Use getrandom instead of /dev/urandom when available

Summary:
[Folly] Use `getrandom` instead of `/dev/urandom` when available.

The C library function `getrandom` is available on linux >= 3.17 and glibc >= 2.25.

Reviewed By: ot, simpkins, luciang

Differential Revision: D19211519

fbshipit-source-id: 07e7e856fc81d1f04efca4a6cfa0b76ada08f443

CMake/FollyConfigChecks.cmake

@@ -89,6 +89,7 @@ check_symbol_exists(pthread_atfork pthread.h FOLLY_HAVE_PTHREAD_ATFORK)
 # it fails complaining that there are multiple overloaded versions of memrchr()
 check_function_exists(memrchr FOLLY_HAVE_MEMRCHR)
 check_symbol_exists(accept4 sys/socket.h FOLLY_HAVE_ACCEPT4)
+check_symbol_exists(getrandom sys/random.h FOLLY_HAVE_GETRANDOM)
 check_symbol_exists(preadv sys/uio.h FOLLY_HAVE_PREADV)
 check_symbol_exists(pwritev sys/uio.h FOLLY_HAVE_PWRITEV)
 check_symbol_exists(clock_gettime time.h FOLLY_HAVE_CLOCK_GETTIME)

CMake/folly-config.h.cmake

@@ -44,6 +44,7 @@
 
 #cmakedefine FOLLY_HAVE_MEMRCHR 1
 #cmakedefine FOLLY_HAVE_ACCEPT4 1
+#cmakedefine01 FOLLY_HAVE_GETRANDOM
 #cmakedefine FOLLY_HAVE_PREADV 1
 #cmakedefine FOLLY_HAVE_PWRITEV 1
 #cmakedefine FOLLY_HAVE_CLOCK_GETTIME 1

folly/Random.cpp

@@ -26,6 +26,8 @@
 #include <folly/FileUtil.h>
 #include <folly/SingletonThreadLocal.h>
 #include <folly/ThreadLocal.h>
+#include <folly/detail/FileUtilDetail.h>
+#include <folly/portability/Config.h>
 #include <folly/portability/SysTime.h>
 #include <folly/portability/Unistd.h>
 #include <glog/logging.h>
@@ -34,6 +36,10 @@
 #include <wincrypt.h> // @manual
 #endif
 
+#if FOLLY_HAVE_GETRANDOM
+#include <sys/random.h>
+#endif
+
 namespace folly {
 
 namespace {
@@ -58,10 +64,24 @@ void readRandomDevice(void* data, size_t size) {
   CHECK(size <= std::numeric_limits<DWORD>::max());
   PCHECK(CryptGenRandom(cryptoProv, (DWORD)size, (BYTE*)data));
 #else
-  // Keep the random device open for the duration of the program.
-  static int randomFd = ::open("/dev/urandom", O_RDONLY | O_CLOEXEC);
-  PCHECK(randomFd >= 0);
-  auto bytesRead = readFull(randomFd, data, size);
+  ssize_t bytesRead = 0;
+  auto gen = [](int, void* buf, size_t buflen) {
+#if FOLLY_HAVE_GETRANDOM
+    auto flags = 0u;
+    return ::getrandom(buf, buflen, flags);
+#else
+    [](...) {}(buf, buflen);
+    errno = ENOSYS;
+    return -1;
+#endif
+  };
+  bytesRead = fileutil_detail::wrapFull(gen, -1, data, size);
+  if (bytesRead == -1 && errno == ENOSYS) {
+    // Keep the random device open for the duration of the program.
+    static int randomFd = ::open("/dev/urandom", O_RDONLY | O_CLOEXEC);
+    PCHECK(randomFd >= 0);
+    bytesRead = readFull(randomFd, data, size);
+  }
   PCHECK(bytesRead >= 0);
   CHECK_EQ(size_t(bytesRead), size);
 #endif