Re: [PATCH kvmtool 0/2] Fix virtio/rng handling in low entropy situations

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Wed, Apr 19, 2023 at 02:58:32PM +0100, Jean-Philippe Brucker wrote:
> On Thu, Apr 13, 2023 at 05:57:55PM +0100, Andre Przywara wrote:
> > I am not sure we now really need patch 2 anymore (originally I had this
> > one before I switched to /dev/urandom). I *think* even a read from
> > /dev/urandom can return early (because of a signal, for instance), so
> > a return with 0 bytes read seems possible.
> 
> Given that this should be very rare, maybe a simple loop would be better
> than switching the blocking mode?  It's certainly a good idea to apply the
> "MUST" requirements from virtio.

Digging a bit more, the manpage [1] is helpful:

	The O_NONBLOCK flag has no effect when opening /dev/urandom.
	When calling read(2) for the device /dev/urandom, reads of up to
	256 bytes will return as many bytes as are requested and will not
	be interrupted by a signal handler. Reads with a buffer over
	this limit may return less than the requested number of bytes or
	fail with the error EINTR, if interrupted by a signal handler.

So I guess you can also drop the O_NONBLOCK flag in patch 1. And for the
second one, maybe we could fallback to a 256 bytes read if the first one
fails

Thanks,
Jean

[1] https://man7.org/linux/man-pages/man4/urandom.4.html