df: report correct device in presence of eclipsed mounts

* src/df.c (last_device_for_mount): A new function to identify
the last device mounted for a mount point.
(get_disk): Use the above to discard mount entries for a device,
where a later mount entry uses a different device name than
that of the user specified device.
* tests/df/over-mount-device.sh: A new root test.
* tests/local.mk: Reference the new test.
* NEWS: Reword for all these related recent fixes.
Discussed at: http://bugs.gnu.org/16539#69

NEWS

@@ -44,10 +44,11 @@ GNU coreutils NEWS                                    -*- outline -*-
 
   [These dd bugs were present in "the beginning".]
 
-  df now elides duplicates for virtual file systems like tmpfs.
-  Displays the correct device details for points mounted multiple times.
-  Displays placeholder values for inaccessible file systems,
-  rather than error messages or values for the wrong file system.
+  df has more fixes related to the newer dynamic representation of file systems:
+  Duplicates are elided for virtual file systems like tmpfs.
+  Details for the correct device are output for points mounted multiple times.
+  Placeholder values are output for inaccessible file systems, rather than
+  than error messages or values for the wrong file system.
   [These bugs were present in "the beginning".]
 
   du now silently ignores directory cycles introduced with bind mounts.

src/df.c

@@ -1114,6 +1114,33 @@ get_dev (char const *disk, char const *mount_point, char const* file,
   free (dev_name);
 }
 
+/* Scan the mount list returning the _last_ device found for MOUNT.
+   NULL is returned if MOUNT not found.  The result is malloced.  */
+static char *
+last_device_for_mount (char const* mount)
+{
+  struct mount_entry const *me;
+  struct mount_entry const *le = NULL;
+
+  for (me = mount_list; me; me = me->me_next)
+    {
+      if (STREQ (me->me_mountdir, mount))
+        le = me;
+    }
+
+  if (le)
+    {
+      char *devname = le->me_devname;
+      char *canon_dev = canonicalize_file_name (devname);
+      if (canon_dev && IS_ABSOLUTE_FILE_NAME (canon_dev))
+        return canon_dev;
+      free (canon_dev);
+      return xstrdup (le->me_devname);
+    }
+  else
+    return NULL;
+}
+
 /* If DISK corresponds to a mount point, show its usage
    and return true.  Otherwise, return false.  */
 static bool
@@ -1122,6 +1149,7 @@ get_disk (char const *disk)
   struct mount_entry const *me;
   struct mount_entry const *best_match = NULL;
   bool best_match_accessible = false;
+  bool eclipsed_device = false;
   char const *file = disk;
 
   char *resolved = canonicalize_file_name (disk);
@@ -1139,9 +1167,12 @@ get_disk (char const *disk)
 
       if (STREQ (disk, devname))
         {
+          char *last_device = last_device_for_mount (me->me_mountdir);
+          eclipsed_device = last_device && ! STREQ (last_device, devname);
           size_t len = strlen (me->me_mountdir);
 
-          if (! best_match_accessible || len < best_match_len)
+          if (! eclipsed_device
+              && (! best_match_accessible || len < best_match_len))
             {
               struct stat disk_stats;
               bool this_match_accessible = false;
@@ -1159,6 +1190,8 @@ get_disk (char const *disk)
                     best_match_len = len;
                 }
             }
+
+          free (last_device);
         }
 
       free (canon_dev);
@@ -1173,6 +1206,13 @@ get_disk (char const *disk)
                best_match->me_remote, NULL, false);
       return true;
     }
+  else if (eclipsed_device)
+    {
+      error (0, 0, _("cannot access %s: over-mounted by another device"),
+             quote (file));
+      exit_status = EXIT_FAILURE;
+      return true;
+    }
 
   return false;
 }

tests/df/over-mount-device.sh

@@ -0,0 +1,57 @@
+#!/bin/sh
+# Ensure that df /dev/loop0 errors out if overmounted by another device
+
+# Copyright (C) 2014 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ df
+require_root_
+
+cwd=$(pwd)
+cleanup_() { cd /; umount "$cwd/mnt"; umount "$cwd/mnt"; }
+
+skip=0
+
+# Create 2 file systems
+for i in 1 2; do
+  dd if=/dev/zero of=blob$i bs=8192 count=200 > /dev/null 2>&1 \
+                                             || skip=1
+  mkfs -t ext2 -F blob$i \
+    || skip_ "failed to create ext2 file system"
+done
+
+# Mount both at the same place (eclipsing the first)
+mkdir mnt                                    || skip=1
+mount -oloop blob1 mnt                       || skip=1
+eclipsed_dev=$(df --o=source mnt | tail -n1) || skip=1
+mount -oloop blob2 mnt                       || skip=1
+
+test $skip = 1 \
+  && skip_ "insufficient mount/ext2 support"
+
+df . || skip_ "failed to lookup the device for the current dir"
+
+echo "df: cannot access '$eclipsed_dev': over-mounted by another device" > exp
+
+# We should get an error for the eclipsed device and continue
+df $eclipsed_dev . > out 2> err && fail=1
+
+# header and single entry in output
+test $(wc -l < out) = 2 || fail=1
+
+compare exp err || fail=1
+
+Exit $fail

tests/local.mk

@@ -115,6 +115,7 @@ all_root_tests =				\
   tests/cp/sparse-fiemap.sh			\
   tests/dd/skip-seek-past-dev.sh		\
   tests/df/problematic-chars.sh			\
+  tests/df/over-mount-device.sh			\
   tests/du/bind-mount-dir-cycle.sh		\
   tests/id/setgid.sh				\
   tests/install/install-C-root.sh		\