Re: [PATCH] virtio-rng: return available data with O_NONBLOCK

[Martin Wilck <mwilck@suse.com>]

On Wed, 2020-07-15 at 00:00 +0200, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
> 
> If a program opens /dev/hwrng with O_NONBLOCK and uses poll() and
> non-blocking read() to retrieve random data, it ends up in a tight
> loop with poll() always returning POLLIN and read() returning EAGAIN.
> This repeats forever until some process makes a blocking read() call.
> The reason is that virtio_read() always returns 0 in non-blocking
> mode,
> even if data is available.
> 
> The following test program illustrates the behavior.

...

> This can be observed in the real word e.g. with nested qemu/KVM
> virtual
> machines, if both the "outer" and "inner" VMs have a virtio-rng
> device.
> If the "inner" VM requests random data, qemu running in the "outer"
> VM
> uses this device in a non-blocking manner like the test program
> above.
> 
> Fix it by returning available data if it exists.
> 
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  drivers/char/hw_random/virtio-rng.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/char/hw_random/virtio-rng.c
> b/drivers/char/hw_random/virtio-rng.c
> index 79a6e47b5fbc..94806308d814 100644
> --- a/drivers/char/hw_random/virtio-rng.c
> +++ b/drivers/char/hw_random/virtio-rng.c
> @@ -59,6 +59,9 @@ static int virtio_read(struct hwrng *rng, void
> *buf, size_t size, bool wait)
>  	if (vi->hwrng_removed)
>  		return -ENODEV;
>  
> +	if (vi->data_avail >= size || (vi->data_avail && !wait))
> +		return vi->data_avail;
> +
>  	if (!vi->busy) {
>  		vi->busy = true;
>  		reinit_completion(&vi->have_data);

This patch was nonsense. I'm sorry. Looking into it again.

Martin