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    Subject[PATCH v3 1/7] random: Simplify API for random address requests
    Date
    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.

    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>
    ---
    Changes from v2:
    - s/randomize_addr/randomize_page/ (Kees Cook)
    - PAGE_ALIGN(start) if it wasn't (Kees Cook, Michael Ellerman)

    drivers/char/random.c | 33 +++++++++++++++++++++++++++++++++
    include/linux/random.h | 1 +
    2 files changed, 34 insertions(+)

    diff --git a/drivers/char/random.c b/drivers/char/random.c
    index 0158d3bff7e5..61cb434e3bea 100644
    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -1840,6 +1840,39 @@ randomize_range(unsigned long start, unsigned long end, unsigned long len)
    return PAGE_ALIGN(get_random_int() % range + start);
    }

    +/**
    + * randomize_page - 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.
    + *
    + * If @start + @range would overflow, @range is capped.
    + *
    + * NOTE: Historical use of randomize_range, which this replaces, presumed that
    + * @start was already page aligned. We now align it regardless.
    + *
    + * Return: A page aligned address within [start, start + range). On error,
    + * @start is returned.
    + */
    +unsigned long
    +randomize_page(unsigned long start, unsigned long range)
    +{
    + if (!PAGE_ALIGNED(start)) {
    + range -= PAGE_ALIGN(start) - start;
    + start = PAGE_ALIGN(start);
    + }
    +
    + if (start > ULONG_MAX - range)
    + range = ULONG_MAX - start;
    +
    + range >>= PAGE_SHIFT;
    +
    + if (range == 0)
    + return start;
    +
    + return start + (get_random_long() % range << PAGE_SHIFT);
    +}
    +
    /* 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..098fec690d65 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_page(unsigned long start, unsigned long range);

    u32 prandom_u32(void);
    void prandom_bytes(void *buf, size_t nbytes);
    --
    2.9.2
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