linux-kernel - Re: [PATCH] irqchip/apple-aic: application of sizeof() to a pointer

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Message-ID: <87pmmuyyev.wl-maz@kernel.org>
Date:   Thu, 10 Mar 2022 09:00:56 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Guo Zhengkui <guozhengkui@...o.com>
Cc:     Hector Martin <marcan@...can.st>, Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Thomas Gleixner <tglx@...utronix.de>,
        "moderated list:ARM/APPLE MACHINE SUPPORT" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:IRQCHIP DRIVERS" <linux-kernel@...r.kernel.org>,
        "zhengkui_guo@...look.com" <zhengkui_guo@...look.com>
Subject: Re: [PATCH] irqchip/apple-aic: application of sizeof() to a pointer

On Thu, 10 Mar 2022 08:48:07 +0000,
Guo Zhengkui <guozhengkui@...o.com> wrote:
> 
> On 2022/3/10 16:27, Marc Zyngier wrote:
> > On Thu, 10 Mar 2022 05:02:38 +0000,
> > Guo Zhengkui <guozhengkui@...o.com> wrote:
> >> 
> >> `ic->fiq_aff[fiq]` is a pointer to the unnamed struct.
> >> `sizeof(ic->fiq_aff[fiq])` intends to get the size of this pointer. But
> >> readers may get confused. `sizeof(unsigned long)` makes more sense because
> >> `unsigned long` has the same size of pointer.
> > 
> > More sense? It really depends on who reads the code.
> > 
> >> 
> >> reference:
> >> https://lore.kernel.org/all/Ya%2FeUbdN1+ABFVWf@rowland.harvard.edu/
> >> https://lore.kernel.org/all/YbBGGI9wQenI4kP7@kroah.com/
> >> https://lore.kernel.org/all/20211209062441.9856-1-guozhengkui@vivo.com/
> >> 
> >> Signed-off-by: Guo Zhengkui <guozhengkui@...o.com>
> >> ---
> >>   drivers/irqchip/irq-apple-aic.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >> 
> >> diff --git a/drivers/irqchip/irq-apple-aic.c b/drivers/irqchip/irq-apple-aic.c
> >> index b40199c6625e..23098b469b1a 100644
> >> --- a/drivers/irqchip/irq-apple-aic.c
> >> +++ b/drivers/irqchip/irq-apple-aic.c
> >> @@ -810,7 +810,7 @@ static void build_fiq_affinity(struct aic_irq_chip *ic, struct device_node *aff)
> >>   	if (WARN_ON(n < 0))
> >>   		return;
> >>   -	ic->fiq_aff[fiq] = kzalloc(sizeof(ic->fiq_aff[fiq]),
> >> GFP_KERNEL);
> >> +	ic->fiq_aff[fiq] = kzalloc(sizeof(unsigned long), GFP_KERNEL);
> > 
> > And by doing that, you are making even more difficult to spot the
> > glaring bug that is in front of your eyes: we're not trying to
> > allocate a pointer, but to allocate the full data structure.
> > 
> 
> Oh, I surely made a big mistake...

Big mistake? No. You patch didn't change the behaviour of the code.
But in the future, you want to pay more attention to what the author
is trying to achieve: how is the data used, for example. Blindly
generalising an advice given out of context often leads to bad
patches.

> 
> > Nothing went wrong as a 64bit allocation is plenty when you need at
> > most 10 bits, but jeez, what a howler. I'm stashing the fixlet below
> > on top.
> > 
> 
> So, will you send this new patch by yourself?

I've directly applied it[1].

Thanks,

	M.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/commit/?h=irq/irqchip-next&id=dc29812dbc873ae00bf19a4b8661984d7cce7a2e

-- 
Without deviation from the norm, progress is not possible.