From: ebiederm@xmission.com (Eric W. Biederman)
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>, Daniel Cashman <dcashman@android.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@kernel.org>,
        "linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        Jonathan Corbet <corbet@lwn.net>, Don Zickus <dzickus@redhat.com>,
        Heinrich Schuchardt <xypron.glpk@gmx.de>, jpoimboe@redhat.com,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        n-horiguchi@ah.jp.nec.com, Andrea Arcangeli <aarcange@redhat.com>,
        Mel Gorman <mgorman@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        David Rientjes <rientjes@google.com>, Linux-MM <linux-mm@kvack.org>,
        "linux-doc\@vger.kernel.org" <linux-doc@vger.kernel.org>,
        Mark Salyzyn <salyzyn@android.com>,
        Jeffrey Vander Stoep <jeffv@google.com>,
        Nick Kralevich <nnk@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, "H. Peter Anvin" <hpa@zytor.com>,
        "x86\@kernel.org" <x86@kernel.org>, Hector Marco <hecmargi@upv.es>,
        Borislav Petkov <bp@suse.de>, Daniel Cashman <dcashman@google.com>
References: <1448578785-17656-1-git-send-email-dcashman@android.com>
	<1448578785-17656-2-git-send-email-dcashman@android.com>
	<20151130155412.b1a087f4f6f4d4180ab4472d@linux-foundation.org>
	<20151130160118.e43a2e53a59e347a95a94d5c@linux-foundation.org>
	<CAGXu5jK7UzjBxXKQajxhLv-uLk_xQXR_FHOsmW6RLJNeK_-dZg@mail.gmail.com>
	<20151130161811.592c205d8dc7b00f44066a37@linux-foundation.org>
Date: Mon, 30 Nov 2015 18:55:23 -0600
In-Reply-To: <20151130161811.592c205d8dc7b00f44066a37@linux-foundation.org>
	(Andrew Morton's message of "Mon, 30 Nov 2015 16:18:11 -0800")
Message-ID: <87610jugw4.fsf@x220.int.ebiederm.org>
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Subject: Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.
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Andrew Morton <akpm@linux-foundation.org> writes:

> On Mon, 30 Nov 2015 16:04:36 -0800 Kees Cook <keescook@chromium.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@linux-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.

No large objects except we do seem to have values that are stashed
in extra1 that are cast to non-const types.

Any chance you will do the work to hunt all of those down and modify
the casts to preserve const or to remove the casts entirely?

Eric


> Cc: Kees Cook <keescook@chromium.org>
> Cc: Daniel Cashman <dcashman@android.com>
> Signed-off-by: Andrew Morton <akpm@linux-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
> _
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