[standalone] Use fewer syscalls to get entropy on Linux and Mac.

TEST=ci
Change-Id: Ib4737145a34b1d5e4aeab4377f918745abe8e1fa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/452220
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

Author: rmacnak-google

runtime/bin/crypto_linux.cc

@@ -5,8 +5,9 @@
 #include "platform/globals.h"
 #if defined(DART_HOST_OS_LINUX) || defined(DART_HOST_OS_ANDROID)
 
-#include <errno.h>  // NOLINT
-#include <fcntl.h>  // NOLINT
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/syscall.h>
 
 #include "bin/crypto.h"
 #include "bin/fdutils.h"
@@ -15,7 +16,7 @@
 namespace dart {
 namespace bin {
 
-bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
+static bool GetRandomFromDev(intptr_t count, uint8_t* buffer) {
   ThreadSignalBlocker signal_blocker(SIGPROF);
   intptr_t fd = TEMP_FAILURE_RETRY_NO_SIGNAL_BLOCKER(
       open("/dev/urandom", O_RDONLY | O_CLOEXEC));
@@ -38,6 +39,25 @@ bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
   return true;
 }
 
+bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
+  intptr_t bytes_read = 0;
+  do {
+    ssize_t res;
+    do {
+      res = syscall(__NR_getrandom, buffer + bytes_read, count - bytes_read,
+                    /*flags=*/0);
+    } while (res == -1 && errno == EINTR);
+    if (res == -1) {
+      if (errno == ENOSYS) {
+        return GetRandomFromDev(count, buffer);
+      }
+      return false;
+    }
+    bytes_read += res;
+  } while (bytes_read < count);
+  return true;
+}
+
 }  // namespace bin
 }  // namespace dart
 

runtime/bin/crypto_macos.cc

@@ -5,8 +5,9 @@
 #include "platform/globals.h"
 #if defined(DART_HOST_OS_MACOS)
 
-#include <errno.h>  // NOLINT
-#include <fcntl.h>  // NOLINT
+#include <errno.h>
+#include <fcntl.h>
+#include <sys/random.h>
 
 #include "bin/crypto.h"
 #include "bin/fdutils.h"
@@ -16,23 +17,18 @@ namespace dart {
 namespace bin {
 
 bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
-  intptr_t fd = TEMP_FAILURE_RETRY(open("/dev/urandom", O_RDONLY | O_CLOEXEC));
-  if (fd < 0) {
-    return false;
-  }
   intptr_t bytes_read = 0;
   do {
-    int res =
-        TEMP_FAILURE_RETRY(read(fd, buffer + bytes_read, count - bytes_read));
+    intptr_t chunk_size = count - bytes_read;
+    if (chunk_size > 256) {
+      chunk_size = 256;
+    }
+    int res = getentropy(buffer + bytes_read, chunk_size);
     if (res < 0) {
-      int err = errno;
-      close(fd);
-      errno = err;
       return false;
     }
-    bytes_read += res;
+    bytes_read += chunk_size;
   } while (bytes_read < count);
-  close(fd);
   return true;
 }