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

Jason Cooper
2016-07-26T15:55:49Z

On Mon, Jul 25, 2016 at 09:44:27PM -0700, Kees Cook wrote:
> On Mon, Jul 25, 2016 at 8:01 PM, Jason Cooper <jason@lakedaemon.net> 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)
>
> Also, this series isn't bisectable since randomize_range gets removed
> here before the callers are updated. Perhaps add a macro that calls
> randomize_addr with a BUG_ON for len != 0? (And then remove it in the
> last patch?)

No, I was thinking just add randomize_addr() in the first patch, convert
all the callers, then the last patch would remove randomize_range().

That way the last patch can be a cleanup in a later merge window if
needed.

thx,

Jason.