Re: [PATCH] divide by 3*sizeof(u32) when computing array_size

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: David Laight <David.Laight@ACULAB.COM>
Cc: "Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Salah Triki" <salah.triki@gmail.com>,
	"fabrice.gasnier@foss.st.com" <fabrice.gasnier@foss.st.com>,
	"thierry.reding@gmail.com" <thierry.reding@gmail.com>,
	"lee.jones@linaro.org" <lee.jones@linaro.org>,
	"mcoquelin.stm32@gmail.com" <mcoquelin.stm32@gmail.com>,
	"alexandre.torgue@foss.st.com" <alexandre.torgue@foss.st.com>,
	"linux-pwm@vger.kernel.org" <linux-pwm@vger.kernel.org>,
	"linux-stm32@st-md-mailman.stormreply.com"
	<linux-stm32@st-md-mailman.stormreply.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] divide by 3*sizeof(u32) when computing array_size
Date: Tue, 13 Jul 2021 12:22:53 +0100	[thread overview]
Message-ID: <20210713112253.GH22278@shell.armlinux.org.uk> (raw)
In-Reply-To: <012ccfea2a564274bd9d2e1cfc130873@AcuMS.aculab.com>

On Tue, Jul 13, 2021 at 11:07:00AM +0000, David Laight wrote:
> From: Russell King
> > Sent: 13 July 2021 10:20
> ....
> > I would also note that the code relies on there being no padding in
> > struct stm32_breakinput - it should be noted that a strict
> > interpretation of the C standard allows padding to be added anywhere
> > to a structure - at the start, end or between members.
> 
> I'm pretty certain I remember that padding before the first member
> isn't allowed.

You may be right there.

> In any case the kernel generally assumes there is no extra padding.
> (eg for structures that map hardware registers.)

That's incorrect. Places where we care either generally end up with
__packed or are carefully layed out to ensure members are naturally
aligned to reduce the likelyhood of it. 32-bit OABI ARM has been
particularly "fun" in this respect.

> For big structures it is worth adding a compile-time check of
> the structure size - but not really for three u32.

Sorry, structure size has absolutely nothing to do with whether it's
a good idea to have a compile-time check. The deciding factor is
whether the code relies on some property such as it being a certain
size. Such as in this exact case. If you grep for "BUILD_BUG_ON.*sizeof"
in fs/ for example, this illustrates the point rather well.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!