diff --git a/rng/async/mirage_crypto_rng_async.ml b/rng/async/mirage_crypto_rng_async.ml
index 74335f60..ef4d282d 100644
--- a/rng/async/mirage_crypto_rng_async.ml
+++ b/rng/async/mirage_crypto_rng_async.ml
@@ -34,7 +34,7 @@ let periodically_collect_getrandom_entropy time_source span =
       let idx = ref 0 in
       let f () =
         incr idx;
-        String.sub random ~pos:(per_pool * (pred !idx)) ~len:per_pool
+        Ok (String.sub random ~pos:(per_pool * (pred !idx)) ~len:per_pool)
       in
       Entropy.feed_pools None source f)
 
diff --git a/rng/eio/mirage_crypto_rng_eio.ml b/rng/eio/mirage_crypto_rng_eio.ml
index e6095713..811f5b15 100644
--- a/rng/eio/mirage_crypto_rng_eio.ml
+++ b/rng/eio/mirage_crypto_rng_eio.ml
@@ -32,7 +32,7 @@ let periodically_feed_entropy env delta source =
     let idx = ref 0 in
     let f () =
       incr idx;
-      String.sub random (per_pool * (pred !idx)) per_pool
+      Ok (String.sub random (per_pool * (pred !idx)) per_pool)
     in
     Entropy.feed_pools None source f
   in
diff --git a/rng/entropy.ml b/rng/entropy.ml
index 8fe710be..036a2c9a 100644
--- a/rng/entropy.ml
+++ b/rng/entropy.ml
@@ -30,8 +30,8 @@
 module Cpu_native = struct
 
   external cycles : unit -> int  = "mc_cycle_counter" [@@noalloc]
-  external rdseed : unit -> int  = "mc_cpu_rdseed" [@@noalloc]
-  external rdrand : unit -> int  = "mc_cpu_rdrand" [@@noalloc]
+  external rdseed : bytes -> int -> bool  = "mc_cpu_rdseed" [@@noalloc]
+  external rdrand : bytes -> int -> bool  = "mc_cpu_rdrand" [@@noalloc]
   external rng_type : unit -> int  = "mc_cpu_rng_type" [@@noalloc]
 
   let cpu_rng =
@@ -119,10 +119,9 @@ let cpu_rng_bootstrap =
   | None -> Error `Not_supported
   | Some insn ->
     let cpu_rng_bootstrap id =
-      let r = cpu_rng insn () in
-      if r = 0 then failwith "Mirage_crypto_rng.Entropy: 0 is a bad CPU RNG value";
       let buf = Bytes.create 10 in
-      Bytes.set_int64_le buf 2 (Int64.of_int r);
+      let r = cpu_rng insn buf 2 in
+      if not r then failwith "Mirage_crypto_rng.Entropy: CPU RNG broken";
       write_header id buf;
       Bytes.unsafe_to_string buf
     in
@@ -150,7 +149,11 @@ let feed_pools g source f =
   let g = match g with None -> Some (Rng.default_generator ()) | Some g -> Some g in
   let `Acc handle = Rng.accumulate g source in
   for _i = 0 to pred (Rng.pools g) do
-    handle (f ())
+    match f () with
+    | Ok data -> handle data
+    | Error `No_random_available ->
+      (* should we log a message? *)
+      ()
   done
 
 let cpu_rng =
@@ -165,8 +168,10 @@ let cpu_rng =
       in
       let f () =
         let buf = Bytes.create 8 in
-        Bytes.set_int64_le buf 0 (Int64.of_int (randomf ()));
-        Bytes.unsafe_to_string buf
+        if randomf buf 0 then
+          Ok (Bytes.unsafe_to_string buf)
+        else
+          Error `No_random_available
       in
       fun () -> feed_pools g source f
     in
diff --git a/rng/lwt/mirage_crypto_rng_lwt.ml b/rng/lwt/mirage_crypto_rng_lwt.ml
index 9730e90b..001e8460 100644
--- a/rng/lwt/mirage_crypto_rng_lwt.ml
+++ b/rng/lwt/mirage_crypto_rng_lwt.ml
@@ -19,7 +19,7 @@ let getrandom_task delta source =
     let idx = ref 0 in
     let f () =
       incr idx;
-      String.sub random (per_pool * (pred !idx)) per_pool
+      Ok (String.sub random (per_pool * (pred !idx)) per_pool)
     in
     Entropy.feed_pools None source f
   in
diff --git a/rng/miou/mirage_crypto_rng_miou_unix.ml b/rng/miou/mirage_crypto_rng_miou_unix.ml
index 82ecdbf7..b17bc593 100644
--- a/rng/miou/mirage_crypto_rng_miou_unix.ml
+++ b/rng/miou/mirage_crypto_rng_miou_unix.ml
@@ -18,7 +18,10 @@ let getrandom delta source =
     let size = per_pool * pools None in
     let random = Mirage_crypto_rng_unix.getrandom size in
     let idx = ref 0 in
-    let fn () = incr idx; String.sub random (per_pool * (pred !idx)) per_pool in
+    let fn () =
+      incr idx;
+      Ok (String.sub random (per_pool * (pred !idx)) per_pool)
+    in
     Entropy.feed_pools None source fn in
   periodic fn delta
 
diff --git a/rng/mirage_crypto_rng.mli b/rng/mirage_crypto_rng.mli
index a483331d..ee60980a 100644
--- a/rng/mirage_crypto_rng.mli
+++ b/rng/mirage_crypto_rng.mli
@@ -136,14 +136,14 @@ module Entropy : sig
 
   (** {1 Periodic pulled sources} *)
 
-  val feed_pools : g option -> source -> (unit -> string) -> unit
+  val feed_pools : g option -> source -> (unit -> (string, [ `No_random_available ]) result) -> unit
   (** [feed_pools g source f] feeds all pools of [g] using [source] by executing
       [f] for each pool. *)
 
   val cpu_rng : (g option -> unit -> unit, [`Not_supported]) Result.t
   (** [cpu_rng g] uses the CPU RNG (rdrand or rdseed) to feed all pools
       of [g]. It uses {!feed_pools} internally. If neither rdrand nor rdseed
-      are available, [fun () -> ()] is returned. *)
+      are available, [`Not_supported] is returned. *)
 
   (**/**)
   val id : source -> int
diff --git a/src/native/entropy_cpu_stubs.c b/src/native/entropy_cpu_stubs.c
index cef0c1cf..37fe794d 100644
--- a/src/native/entropy_cpu_stubs.c
+++ b/src/native/entropy_cpu_stubs.c
@@ -15,11 +15,27 @@
 #define random_t unsigned long long
 #define _rdseed_step _rdseed64_step
 #define _rdrand_step _rdrand64_step
+#define fill_bytes(buf, off, data) {                    \
+    (_bp_uint8_off(buf, off))[0] = (uint8_t)((data) >> 56);     \
+    (_bp_uint8_off(buf, off))[1] = (uint8_t)((data) >> 48);     \
+    (_bp_uint8_off(buf, off))[2] = (uint8_t)((data) >> 40);     \
+    (_bp_uint8_off(buf, off))[3] = (uint8_t)((data) >> 32);     \
+    (_bp_uint8_off(buf, off))[4] = (uint8_t)((data) >> 24);     \
+    (_bp_uint8_off(buf, off))[5] = (uint8_t)((data) >> 16);     \
+    (_bp_uint8_off(buf, off))[6] = (uint8_t)((data) >> 8);      \
+    (_bp_uint8_off(buf, off))[7] = (uint8_t)((data));           \
+  }
 
 #elif defined (__i386__)
 #define random_t unsigned int
 #define _rdseed_step _rdseed32_step
 #define _rdrand_step _rdrand32_step
+#define fill_bytes(buf, off, data) {                    \
+    (_bp_uint8_off(buf, off))[0] = (uint8_t)((data) >> 24);     \
+    (_bp_uint8_off(buf, off))[1] = (uint8_t)((data) >> 16);     \
+    (_bp_uint8_off(buf, off))[2] = (uint8_t)((data) >> 8);      \
+    (_bp_uint8_off(buf, off))[3] = (uint8_t)((data));           \
+  }
 
 #endif
 #endif /* __i386__ || __x86_64__ */
@@ -229,29 +245,31 @@ static void detect (void) {
 #endif
 }
 
-CAMLprim value mc_cpu_rdseed (value __unused(unit)) {
+CAMLprim value mc_cpu_rdseed (value buf, value off) {
 #ifdef __mc_ENTROPY__
   random_t r = 0;
   int ok = 0;
   int i = RETRIES;
   do { ok = _rdseed_step (&r); _mm_pause (); } while ( !(ok | !--i) );
-  return Val_long(r);
+  fill_bytes(buf, off, r);
+  return Val_bool (ok);
 #else
   /* ARM: CPU-assisted randomness here. */
-  return Val_long (0);
+  return Val_bool (0);
 #endif
 }
 
-CAMLprim value mc_cpu_rdrand (value __unused(unit)) {
+CAMLprim value mc_cpu_rdrand (value buf, value off) {
 #ifdef __mc_ENTROPY__
   random_t r = 0;
   int ok = 0;
   int i = RETRIES;
   do { ok = _rdrand_step (&r); } while ( !(ok | !--i) );
-  return Val_long(r);
+  fill_bytes(buf, off, r);
+  return Val_bool (ok);
 #else
   /* ARM: CPU-assisted randomness here. */
-  return Val_long (0);
+  return Val_bool (0);
 #endif
 }