > SCTP_MAX_STREAM)
> > > +			goto out;

No, smaller as in not requiring the compiler to add instructions
to mask (or worse sign extend) the result of the arithmetic expression
to less than the number of bits in an 'int' when the result of the
expression is to be kept in a register.

The x86 is about the only modern cpu that has 8 and 16 bit arithmetic.
For everything else you really don't want to do arithmetic on char
and short unless you really want the wrapping to happen.

	David