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

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Date:	Mon, 30 Nov 2015 16:18:11 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Daniel Cashman <dcashman@...roid.com>,
	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 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
_

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