getrandom: add helper for getrandom() syscall

getrandom() was introduced in version 3.17 of the Linux kernel.
By default getrandom() block until /dev/urandom pool has been initialized
and then read from it.
Read buffer is 256 bytes so getrandom() calls always succeed.

First usage will be to save a seed for /dev/urandom.

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/getrandom.c b/getrandom.c
new file mode 100644
index 0000000..9671202
--- /dev/null
+++ b/getrandom.c
@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2016 Etienne Champetier <champetier.etienne@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+#define _GNU_SOURCE
+#include <errno.h>
+#include <linux/random.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#define ERROR_EXIT(fmt, ...) do { \
+        fprintf(stderr, fmt, ## __VA_ARGS__); \
+        return EXIT_FAILURE; \
+        } while (0)
+
+static int usage(char *name)
+{
+    fprintf(stderr, "Usage: %s <nb>\n", name);
+    fprintf(stderr, " => return <nb> bytes from getrandom()\n");
+    return EXIT_FAILURE;
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc != 2)
+        return usage(argv[0]);
+
+    if (isatty(STDOUT_FILENO))
+        ERROR_EXIT("Not outputting random to a tty\n");
+
+    int nbtot = atoi(argv[1]);
+    if (nbtot < 1)
+        ERROR_EXIT("Invalid <nb> param (must be > 0)\n");
+
+    char buf[256];
+    int len = sizeof(buf);
+    while (nbtot > 0) {
+        if (nbtot <= sizeof(buf))
+            len = nbtot;
+        if (syscall(SYS_getrandom, buf, len, 0) == -1)
+            ERROR_EXIT("getrandom() failed: %s\n", strerror(errno));
+        if (write(STDOUT_FILENO, buf, len) != len)
+            ERROR_EXIT("write() failed: %s\n", strerror(errno));
+        nbtot -= sizeof(buf);
+    }
+}