linux-kernel - Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap

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Message-ID: <565CEE3D.4070008@android.com>
Date:	Mon, 30 Nov 2015 16:47:57 -0800
From:	Daniel Cashman <dcashman@...roid.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Jonathan Corbet <corbet@....net>,
	Don Zickus <dzickus@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Heinrich Schuchardt <xypron.glpk@....de>, jpoimboe@...hat.com,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	n-horiguchi@...jp.nec.com, Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Rientjes <rientjes@...gle.com>,
	Linux-MM <linux-mm@...ck.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Mark Salyzyn <salyzyn@...roid.com>,
	Jeffrey Vander Stoep <jeffv@...gle.com>,
	Nick Kralevich <nnk@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>, Hector Marco <hecmargi@....es>,
	Borislav Petkov <bp@...e.de>,
	Daniel Cashman <dcashman@...gle.com>
Subject: Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base
 ASLR.

On 11/30/15 4:18 PM, Andrew Morton wrote:
> On Mon, 30 Nov 2015 16:04:36 -0800 Kees Cook <keescook@...omium.org> wrote:
> 
>>>>> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
>>>>> +   {
>>>>> +           .procname       = "mmap_rnd_bits",
>>>>> +           .data           = &mmap_rnd_bits,
>>>>> +           .maxlen         = sizeof(mmap_rnd_bits),
>>>>> +           .mode           = 0600,
>>>>> +           .proc_handler   = proc_dointvec_minmax,
>>>>> +           .extra1         = (void *) &mmap_rnd_bits_min,
>>>>> +           .extra2         = (void *) &mmap_rnd_bits_max,
>>>>
>>>> hm, why the typecasts?  They're unneeded and are omitted everywhere(?)
>>>> else in kernel/sysctl.c.
>>>
>>> Oh.  Casting away constness.
>>>
>>> What's the thinking here?  They can change at any time so they aren't
>>> const so we shouldn't declare them to be const?
>>
>> The _min and _max values shouldn't be changing: they're decided based
>> on the various CONFIG options that calculate the valid min/maxes. Only
>> mmap_rnd_bits itself should be changing.
> 
> hmpf.
> 
> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: include/linux/sysctl.h: make ctl_table.extra1/2 const
> 
> Nothing should be altering these values.  Declare the pointed-to values to
> be const so we can actually use const values.
> 
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Daniel Cashman <dcashman@...roid.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>  include/linux/sysctl.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff -puN include/linux/sysctl.h~a include/linux/sysctl.h
> --- a/include/linux/sysctl.h~a
> +++ a/include/linux/sysctl.h
> @@ -111,8 +111,8 @@ struct ctl_table
>  	struct ctl_table *child;	/* Deprecated */
>  	proc_handler *proc_handler;	/* Callback for text formatting */
>  	struct ctl_table_poll *poll;
> -	void *extra1;
> -	void *extra2;
> +	const void *extra1;
> +	const void *extra2;
>  };
>  
>  struct ctl_node {
> diff -puN kernel/sysctl.c~a kernel/sysctl.c
> _

This looks good to me, thanks!  I hadn't gone through all of the uses of
extra1 and extra2, nor did I think the change herein was conceptually a
part of mine, so I casted-away in order to indicate intent, while
leaving the change here as a possibility if ctl_table could be altered
for all use-cases.

Thank You,
Dan

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