Use getrandom() if available in json_c_get_random_seed

Lower overhead than opening & reading from /dev/urandom, and works
in chroots and other situtations where /dev/urandom is not available.
Falls back to existing methods when kernel doesn't support the syscall.

CMakeLists.txt

@@ -144,6 +144,7 @@ check_include_file(stdint.h         HAVE_STDINT_H)
 check_include_file(stdlib.h         HAVE_STDLIB_H)
 check_include_file(sys/cdefs.h      HAVE_SYS_CDEFS_H)
 check_include_file(sys/param.h      HAVE_SYS_PARAM_H)
+check_include_file(sys/random.h     HAVE_SYS_RANDOM_H)
 check_include_file(sys/stat.h       HAVE_SYS_STAT_H)
 check_include_file(xlocale.h        HAVE_XLOCALE_H)
 
@@ -190,6 +191,9 @@ endif()
 if (HAVE_SYSLOG_H)
     check_symbol_exists(vsyslog     "syslog.h" HAVE_VSYSLOG)
 endif()
+if (HAVE_SYS_RANDOM_H)
+    check_symbol_exists(getrandom   "sys/random.h" HAVE_GETRANDOM)
+endif()
 if (HAVE_SYS_RESOURCE_H)
     check_symbol_exists(getrusage   "sys/resource.h" HAVE_GETRUSAGE)
 endif()

cmake/config.h.in

@@ -56,6 +56,9 @@
 /* Define to 1 if you have the <sys/param.h> header file. */
 #cmakedefine HAVE_SYS_PARAM_H @HAVE_SYS_PARAM_H@
 
+/* Define to 1 if you have the <sys/random.h> header file. */
+#cmakedefine HAVE_SYS_RANDOM_H
+
 /* Define to 1 if you have the <sys/resource.h> header file. */
 #cmakedefine HAVE_SYS_RESOURCE_H
 
@@ -140,6 +143,9 @@
 /* Define to 1 if you have the `vsyslog' function. */
 #cmakedefine HAVE_VSYSLOG @HAVE_VSYSLOG@
 
+/* Define if you have the `getrandom' function. */
+#cmakedefine HAVE_GETRANDOM
+
 /* Define if you have the `getrusage' function. */
 #cmakedefine HAVE_GETRUSAGE
 

random_seed.c

@@ -155,6 +155,40 @@ retry:
 
 #endif /* defined ENABLE_RDRAND */
 
+#ifdef HAVE_GETRANDOM
+
+#include <stdlib.h>
+#ifdef HAVE_SYS_RANDOM_H
+#include <sys/random.h>
+#endif
+
+static int get_getrandom_seed(void)
+{
+	DEBUG_SEED("get_dev_random_seed");
+
+	int r;
+	ssize_t ret;
+
+	do {
+		ret = getrandom(&r, sizeof(r), 0);
+	} while ((ret == -1) && (errno == EINTR));
+
+	if (ret == -1)
+	{
+		if (errno == ENOSYS) /* syscall not available in kernel */
+			return -1;
+
+		fprintf(stderr, "error from getrandom(): %s", strerror(errno));
+		exit(1);
+	}
+
+	if (ret != sizeof(r))
+		return -1;
+
+	return r;
+}
+#endif /* defined HAVE_GETRANDOM */
+
 /* has_dev_urandom */
 
 #if defined(__APPLE__) || defined(__unix__) || defined(__linux__)
@@ -283,6 +317,13 @@ int json_c_get_random_seed(void)
 	if (has_rdrand())
 		return get_rdrand_seed();
 #endif
+#ifdef HAVE_GETRANDOM
+	{
+		int seed = get_getrandom_seed();
+		if (seed != -1)
+			return seed;
+	}
+#endif
 #if defined HAVE_DEV_RANDOM && HAVE_DEV_RANDOM
 	if (has_dev_urandom())
 		return get_dev_random_seed();