Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[Andrew Morton <akpm@linux-foundation.org>]

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.

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
_