df: improve mount point selection with inaccurate mount list

v8.23 has a test failure on Fedora rawhide build servers
in tests/df/skip-duplicate.sh.  This was due to no '/'
entry being output by df.  That was due to an inaccurate
/proc/mounts on the build environment as stat(/mnt/point)
identified all these /proc/mounts entries as having the
same device id:

  /                    rootfs
  /                    /dev/md1
  /dev                 devtmpfs
  /run                 tmpfs
  /boot                /dev/md0
  /proc/filesystems    /dev/md1

Since the device name on the right changes for a given id,
that causes the entries to be continually replaced, thus
resulting in no '/' entry.  I'm guessing this is due to
the mock environment bind mounting unneeded or sensitive
items to a dummy file on the host / (/dev/md1) though
have not looked into those details.

So rather than relying on an accurate /proc/mounts,
the attached patch takes a more conservative replacement
approach and only swaps a new device entry when the
mount point matches.  That should handle all practical
cases while also avoiding this situation.

* src/df.c (filter_mount_list): Only replace entries with
different device names when the mount point also matches.

1 files changed, 8 insertions, 3 deletions

diff --git a/src/df.c b/src/df.c
index 3ef5d33b0..e907b9409 100644
--- a/src/df.c
+++ b/src/df.c
@@ -640,13 +640,18 @@ filter_mount_list (bool devices_only)
 
           if (devlist)
             {
-              /* ...let the shorter mountdir win.  */
+                  /* let "real" devices with '/' in the name win.  */
               if ((strchr (me->me_devname, '/')
                    && ! strchr (devlist->me->me_devname, '/'))
+                  /* let a shorter mountdir win.  */
                   || (strlen (devlist->me->me_mountdir)
                       > strlen (me->me_mountdir))
-                  /* or one overmounted on a different device.  */
-                  || ! STREQ (devlist->me->me_devname, me->me_devname))
+                  /* let an entry overmounted on a different device win...  */
+                  || (! STREQ (devlist->me->me_devname, me->me_devname)
+                      /* ... but only when matching an exsiting mount point, to
+                      avoid problematic replacement when given inaccurate mount
+                      lists, seen with some chroot environments for example.  */
+                      && STREQ (me->me_mountdir, devlist->me->me_mountdir)))
                 {
                   /* Discard mount entry for existing device.  */
                   discard_me = devlist->me;