rng-random: add an RNG backend that uses /dev/random (v3)

The filename can be overridden but it expects a non-blocking source of entropy.
A typical invocation would be:

qemu -object rng-random,id=rng0 -device virtio-rng-pci,rng=rng0

This can also be used with /dev/urandom by using the command line:

qemu -object rng-random,filename=/dev/urandom,id=rng0 \
     -device virtio-rng-pci,rng=rng0

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
v1 -> v2
 - merged header split patch into this one
v2 -> v3
 - bug fix in rng-random (Paolo)

backends/Makefile.objs

-common-obj-y += rng.o
+common-obj-y += rng.o rng-random.o

backends/rng-random.c

+/*
+ * QEMU Random Number Generator Backend
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Anthony Liguori   <aliguori@us.ibm.com>
+ *
+ * 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/rng-random.h"
+#include "qemu/rng.h"
+#include "qerror.h"
+#include "main-loop.h"
+
+struct RndRandom
+{
+    RngBackend parent;
+
+    int fd;
+    char *filename;
+
+    EntropyReceiveFunc *receive_func;
+    void *opaque;
+    size_t size;
+};
+
+/**
+ * A simple and incomplete backend to request entropy from /dev/random.
+ *
+ * This backend exposes an additional "filename" property that can be used to
+ * set the filename to use to open the backend.
+ */
+
+static void entropy_available(void *opaque)
+{
+    RndRandom *s = RNG_RANDOM(opaque);
+    uint8_t buffer[s->size];
+    ssize_t len;
+
+    len = read(s->fd, buffer, s->size);
+    g_assert(len != -1);
+
+    s->receive_func(s->opaque, buffer, len);
+    s->receive_func = NULL;
+
+    qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
+}
+
+static void rng_random_request_entropy(RngBackend *b, size_t size,
+                                        EntropyReceiveFunc *receive_entropy,
+                                        void *opaque)
+{
+    RndRandom *s = RNG_RANDOM(b);
+
+    if (s->receive_func) {
+        s->receive_func(s->opaque, NULL, 0);
+    }
+
+    s->receive_func = receive_entropy;
+    s->opaque = opaque;
+    s->size = size;
+
+    qemu_set_fd_handler(s->fd, entropy_available, NULL, s);
+}
+
+static void rng_random_opened(RngBackend *b, Error **errp)
+{
+    RndRandom *s = RNG_RANDOM(b);
+
+    if (s->filename == NULL) {
+        error_set(errp, QERR_INVALID_PARAMETER_VALUE,
+                  "filename", "a valid filename");
+    } else {
+        s->fd = open(s->filename, O_RDONLY | O_NONBLOCK);
+
+        if (s->fd == -1) {
+            error_set(errp, QERR_OPEN_FILE_FAILED, s->filename);
+        }
+    }
+}
+
+static char *rng_random_get_filename(Object *obj, Error **errp)
+{
+    RndRandom *s = RNG_RANDOM(obj);
+
+    if (s->filename) {
+        return g_strdup(s->filename);
+    }
+
+    return NULL;
+}
+
+static void rng_random_set_filename(Object *obj, const char *filename,
+                                 Error **errp)
+{
+    RngBackend *b = RNG_BACKEND(obj);
+    RndRandom *s = RNG_RANDOM(obj);
+
+    if (b->opened) {
+        error_set(errp, QERR_PERMISSION_DENIED);
+        return;
+    }
+
+    if (s->filename) {
+        g_free(s->filename);
+    }
+
+    s->filename = g_strdup(filename);
+}
+
+static void rng_random_init(Object *obj)
+{
+    RndRandom *s = RNG_RANDOM(obj);
+
+    object_property_add_str(obj, "filename",
+                            rng_random_get_filename,
+                            rng_random_set_filename,
+                            NULL);
+
+    s->filename = g_strdup("/dev/random");
+}
+
+static void rng_random_finalize(Object *obj)
+{
+    RndRandom *s = RNG_RANDOM(obj);
+
+    qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
+
+    if (s->fd != -1) {
+        close(s->fd);
+    }
+
+    g_free(s->filename);
+}
+
+static void rng_random_class_init(ObjectClass *klass, void *data)
+{
+    RngBackendClass *rbc = RNG_BACKEND_CLASS(klass);
+
+    rbc->request_entropy = rng_random_request_entropy;
+    rbc->opened = rng_random_opened;
+}
+
+static TypeInfo rng_random_info = {
+    .name = TYPE_RNG_RANDOM,
+    .parent = TYPE_RNG_BACKEND,
+    .instance_size = sizeof(RndRandom),
+    .class_init = rng_random_class_init,
+    .instance_init = rng_random_init,
+    .instance_finalize = rng_random_finalize,
+};
+
+static void register_types(void)
+{
+    type_register_static(&rng_random_info);
+}
+
+type_init(register_types);

include/qemu/rng-random.h

+/*
+ * QEMU Random Number Generator Backend
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Anthony Liguori   <aliguori@us.ibm.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+#ifndef QEMU_RNG_RANDOM_H
+#define QEMU_RNG_RANDOM_H
+
+#include "qemu/object.h"
+
+#define TYPE_RNG_RANDOM "rng-random"
+#define RNG_RANDOM(obj) OBJECT_CHECK(RndRandom, (obj), TYPE_RNG_RANDOM)
+
+typedef struct RndRandom RndRandom;
+
+#endif