rng: add request queue support to rng-random

Requests are now created in the RngBackend parent class and the
code path is shared by both rng-egd and rng-random.

This commit fixes the rng-random implementation which processed
only one request at a time and simply discarded all but the most
recent one. In the guest this manifested as delayed completion
of reads from virtio-rng, i.e. a read was completed only after
another read was issued.

By switching rng-random to use the same request queue as rng-egd,
the unsafe stack-based allocation of the entropy buffer is
eliminated and replaced with g_malloc.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Message-Id: <1456994238-9585-5-git-send-email-lprosek@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>

backends/rng-egd.c

@@ -27,20 +27,10 @@ typedef struct RngEgd
     char *chr_name;
 } RngEgd;
 
-static void rng_egd_request_entropy(RngBackend *b, size_t size,
+static void rng_egd_request_entropy(RngBackend *b, RngRequest *req)
-                                    EntropyReceiveFunc *receive_entropy,
-                                    void *opaque)
 {
     RngEgd *s = RNG_EGD(b);
-    RngRequest *req;
+    size_t size = req->size;
-
-    req = g_malloc(sizeof(*req));
-
-    req->offset = 0;
-    req->size = size;
-    req->receive_entropy = receive_entropy;
-    req->opaque = opaque;
-    req->data = g_malloc(req->size);
 
     while (size > 0) {
         uint8_t header[2];
@@ -54,8 +44,6 @@ static void rng_egd_request_entropy(RngBackend *b, size_t size,
 
         size -= len;
     }
-
-    s->parent.requests = g_slist_append(s->parent.requests, req);
 }
 
 static int rng_egd_chr_can_read(void *opaque)

backends/rng-random.c

@@ -22,10 +22,6 @@ struct RndRandom
 
     int fd;
     char *filename;
-
-    EntropyReceiveFunc *receive_func;
-    void *opaque;
-    size_t size;
 };
 
 /**
@@ -38,36 +34,35 @@ struct RndRandom
 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);
+    while (s->parent.requests != NULL) {
-    if (len < 0 && errno == EAGAIN) {
+        RngRequest *req = s->parent.requests->data;
-        return;
+        ssize_t len;
+
+        len = read(s->fd, req->data, req->size);
+        if (len < 0 && errno == EAGAIN) {
+            return;
+        }
+        g_assert(len != -1);
+
+        req->receive_entropy(req->opaque, req->data, len);
+
+        rng_backend_finalize_request(&s->parent, req);
     }
-    g_assert(len != -1);
-
-    s->receive_func(s->opaque, buffer, len);
-    s->receive_func = NULL;
 
+    /* We've drained all requests, the fd handler can be reset. */
     qemu_set_fd_handler(s->fd, NULL, NULL, NULL);
 }
 
-static void rng_random_request_entropy(RngBackend *b, size_t size,
+static void rng_random_request_entropy(RngBackend *b, RngRequest *req)
-                                        EntropyReceiveFunc *receive_entropy,
-                                        void *opaque)
 {
     RndRandom *s = RNG_RANDOM(b);
 
-    if (s->receive_func) {
+    if (s->parent.requests == NULL) {
-        s->receive_func(s->opaque, NULL, 0);
+        /* If there are no pending requests yet, we need to
+         * install our fd handler. */
+        qemu_set_fd_handler(s->fd, entropy_available, NULL, s);
     }
-
-    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)

backends/rng.c

@@ -20,9 +20,20 @@ void rng_backend_request_entropy(RngBackend *s, size_t size,
                                  void *opaque)
 {
     RngBackendClass *k = RNG_BACKEND_GET_CLASS(s);
+    RngRequest *req;
 
     if (k->request_entropy) {
-        k->request_entropy(s, size, receive_entropy, opaque);
+        req = g_malloc(sizeof(*req));
+
+        req->offset = 0;
+        req->size = size;
+        req->receive_entropy = receive_entropy;
+        req->opaque = opaque;
+        req->data = g_malloc(req->size);
+
+        k->request_entropy(s, req);
+
+        s->requests = g_slist_append(s->requests, req);
     }
 }
 

include/sysemu/rng.h

@@ -45,8 +45,7 @@ struct RngBackendClass
 {
     ObjectClass parent_class;
 
-    void (*request_entropy)(RngBackend *s, size_t size,
+    void (*request_entropy)(RngBackend *s, RngRequest *req);
-                            EntropyReceiveFunc *receive_entropy, void *opaque);
 
     void (*opened)(RngBackend *s, Error **errp);
 };