netdev - Re: [PATCH net v2] net: netfilter: Fix port selection of FTP for NF_NAT_RANGE_PROTO

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Date:   Wed, 8 Sep 2021 12:22:44 +1000
From:   Duncan Roe <duncan_roe@...usnet.com.au>
To:     Jan Engelhardt <jengelh@...i.de>
Cc:     Florian Westphal <fw@...len.de>,
        Cole Dishington <Cole.Dishington@...iedtelesis.co.nz>,
        pablo@...filter.org, kadlec@...filter.org, davem@...emloft.net,
        kuba@...nel.org, shuah@...nel.org, linux-kernel@...r.kernel.org,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org,
        Anthony Lineham <anthony.lineham@...iedtelesis.co.nz>,
        Scott Parlane <scott.parlane@...iedtelesis.co.nz>,
        Blair Steven <blair.steven@...iedtelesis.co.nz>
Subject: Re: [PATCH net v2] net: netfilter: Fix port selection of FTP for
 NF_NAT_RANGE_PROTO_SPECIFIED

On Tue, Sep 07, 2021 at 05:11:42PM +0200, Jan Engelhardt wrote:
>
> On Tuesday 2021-09-07 15:54, Florian Westphal wrote:
> >> -	/* Try to get same port: if not, try to change it. */
> >> -	for (port = ntohs(exp->saved_proto.tcp.port); port != 0; port++) {
> >> -		int ret;
> >> +	if (htons(nat->range_info.min_proto.all) == 0 ||
> >> +	    htons(nat->range_info.max_proto.all) == 0) {
> >
> >Either use if (nat->range_info.min_proto.all || ...
> >
> >or use ntohs().  I will leave it up to you if you prefer
> >ntohs(nat->range_info.min_proto.all) == 0 or
> >nat->range_info.min_proto.all == ntohs(0).
>
> If one has the option, one should always prefer to put htons/htonl on
> the side with the constant literal;
> Propagation of constants and compile-time evaluation is the target.
>
> That works for some other functions as well (e.g.
> strlen("fixedstring")).

When comparing against constant zero, why use htons/htonl at all?

Cheers ... Duncan.