lib: Use arc4random if present

configure.ac

@@ -324,6 +324,8 @@ DOVECOT_FDATASYNC
 DOVECOT_LIBCAP
 DOVECOT_LIBWRAP
 
+DOVECOT_ARC4RANDOM
+
 AC_DEFINE(PACKAGE_WEBPAGE, "http://www.dovecot.org/", [Support URL])
 
 dnl * after -lsocket and -lnsl tests, inet_aton() may be in them

m4/arc4random.m4

@@ -0,0 +1,11 @@
+AC_DEFUN([DOVECOT_ARC4RANDOM], [
+  AC_CHECK_FUNC([arc4random], [
+    AC_DEFINE([HAVE_ARC4RANDOM], [1], [Define this if you arc4random()])
+  ], [
+      AC_CHECK_LIB([bsd], [arc4random], [
+        LIBS="$LIBS -lbsd"
+        AC_DEFINE([HAVE_ARC4RANDOM], [1], [Define this if you arc4random()])
+        AC_DEFINE([HAVE_LIBBSD], [1], [Define this if you have libbsd])
+      ])
+  ])
+])

src/lib/rand.c

@@ -31,3 +31,14 @@ void rand_set_seed(unsigned int s)
 
 	srand(seed);
 }
+
+#ifdef HAVE_ARC4RANDOM
+#ifdef HAVE_LIBBSD
+#include <bsd/stdlib.h>
+#endif
+
+/* this returns [0,RAND_MAX), to keep it compatible with rand() */
+int arc4random_rand(void) {
+	return (int)(arc4random() % ((unsigned)RAND_MAX + 1));
+}
+#endif

src/lib/rand.h

@@ -16,4 +16,11 @@ unsigned int rand_get_last_seed(void);
 /* Actually seed the prng (could add char* for name of function?) */
 void rand_set_seed(unsigned int s);
 
+#ifdef HAVE_ARC4RANDOM
+
+int arc4random_rand(void);
+#define rand arc4random_rand
+
+#endif
+
 #endif

src/lib/randgen.c

@@ -61,10 +61,24 @@ void random_deinit(void)
 	i_close_fd(&urandom_fd);
 }
 
+#ifdef HAVE_ARC4RANDOM
+#ifdef HAVE_LIBBSD
+#include <bsd/stdlib.h>
+#endif
+
+void random_fill_weak(void *buf, size_t size)
+{
+	arc4random_buf(buf, size);
+}
+
+#else
+
 void random_fill_weak(void *buf, size_t size)
 {
 	unsigned char *cbuf = buf;
 
 	for (; size > 0; size--)
 		*cbuf++ = (unsigned char)rand();
 }
+
+#endif