Move csrand() to a new file csrand.c

A set of APIs similar to arc4random(3) is complex enough to deserve its
own file.

Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Cc: Björn Esser <besser82@fedoraproject.org>
Cc: Yann Droneaud <ydroneaud@opteya.com>
Cc: Joseph Myers <joseph@codesourcery.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

libmisc/csrand.c

@@ -0,0 +1,61 @@
+#include <config.h>
+
+#ident "$Id$"
+
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#if HAVE_SYS_RANDOM_H
+#include <sys/random.h>
+#endif
+#include "bit.h"
+#include "prototypes.h"
+#include "shadowlog.h"
+
+
+/*
+ * Return a uniformly-distributed CS random u_long value.
+ */
+unsigned long
+csrand(void)
+{
+	FILE           *fp;
+	unsigned long  r;
+
+#ifdef HAVE_GETENTROPY
+	/* getentropy may exist but lack kernel support.  */
+	if (getentropy(&r, sizeof(r)) == 0)
+		return r;
+#endif
+
+#ifdef HAVE_GETRANDOM
+	/* Likewise getrandom.  */
+	if (getrandom(&r, sizeof(r), 0) == sizeof(r))
+		return r;
+#endif
+
+#ifdef HAVE_ARC4RANDOM_BUF
+	/* arc4random_buf can never fail.  */
+	arc4random_buf(&r, sizeof(r));
+	return r;
+#endif
+
+	/* Use /dev/urandom as a last resort.  */
+	fp = fopen("/dev/urandom", "r");
+	if (NULL == fp) {
+		goto fail;
+	}
+
+	if (fread(&r, sizeof(r), 1, fp) != 1) {
+		fclose(fp);
+		goto fail;
+	}
+
+	fclose(fp);
+	return r;
+
+fail:
+	fprintf(log_get_logfd(), _("Unable to obtain random bytes.\n"));
+	exit(1);
+}