Messages in this thread Patch in this message | | | From | Yann Droneaud <> | Date | Fri, 29 Jul 2016 10:59:14 +0200 | Subject | Re: [PATCH 1/7] random: Simplify API for random address requests |
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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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