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

[Daniel Cashman]

On 11/30/15 4:18 PM, Andrew Morton wrote:
> On Mon, 30 Nov 2015 16:04:36 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
> 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@xxxxxxxxxxxx>
> Cc: Daniel Cashman <dcashman@xxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> 
>  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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