Re: [Qemu-devel] [PATCH 1/1] rng backend: open backend in blocking mode

[Amit Shah <amit.shah@redhat.com>]

On (Mon) 01 Apr 2013 [09:02:46], Anthony Liguori wrote:
> Amit Shah <amit.shah@redhat.com> writes:
> 
> > Opening backends in non-blocking mode isn't necessary, we don't do
> > anything while waiting for data.
> >
> > This also excuses us from checking for EAGAIN, which for the default
> > random backend, is a very common return error type.
> 
> It's not common...  It really shouldn't happen however.

EAGAIN is common when a file is opened in non-blocking mode.  Needs to
be made verbose?

> >  Starting the guest
> > with '-device virtio-rng-pci', issuing a 'cat /dev/hwrng' in the guest
> > while also doing 'cat /dev/random' on the host causes
> 
> You are essentially cat'ing the same device twice.  What's happening is
> that there is entropy available in /dev/random so a select()
> notification happens but before we are able to read() it, the cat of
> /dev/hwrng ends up consuming that entropy.
> 
> This would never happen with a socket, for instance.  /dev/random is
> special in there are multiple readers.
> 
> >
> > backends/rng-random.c:44:entropy_available: assertion failed: (len != -1)
> >
> > without this fix.
> 
> This fix would cause QEMU to block indefinitely which I don't think is
> very good behavior.  I think a better solution would be:
> 
> diff --git a/backends/rng-random.c b/backends/rng-random.c
> index acd20af..9fde566 100644
> --- a/backends/rng-random.c
> +++ b/backends/rng-random.c
> @@ -41,6 +41,9 @@ static void entropy_available(void *opaque)
>      ssize_t len;
>  
>      len = read(s->fd, buffer, s->size);
> +    if (len == -1 && errno == EINTR) {
> +        return;
> +    }

This has to be an additional fix on top of this one.  EAGAIN has to be
handled if we want to allow nonblocking reads, and there doesn't seem
to be any reason to have these reads be non-blocking.

OTOH, I also think we could use the glib functions for file IO, since
handling EINTR in each open-coded read call isn't always fun.

		Amit