linux-kernel - Re: Question on FIELD

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Date:   Thu, 11 Oct 2018 21:13:10 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     John Garry <john.garry@...wei.com>, Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        jakub.kicinski@...ronome.com, yamada.masahiro@...ionext.com,
        Arnd Bergmann <arnd@...db.de>, viro@...iv.linux.org.uk,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     linux-wireless@...r.kernel.org, nbd@....name
Subject: Re: Question on FIELD_PREP() for static array

On Thu, 2018-10-11 at 18:26 +0100, John Garry wrote:
> > > Yeah, I have a patch now to reference it, but I don't see anything from
> > > -Wvla with gcc 8.1?
> > 
> > I'm using a 7.3.1-based toolchain
> > 
> > > 
> > > See
> > > https://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git/commit/?id=3d7af878357acd9e37fc156928106f1a969c8942
> > > 
> > > and its parent.
> > > 
> > > Do you see -Wvla warnings there? Any idea how I could reproduce them?
> > 
> > I'll try it, thanks
> 
> 3d7af878357acd9e37f builds ok. However I am using 20181010-next (I'm not 
> sure what yours is based on), and I just noticed that it includes this 
> new guy:
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/Makefile?h=next-20181011&id=bc5701d8e27fd8beaf895176982fc6a97878f3b

Right, but I added -Wvla by adding subdir-ccflags-y to my Makefile for
this test, and I don't see the warning.

I tried with gcc 7.3.1 too now (Fedora 27) and it doesn't provoke the
warning, even if apply bc5701d8e27fd (manually). I did ensure with V=1
that it shows up on the compiler command line.

This is on x86-64, are you using something else?

Hmm.

However, I have another trick:

#define __NLA_ENSURE(condition) (0 * sizeof(struct { unsigned int x:1 - 2*!(condition);}))

or, in this context,

#define BUILD_BUG_ON_RET_ZERO(cond) (0 * sizeof(struct { unsigned int x:1 - 2*!(condition);}))


What do you think?

johannes