lib: Use best available random number generator

The best available method is used from short list of
getrandom, arc4random or reading /dev/urandom

src/lib/randgen.c

@@ -6,28 +6,56 @@
 #include <unistd.h>
 #include <fcntl.h>
 
+/* get randomness from either getrandom, arc4random or /dev/urandom */
+
+#if defined(HAVE_GETRANDOM) && HAVE_DECL_GETRANDOM != 0
+#  include <sys/random.h>
+#  define USE_GETRANDOM
+#elif defined(HAVE_ARC4RANDOM)
+#  if defined(HAVE_LIBBSD)
+#    include <bsd/stdlib.h>
+#  endif
+#  define USE_ARC4RANDOM
+#else
+#  define USE_RANDOM_DEV
+#endif
+
 static int init_refcount = 0;
+#if defined(USE_RANDOM_DEV)
 static int urandom_fd;
+#endif
 
 void random_fill(void *buf, size_t size)
 {
-	size_t pos;
-	ssize_t ret;
-
 	i_assert(init_refcount > 0);
 	i_assert(size < SSIZE_T_MAX);
 
+#if defined(USE_ARC4RANDOM)
+	arc4random_buf(buf, size);
+#else
+	size_t pos;
+	ssize_t ret;
+
 	for (pos = 0; pos < size; ) {
+#  if defined(USE_GETRANDOM)
+		ret = getrandom(buf, size - pos, 0);
+#  else
 		ret = read(urandom_fd, (char *) buf + pos, size - pos);
+#  endif
 		if (unlikely(ret <= 0)) {
 			if (ret == 0)
-				i_fatal("EOF when reading from "DEV_URANDOM_PATH);
+				i_fatal("read("DEV_URANDOM_PATH") failed: EOF");
 			else if (errno != EINTR)
+#  if defined(USE_RANDOM_DEV)
 				i_fatal("read("DEV_URANDOM_PATH") failed: %m");
+#  elif defined(USE_GETRANDOM)
+				i_fatal("getrandom() failed: %m");
+#  endif
 		} else {
 			pos += ret;
 		}
 	}
+#endif /* defined(USE_ARC4RANDOM) */
 }
 
 void random_init(void)
@@ -36,49 +64,27 @@ void random_init(void)
 
 	if (init_refcount++ > 0)
 		return;
-
+#if defined(USE_RANDOM_DEV)
 	urandom_fd = open(DEV_URANDOM_PATH, O_RDONLY);
 	if (urandom_fd == -1) {
 		if (errno == ENOENT) {
-			i_fatal(DEV_URANDOM_PATH" doesn't exist, "
+			i_fatal("open("DEV_URANDOM_PATH") failed: doesn't exist,"
 				"currently we require it");
 		} else {
-			i_fatal("Can't open "DEV_URANDOM_PATH": %m");
+			i_fatal("open("DEV_URANDOM_PATH") failed: %m");
 		}
 	}
-
+	fd_close_on_exec(urandom_fd, TRUE);
+#endif
 	random_fill(&seed, sizeof(seed));
 	srand(seed);
-
-	fd_close_on_exec(urandom_fd, TRUE);
 }
 
 void random_deinit(void)
 {
 	if (--init_refcount > 0)
 		return;
-
+#if defined(USE_RANDOM_DEV)
 	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

src/lib/randgen.h

@@ -1,14 +1,12 @@
 #ifndef RANDGEN_H
 #define RANDGEN_H
 
-/* Fill given buffer with semi-strong randomness, usually from /dev/urandom. */
+/* Fill given buffer with semi-strong randomness */
 void random_fill(void *buf, size_t size);
-/* Fill given buffer with weak randomness, ie. with rand(). This is better if
-   no real randomness is required, as reading from /dev/urandom usually also
-   consumes /dev/random entropy, which may disturb other processes. */
-void random_fill_weak(void *buf, size_t size);
+#define random_fill_weak(buf,size) random_fill((buf), (size))
 
-/* may be called multiple times */
+/* may be called multiple times,
+   and are called by default in lib_init */
 void random_init(void);
 void random_deinit(void);