Re: [Qemu-devel] [PATCH v5 2/4] rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

[Laurent Vivier <lvivier@redhat.com>]

On 14/05/2019 18:27, Laurent Vivier wrote:
> Add a new RNG backend using QEMU builtin getrandom function.
> 
> It can be created and used with something like:
> 
>      ... -object rng-builtin,id=rng0 -device virtio-rng,rng=rng0 ...
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Reviewed-by: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>   backends/Makefile.objs |  2 +-
>   backends/rng-builtin.c | 54 ++++++++++++++++++++++++++++++++++++++++++
>   qemu-options.hx        |  7 ++++++
>   3 files changed, 62 insertions(+), 1 deletion(-)
>   create mode 100644 backends/rng-builtin.c
> 
> diff --git a/backends/Makefile.objs b/backends/Makefile.objs
> index 981e8e122f2c..f0691116e86e 100644
> --- a/backends/Makefile.objs
> +++ b/backends/Makefile.objs
> @@ -1,4 +1,4 @@
> -common-obj-y += rng.o rng-egd.o
> +common-obj-y += rng.o rng-egd.o rng-builtin.o
>   common-obj-$(CONFIG_POSIX) += rng-random.o
>   
>   common-obj-$(CONFIG_TPM) += tpm.o
> diff --git a/backends/rng-builtin.c b/backends/rng-builtin.c
> new file mode 100644
> index 000000000000..6b9543a7de87
> --- /dev/null
> +++ b/backends/rng-builtin.c
> @@ -0,0 +1,54 @@
> +/*
> + * QEMU Builtin Random Number Generator Backend
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "sysemu/rng.h"
> +#include "qemu/main-loop.h"
> +#include "qemu/guest-random.h"
> +
> +#define TYPE_RNG_BUILTIN "rng-builtin"
> +#define RNG_BUILTIN(obj) OBJECT_CHECK(RngBuiltin, (obj), TYPE_RNG_BUILTIN)
> +
> +typedef struct RngBuiltin {
> +    RngBackend parent;
> +} RngBuiltin;
> +
> +static void rng_builtin_request_entropy(RngBackend *b, RngRequest *req)
> +{
> +    RngBuiltin *s = RNG_BUILTIN(b);
> +
> +    while (!QSIMPLEQ_EMPTY(&s->parent.requests)) {
> +        RngRequest *req = QSIMPLEQ_FIRST(&s->parent.requests);
> +
> +        qemu_guest_getrandom_nofail(req->data, req->size);
> +
> +        req->receive_entropy(req->opaque, req->data, req->size);
> +
> +        rng_backend_finalize_request(&s->parent, req);
>

There is a problem here:

rng_backend_request_entropy() calls rng_builtin_request_entropy() and 
then add the request to the queue parent.requests, so receive_entropy() 
is called only on the next call of rng_backend_request_entropy() (and 
parameter req is unused).

Thanks,
Laurent