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    SubjectRe: [PATCH 1/7] random: Simplify API for random address requests
    Hi,

    Le jeudi 28 juillet 2016 à 20:47 +0000, Jason Cooper a écrit :
    > 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 < UINT_MAX.  However, we should match caller
    > expectations to avoid coming up short (ha!) in the future.
    >
    > Address generation within [start, start + range) behavior is
    > preserved.
    >
    > All current callers to randomize_range() chose to use the start
    > address if randomize_range() failed.  Therefore, we simplify things
    > by just returning the start address on error.
    >
    > randomize_range() will be removed once all callers have been
    > converted over to randomize_addr().
    >
    > Signed-off-by: Jason Cooper <jason@lakedaemon.net>
    > ---
    >  drivers/char/random.c  | 26 ++++++++++++++++++++++++++
    >  include/linux/random.h |  1 +
    >  2 files changed, 27 insertions(+)
    >
    > diff --git a/drivers/char/random.c b/drivers/char/random.c
    > index 0158d3bff7e5..3610774bcc53 100644
    > --- a/drivers/char/random.c
    > +++ b/drivers/char/random.c
    > @@ -1840,6 +1840,32 @@ randomize_range(unsigned long start, unsigned
    > long end, unsigned long len)
    >   return PAGE_ALIGN(get_random_int() % range + start);
    >  }
    >  
    > +/**
    > + * randomize_addr - Generate a random, page aligned address
    > + * @start: The smallest acceptable address the caller will take.
    > + * @range: The size of the area, starting at @start, within which the
    > + * random address must fall.
    > + *
    > + * Before page alignment, the random address generated can be any value from
    > + * @start, to @start + @range - 1 inclusive.
    > + *
    > + * If @start + @range would overflow, @range is capped.
    > + *
    > + * Return: A page aligned address within [start, start + range).

    PAGE_ALIGN(start + range - 1) can be greater than start + range ..

    In the worst case, when start = 0, range = ULONG_MAX, the result would
    be 0.

    In order to stay in the bounds, the start address must be rounded up,
    and the random offset must be rounded down.

    Something I haven't found the time to send was looking like this:

      unsigned long base = PAGE_ALIGN(start);

      range -= (base - start);
      range >>= PAGE_SHIFT;
      return base + ((get_random_int() % range) << PAGE_SHIFT);


    >   On error,
    > + * @start is returned.
    > + */
    > +unsigned long
    > +randomize_addr(unsigned long start, unsigned long range)
    > +{
    > + if (range == 0)
    > + return start;
    > +
    > + if (start > ULONG_MAX - range)
    > + range = ULONG_MAX - start;
    > +
    > + return PAGE_ALIGN(get_random_long() % range + start);
    > +}
    > +
    >  /* Interface for in-kernel drivers of true hardware RNGs.
    >   * Those devices may produce endless random bits and will be throttled
    >   * when our pool is full.
    > diff --git a/include/linux/random.h b/include/linux/random.h
    > index e47e533742b5..f1ca2fa4c071 100644
    > --- a/include/linux/random.h
    > +++ b/include/linux/random.h
    > @@ -35,6 +35,7 @@ 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);


    Regards.

    --
    Yann Droneaud
    OPTEYA

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