Problem: reading from /dev/urandom is clunky

Solution: if available use the getrandom function as it doesn't
require any synchronization, state or cleanup

CMakeLists.txt

@@ -331,6 +331,7 @@ if (NOT CMAKE_CROSSCOMPILING)
   zmq_check_tcp_keepalive ()
   zmq_check_tcp_tipc ()
   zmq_check_pthread_setname ()
+  zmq_check_getrandom ()
 endif ()
 
 if (    CMAKE_SYSTEM_NAME MATCHES "Linux"

acinclude.m4

@@ -829,6 +829,29 @@ int main (int argc, char *argv [])
     AS_IF([test "x$libzmq_cv_tcp_keepalive" = "xyes"], [$1], [$2])
 }])
 
+dnl ################################################################################
+dnl # LIBZMQ_CHECK_GETRANDOM([action-if-found], [action-if-not-found])  #
+dnl # Checks if getrandom is supported                                  #
+dnl ################################################################################
+AC_DEFUN([LIBZMQ_CHECK_GETRANDOM], [{
+    AC_CACHE_CHECK([whether getrandom is supported], [libzmq_cv_getrandom],
+        [AC_TRY_RUN([/* thread-local storage test */
+#include <sys/random.h>
+
+int main (int argc, char *argv [])
+{
+    char buf[4];
+    getrandom(buf, 4, 0);
+}
+        ],
+        [libzmq_cv_getrandom="yes"],
+        [libzmq_cv_getrandom="no"],
+        [libzmq_cv_getrandom="not during cross-compile"]
+        )]
+    )
+    AS_IF([test "x$libzmq_cv_getrandom" = "xyes"], [$1], [$2])
+}])
+
 dnl ################################################################################
 dnl # LIBZMQ_CHECK_POLLER_KQUEUE([action-if-found], [action-if-not-found])         #
 dnl # Checks kqueue polling system                                                 #

builds/cmake/Modules/ZMQSourceRunChecks.cmake

@@ -245,3 +245,19 @@ int main(int argc, char *argv [])
     ZMQ_HAVE_PTHREAD_SET_NAME)
   set(CMAKE_REQUIRED_FLAGS ${SAVE_CMAKE_REQUIRED_FLAGS})
 endmacro()
+
+
+macro(zmq_check_getrandom)
+  message(STATUS "Checking whether getrandom is supported")
+  check_c_source_runs(
+    "
+#include <sys/random.h>
+
+int main (int argc, char *argv [])
+{
+    char buf[4];
+    getrandom(buf, 4, 0);
+}
+"
+    ZMQ_HAVE_GETRANDOM)
+endmacro()

configure.ac

@@ -699,6 +699,12 @@ LIBZMQ_CHECK_TCP_KEEPALIVE([
         [Whether TCP_KEEPALIVE is supported.])
     ])
 
+LIBZMQ_CHECK_GETRANDOM([
+    AC_DEFINE([ZMQ_HAVE_GETRANDOM],
+        [1],
+        [Whether getrandom is supported.])
+    ])
+
 AM_CONDITIONAL(HAVE_FORK, test "x$ac_cv_func_fork" = "xyes")
 
 if test "x$cross_compiling" = "xyes"; then

src/random.cpp

@@ -89,7 +89,7 @@ static zmq::mutex_t random_sync;
 void zmq::random_open (void)
 {
 #if defined (ZMQ_USE_LIBSODIUM) || \
-        (defined (ZMQ_USE_TWEETNACL) && !defined (ZMQ_HAVE_WINDOWS))
+        (defined (ZMQ_USE_TWEETNACL) && !defined (ZMQ_HAVE_WINDOWS) && !defined (ZMQ_HAVE_GETRANDOM))
     scoped_lock_t locker (random_sync);
 
     if (random_refcount == 0) {
@@ -106,7 +106,7 @@ void zmq::random_open (void)
 void zmq::random_close (void)
 {
 #if defined (ZMQ_USE_LIBSODIUM) || \
-        (defined (ZMQ_USE_TWEETNACL) && !defined (ZMQ_HAVE_WINDOWS))
+        (defined (ZMQ_USE_TWEETNACL) && !defined (ZMQ_HAVE_WINDOWS) && !defined (ZMQ_HAVE_GETRANDOM))
     scoped_lock_t locker (random_sync);
     --random_refcount;
 

src/tweetnacl.c

@@ -905,27 +905,38 @@ int sodium_init (void)
 
 #else
 
+#include <unistd.h>
+#include <assert.h>
+
+#ifdef ZMQ_HAVE_GETRANDOM
+#include <sys/random.h>
+#else
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
-#include <unistd.h>
-#include <assert.h>
 
 static int fd = -1;
+#endif
 
 void randombytes (unsigned char *x,unsigned long long xlen)
 {
     int i;
+#ifndef ZMQ_HAVE_GETRANDOM
     //  Require that random_open has already been called, to avoid
     //  race conditions.
     assert (fd != -1);
+#endif
     while (xlen > 0) {
         if (xlen < 1048576)
             i = xlen;
         else
             i = 1048576;
 
+#ifdef ZMQ_HAVE_GETRANDOM
+        i = getrandom (x, i);
+#else
         i = read(fd,x,i);
+#endif
         if (i < 1) {
             sleep (1);
             continue;
@@ -939,16 +950,19 @@ void randombytes (unsigned char *x,unsigned long long xlen)
 int randombytes_close (void)
 {
     int rc = -1;
+#ifndef ZMQ_HAVE_GETRANDOM
     if (fd != -1 && close(fd) == 0) {
         fd = -1;
         rc = 0;
     }
+#endif // ZMQ_HAVE_GETRANDOM
     return rc;
 }
 
 //  Do not call manually! Use random_open from random.hpp
 int sodium_init (void)
 {
+#ifndef ZMQ_HAVE_GETRANDOM
     if (fd == -1) {
         for (;;) {
             int flags = O_RDONLY;
@@ -965,6 +979,7 @@ int sodium_init (void)
         assert (rc != -1);
 #endif
     }
+#endif // ZMQ_HAVE_GETRANDOM
     return 0;
 }