Date: Mon, 30 Nov 2015 15:54:12 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Daniel Cashman <dcashman@android.com>
Cc: linux-kernel@vger.kernel.org, linux@arm.linux.org.uk,
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        dcashman@google.com
Subject: Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base
 ASLR.
Message-Id: <20151130155412.b1a087f4f6f4d4180ab4472d@linux-foundation.org>
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	<1448578785-17656-2-git-send-email-dcashman@android.com>
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On Thu, 26 Nov 2015 14:59:42 -0800 Daniel Cashman <dcashman@android.com> wrote:

> ASLR  only uses as few as 8 bits to generate the random offset for the
> mmap base address on 32 bit architectures. This value was chosen to
> prevent a poorly chosen value from dividing the address space in such
> a way as to prevent large allocations. This may not be an issue on all
> platforms. Allow the specification of a minimum number of bits so that
> platforms desiring greater ASLR protection may determine where to place
> the trade-off.
> 
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>  		.mode		= 0644,
>  		.proc_handler	= proc_doulongvec_minmax,
>  	},
> +#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.


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