Re: [RFC patch 1/6] random: Simplify API for random address requests

From: Jason Cooper <jason@lakedaemon.net>
To: william.c.roberts@intel.com, linux-mm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	kernel-hardening@lists.openwall.com
Cc: linux@arm.linux.org.uk, akpm@linux-foundation.org,
	keescook@chromium.org, tytso@mit.edu, arnd@arndb.de,
	gregkh@linuxfoundation.org, catalin.marinas@arm.com,
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	davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com,
	hpa@zytor.com, x86@kernel.org, viro@zeniv.linux.org.uk,
	nnk@google.com, jeffv@google.com, alyzyn@android.com,
	dcashman@android.com
Subject: Re: [RFC patch 1/6] random: Simplify API for random address requests
Date: Tue, 26 Jul 2016 03:30:32 +0000	[thread overview]
Message-ID: <20160726033032.GD4541@io.lakedaemon.net> (raw)
In-Reply-To: <20160726030201.6775-1-jason@lakedaemon.net>

All,

On Tue, Jul 26, 2016 at 03:01:55AM +0000, Jason Cooper wrote:
> To date, all callers of randomize_range() have set the length to 0, and
> check for a zero return value.  For the current callers, the only way
> to get zero returned is if end <= start.  Since they are all adding a
> constant to the start address, this is unnecessary.
> 
> We can remove a bunch of needless checks by simplifying the API to do
> just what everyone wants, return an address between [start, start +
> range].
> 
> While we're here, s/get_random_int/get_random_long/.  No current call
> site is adversely affected by get_random_int(), since all current range
> requests are < MAX_UINT.  However, we should match caller expectations
> to avoid coming up short (ha!) in the future.
> 
> Signed-off-by: Jason Cooper <jason@lakedaemon.net>
> ---
>  drivers/char/random.c  | 17 ++++-------------
>  include/linux/random.h |  2 +-
>  2 files changed, 5 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0158d3bff7e5..1251cb2cbab2 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1822,22 +1822,13 @@ unsigned long get_random_long(void)
>  EXPORT_SYMBOL(get_random_long);
>  
>  /*
> - * randomize_range() returns a start address such that
> - *
> - *    [...... <range> .....]
> - *  start                  end
> - *
> - * a <range> with size "len" starting at the return value is inside in the
> - * area defined by [start, end], but is otherwise randomized.
> + * randomize_addr() returns a page aligned address within [start, start +
> + * range]
>   */
>  unsigned long
> -randomize_range(unsigned long start, unsigned long end, unsigned long len)
> +randomize_addr(unsigned long start, unsigned long range)
>  {
> -	unsigned long range = end - len - start;
> -
> -	if (end <= start + len)
> -		return 0;
> -	return PAGE_ALIGN(get_random_int() % range + start);
> +	return PAGE_ALIGN(get_random_long() % range + start);
>  }

bah!  old patch file.  This should have been:

if (range == 0)
	return start;
else
	return PAGE_ALIGN(get_random_long() % range + start);

sorry,

Jason.

>  
>  /* Interface for in-kernel drivers of true hardware RNGs.
> diff --git a/include/linux/random.h b/include/linux/random.h
> index e47e533742b5..1ad877a98186 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -34,7 +34,7 @@ extern const struct file_operations random_fops, urandom_fops;
>  
>  unsigned int get_random_int(void);
>  unsigned long get_random_long(void);
> -unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
> +unsigned long randomize_addr(unsigned long start, unsigned long range);
>  
>  u32 prandom_u32(void);
>  void prandom_bytes(void *buf, size_t nbytes);
> -- 
> 2.9.2
> 