Messages in this thread Patch in this message | | | From | Jason Cooper <> | Subject | [PATCH v3 1/7] random: Simplify API for random address requests | Date | Wed, 3 Aug 2016 23:39:07 +0000 |
| |
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
| |