[PATCH] getroot: Correctly handle missing btrfs device identifiers.

["James Johnston" <johnstonj.public@codenest.com>]

From 97bcb9eb8c34329a8ca1ec88fd89296e273ceb24 Mon Sep 17 00:00:00 2001
From: James Johnston <johnstonj.public@codenest.com>
Date: Thu, 17 Mar 2016 15:10:49 +0000
Subject: [PATCH] getroot: Correctly handle missing btrfs device identifiers.

The btrfs driver's BTRFS_IOC_FS_INFO ioctl only tells you the maximum
device ID and the number of devices.  It does not tell you which
device IDs may no longer be allocated - for example, if a device is
later deleted from the file system, its device ID won't be allocated
any more.

You must then probe each device ID with BTRFS_IOC_DEV_INFO to see if
it is a valid device or not.  It is not an error condition for this
ioctl to return ENODEV: it seems to mean that the device was deleted
by the user.

Signed-off-by: James Johnston <johnstonj.public@codenest.com>
---
 grub-core/osdep/linux/getroot.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/grub-core/osdep/linux/getroot.c b/grub-core/osdep/linux/getroot.c
index 10480b6..852f3a2 100644
--- a/grub-core/osdep/linux/getroot.c
+++ b/grub-core/osdep/linux/getroot.c
@@ -242,15 +242,29 @@ grub_find_root_devices_from_btrfs (const char *dir)
       struct btrfs_ioctl_dev_info_args devi;
       memset (&devi, 0, sizeof (devi));
       devi.devid = i;
+      /* We have to probe all possible device IDs, since btrfs doesn't
+	 give us a list of all the valid ones.  */
       if (ioctl (fd, BTRFS_IOC_DEV_INFO, &devi) < 0)
 	{
-	  close (fd);
-	  free (ret);
-	  return NULL;
+	  /* ENODEV is normal, e.g. if the first device is deleted from
+	     an array. */
+	  int last_error = errno;
+	  if (last_error != ENODEV)
+	    {
+	      grub_util_warn(_("btrfs device info could not be read "
+			       "for %s, device %d: errno %d"), dir, i,
+			       last_error);
+	      close (fd);
+	      free (ret);
+	      return NULL;
+	    }
+	}
+      else
+	{
+	  ret[j++] = xstrdup ((char *) devi.path);
+	  if (j >= fsi.num_devices)
+	    break;
 	}
-      ret[j++] = xstrdup ((char *) devi.path);
-      if (j >= fsi.num_devices)
-	break;
     }
   close (fd);
   ret[j] = 0;
-- 
1.9.5.msysgit.1