linux-kernel - Re: [PATCH] kernel: sysctl: use 'unsigned long' type for 'zero' variable

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Date:	Wed, 3 Dec 2014 15:25:24 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Andrey Ryabinin <a.ryabinin@...sung.com>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Nadia.Derbey@...l.net, aquini@...hat.com, davidlohr@...com,
	Joe Perches <joe@...ches.com>, manfred@...orfullife.com,
	avagin@...nvz.org, LKML <linux-kernel@...r.kernel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Dmitry Chernenkov <dmitryc@...gle.com>,
	Andrey Konovalov <andreyknvl@...gle.com>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH] kernel: sysctl: use 'unsigned long' type for 'zero'
 variable

On Wed, 03 Dec 2014 15:41:21 +0300 Andrey Ryabinin <a.ryabinin@...sung.com> wrote:

> 
> Use the 'unsigned long' type for 'zero' variable to fix this.
> Changing type to 'unsigned long' shouldn't affect any other users
> of this variable.
> 
> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> Fixes: ed4d4902ebdd ("mm, hugetlb: remove hugetlb_zero and hugetlb_infinity")
> Signed-off-by: Andrey Ryabinin <a.ryabinin@...sung.com>
> ---
>  kernel/sysctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 15f2511..45c45c9 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -120,7 +120,7 @@ static int sixty = 60;
>  
>  static int __maybe_unused neg_one = -1;
>  
> -static int zero;
> +static unsigned long zero;
>  static int __maybe_unused one = 1;
>  static int __maybe_unused two = 2;
>  static int __maybe_unused four = 4;

Yeah, this is ghastly.

Look at 

	{
		.procname	= "numa_balancing",
		.data		= NULL, /* filled in by handler */
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= sysctl_numa_balancing,
		.extra1		= &zero,
		.extra2		= &one,
	},

Now extra1 points at a long and extra2 points at an int. 
sysctl_numa_balancing() calls proc_dointvec_minmax() and I think your
patch just broke big-endian 64-bit machines.  "sched_autogroup_enabled"
breaks as well.

These sysctl tables drove a big truck straight through the C type
system and enabled all sorts of nasty breakage.

So how do we fix it?  Maybe we could actually use the type system a bit
and do something like

union sysctl_payload {
	int i;
	unsigned long ul;
	...
};

static union zero_int = {
	.i = 0
};

static union zero_ulong = {
	.ul = 0
};

and change proc_dointvec_minmax() and a million other functions to take
`union sysctl_payload *' arguments.  But I haven't thought about it much.

Even for a minimal fix, someone should go through each and every
ctl_table and audit/fix the .extra1/.extra2 types/sizes.  And until
that's done I'm not inclined to apply anything, because at least the
current code appears to work a bit.
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