daemon: list-md-devices: Check if a /dev/md<X> device is valid before returning it.

For reasons not fully understood, if md is linked into the kernel (and
not a module), a /dev/md0 device node is created.  However this is not
a real RAID device.  For example running mdadm --detail /dev/md0 will
fail on it.

Check the /dev/md<X> devices are real RAID devices before returning
them from the list-md-devices API.

configure.ac

@@ -286,6 +286,7 @@ AC_CHECK_HEADERS([\
     byteswap.h \
     endian.h \
     errno.h \
+    linux/raid/md_u.h \
     printf.h \
     sys/inotify.h \
     sys/socket.h \

daemon/md.c

@@ -22,8 +22,16 @@
 #include <stdlib.h>
 #include <string.h>
 #include <inttypes.h>
+#include <unistd.h>
+#include <fcntl.h>
 #include <glob.h>
 
+#ifdef HAVE_LINUX_RAID_MD_U_H
+#include <sys/ioctl.h>
+#include <linux/major.h>
+#include <linux/raid/md_u.h>
+#endif /* HAVE_LINUX_RAID_MD_U_H */
+
 #include "daemon.h"
 #include "actions.h"
 #include "optgroups.h"
@@ -176,6 +184,30 @@ glob_errfunc (const char *epath, int eerrno)
   return 1;
 }
 
+/* Check if 'dev' is a real RAID device, because in the case where md
+ * is linked directly into the kernel (not a module), /dev/md0 is
+ * sometimes created.
+ */
+static int
+is_raid_device (const char *dev)
+{
+  int ret = 1;
+
+#if defined(HAVE_LINUX_RAID_MD_U_H) && defined(GET_ARRAY_INFO)
+  int fd;
+  mdu_array_info_t array;
+
+  fd = open (dev, O_RDONLY);
+  if (fd >= 0) {
+    if (ioctl (fd, GET_ARRAY_INFO, &array) == -1 && errno == ENODEV)
+      ret = 0;
+    close (fd);
+  }
+#endif
+
+  return ret;
+}
+
 char **
 do_list_md_devices (void)
 {
@@ -219,6 +251,11 @@ do_list_md_devices (void)
     n = mempcpy (n, &mds.gl_pathv[i][strlen(PREFIX)], len);
     *n = '\0';
 
+    if (!is_raid_device (dev)) {
+      free (dev);
+      continue;
+    }
+
     if (add_string_nodup (&ret, dev) == -1) goto error;
   }