From: Pablo Neira Ayuso <pablo@netfilter.org>
To: Phil Sutter <phil@nwl.cc>, netfilter-devel@vger.kernel.org
Subject: Re: [iptables PATCH] nft: Use ARRAY_SIZE() macro in nft_strerror()
Date: Thu, 24 Oct 2019 11:29:03 +0200 [thread overview]
Message-ID: <20191024092903.sqvoxwwuflk2h4cn@salvia> (raw)
In-Reply-To: <20191024084503.GF17858@orbyte.nwl.cc>
On Thu, Oct 24, 2019 at 10:45:03AM +0200, Phil Sutter wrote:
> Hi,
>
> On Wed, Oct 23, 2019 at 10:41:49PM +0200, Pablo Neira Ayuso wrote:
> > On Wed, Oct 23, 2019 at 02:16:27PM +0200, Phil Sutter wrote:
> > > Hi Pablo,
> > >
> > > On Wed, Oct 23, 2019 at 01:23:11PM +0200, Pablo Neira Ayuso wrote:
> > > > On Wed, Oct 23, 2019 at 01:20:24PM +0200, Pablo Neira Ayuso wrote:
> > > > > On Fri, Oct 18, 2019 at 05:51:14PM +0200, Phil Sutter wrote:
> > > > > > Variable 'table' is an array of type struct table_struct, so this is a
> > > > > > classical use-case for ARRAY_SIZE() macro.
> > > > > >
> > > > > > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > > > >
> > > > > Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
> > > >
> > > > BTW, probably good to add the array check?
> > > >
> > > > https://sourceforge.net/p/libhx/libhx/ci/master/tree/include/libHX/defs.h#l152
> > >
> > > Copying from kernel sources, do you think that's fine?
> > >
> > > | # ifndef ARRAY_SIZE
> > > | -# define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
> > > | +# define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:(-!!(e)); }))
> > > | +# define __same_type(a, b) \
> > > | + __builtin_types_compatible_p(typeof(a), typeof(b))
> > > | +/* &a[0] degrades to a pointer: a different type from an array */
> > > | +# define __must_be_array(a) \
> > > | + BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
> > > | +# define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x))) + __must_be_array(x)
> > > | # endif
> >
> > At quick glance I would say that's fine.
>
> While testing it, I noticed that gcc has a builtin check already:
>
> | ../include/xtables.h:640:36: warning: division 'sizeof (const uint32_t * {aka const unsigned int *}) / sizeof (uint32_t {aka const unsigned int})' does not compute the number of array elements [-Wsizeof-pointer-div]
> | 640 | # define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
> | | ^
> | nft.c:914:18: note: in expansion of macro 'ARRAY_SIZE'
> | 914 | for (i = 1; i < ARRAY_SIZE(multp); i++) {
> | | ^~~~~~~~~~
> | nft.c:906:25: note: first 'sizeof' operand was declared here
> | 906 | static const uint32_t *multp = mult;
> | | ^~~~~
>
> AFAICT, the only benefit the above brings is that it causes an error
> instead of warning. Do you think we still need it? Maybe instead enable
> -Werror? ;)
If gcc is already checking for this. Warning should be fine.
Regarding -Werror, we would at least need to keep the autogenerated C
code by bison away from it.
IIRC I enabled this in conntrack-tools long time ago, and I started
getting reports on it breaking compilation with new gcc versions that
were actually spewing new warnings. That was stopping users to install
latest, probably -Werror is too agressive?
next prev parent reply other threads:[~2019-10-24 9:29 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-18 15:51 [iptables PATCH] nft: Use ARRAY_SIZE() macro in nft_strerror() Phil Sutter
2019-10-23 11:20 ` Pablo Neira Ayuso
2019-10-23 11:23 ` Pablo Neira Ayuso
2019-10-23 12:16 ` Phil Sutter
2019-10-23 20:41 ` Pablo Neira Ayuso
2019-10-24 8:45 ` Phil Sutter
2019-10-24 9:29 ` Pablo Neira Ayuso [this message]
2019-10-24 9:51 ` Phil Sutter
2019-10-24 10:01 ` Pablo Neira Ayuso
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