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.
Regards
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