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Date: Thu, 18 Aug 2016 13:53:00 +0200
From: LABBE Corentin <clabbe.montjoie@gmail.com>
To: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
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Subject: Re: [PATCH] Add Ingenic JZ4780 hardware RNG driver
Message-ID: <20160818115300.GA6621@Red>
References: <1471448151-20850-1-git-send-email-prasannatsmkumar@gmail.com>
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 <CANc+2y55ZCkauwKNtuuCxLx-WOtm8z+A_EBKsYSjEUdc+ZbZTQ@mail.gmail.com>
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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