sysdep: Add wrapper to get random bytes

Add a wrapper function in sysdep to get random bytes, and required checks
in configure.ac to select how to do it. The configure script tries, in
order, getrandom(), getentropy() and reading from /dev/urandom.

conf/conf.c

@@ -524,6 +524,7 @@ order_shutdown(int gr)
   c->gr_down = gr;
 
   config_commit(c, RECONFIG_HARD, 0);
+  random_close();
   shutting_down = 1;
 }
 

configure.ac

@@ -374,6 +374,10 @@ elif test "$bird_cv_lib_log" != yes ; then
   LIBS="$LIBS $bird_cv_lib_log"
 fi
 
+AC_CHECK_FUNCS(getrandom)
+AC_CHECK_FUNCS(getentropy)
+AC_CHECK_HEADERS(sys/random.h)
+
 if test "$enable_debug" = yes ; then
   AC_DEFINE([DEBUGGING], [1], [Define to 1 if debugging is enabled])
   LDFLAGS="$LDFLAGS -rdynamic"

lib/birdlib.h

@@ -192,5 +192,8 @@ asm(
 /* Pseudorandom numbers */
 
 u32 random_u32(void);
+int random_bytes(char *buf, size_t size);
+void random_close(void);
+void random_init(void);
 
 #endif

sysdep/unix/main.c

@@ -867,6 +867,7 @@ main(int argc, char **argv)
   parse_args(argc, argv);
   log_switch(1, NULL, NULL);
 
+  random_init();
   net_init();
   resource_init();
   timer_init();

sysdep/unix/random.c

@@ -7,6 +7,21 @@
  */
 
 #include <stdlib.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include "sysdep/config.h"
+
+#ifdef HAVE_SYS_STAT_H
+#  include <sys/stat.h>
+#endif
+#ifdef HAVE_LINUX_RANDOM_H
+#  include <linux/random.h>
+#endif
+#if defined(HAVE_SYS_RANDOM_H) && (defined(HAVE_GETRANDOM) || defined(HAVE_GETENTROPY))
+#    include <sys/random.h>
+#endif
 
 #include "nest/bird.h"
 
@@ -19,3 +34,76 @@ random_u32(void)
   rand_high = random();
   return (rand_low & 0xffff) | ((rand_high & 0xffff) << 16);
 }
+
+void
+random_init()
+{
+  char buf;
+  /* get a single random byte to trip any errors early */
+  random_bytes(&buf, sizeof(buf));
+}
+
+#if defined(HAVE_GETRANDOM) || defined(HAVE_GENTROPY)
+int
+random_bytes(char *buf, size_t size)
+{
+  int n;
+  int flags = 0;
+  while (0 < size) {
+#if defined(HAVE_GETRANDOM)
+    n = getrandom(buf, size, flags);
+#else
+    n = getentropy(buf, size);
+#endif
+    if (n < 0) {
+      if (errno == EINTR)
+        continue;
+      die("Couldn't get random bytes: %m");
+    }
+    buf += n;
+    size -= n;
+  }
+
+  return 0;
+}
+
+void random_close(void) {}
+
+#else
+
+static int urandom_fd = -1;
+int random_bytes(char *buf, size_t size)
+{
+  int n;
+
+  if (urandom_fd < 0)
+  {
+    urandom_fd = open("/dev/urandom", O_RDONLY);
+    if (urandom_fd < 0)
+      die("Couldn't open /dev/urandom: %m");
+  }
+
+  do
+  {
+    n = read(urandom_fd, buf, size);
+    if (n <= 0) {
+      if (errno == EINTR)
+        continue;
+      die("Couldn't read from /dev/urandom: %m");
+    }
+    buf += n;
+    size -= n;
+  } while (size > 0);
+
+  return 0;
+}
+
+void
+random_close(void)
+{
+  if (urandom_fd >= 0) {
+    close(urandom_fd);
+    urandom_fd = -1;
+  }
+}
+#endif