On 18/08/16 12:53, LABBE Corentin wrote:
On Thu, Aug 18, 2016 at 10:44:18AM +0530, PrasannaKumar Muralidharan wrote:
+static int jz4780_rng_read(struct hwrng *rng, void *buf, size_t max, bool wait)
+{
+ struct jz4780_rng *jz4780_rng = container_of(rng, struct jz4780_rng,
+ rng);
+ u32 *data = buf;
+ *data = jz4780_rng_readl(jz4780_rng, REG_RNG_DATA);
+ return 4;
+}
If max is less than 4, its bad
Data will be 4 bytes.
No, according to comment in include/linux/hw_random.h "drivers can fill up to max bytes of data"
So you cannot write more than max bytes without risking buffer overflow.
And if max > 4, hwrng client need to recall your read function.
The better example I found is tpm_get_random() in drivers/char/tpm/tpm-interface.c for handling both problem.
Right now the core code will never actually ask a RNG driver for <4
bytes so perhaps it would be better to update the comment in
include/linux/hw_random.h !
For devices with 32-bit RNG registers the extra code to handle a special
case that doesn't actually exist is a waste.
There are 14 drivers in drivers/char/hw_random that support the ->read()
interface but only three of these actually support max == 1 (existing
accepted behavior varies between return 0, return 2, return 4 and return
-EIO).
Daniel.
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