rsync - Aug 2018 - BUG: --link-dest may follow symlinks and failure to hard link a non-regular file is fatal

Hi,

following the instructions on
https://bugzilla.samba.org/createaccount-save.html, I've applied for a
bugzilla account at  bugzilla-maintenance at samba.org but didn't receive a
reply, so I report through this list.

With --link-dest the search for a candidate to link from, follows symlinks in
directory path components, when the path element is a directory in the source
tree but a symlink in the --link-dest tree. This may trigger unwanted side
effects (e.g. automounter mounts). Moreover, the target candidate found might
not be suitable for a hard link, because it may be on another file system. For
regular files, rsync tried so fals back to copy, which might be unwanted (e.g.
going over nfs). For non-regular files, rsync will create the destination file,
but report an error and exit with a non-zero exit status.

This is a program to demonstrate the bug (requires root for the mount):

     #! /bin/sh
     set -x
     if [ -d /tmp/rsync_bug ]; then
         umount /tmp/rsync_bug/mnt
         rm -rf /tmp/rsync_bug
     fi
     mkdir /tmp/rsync_bug
     cd /tmp/rsync_bug
     mkdir mnt
     mount -t tmpfs tmpfs mnt
     ln -s BLABLA mnt/test
     mkdir src
     ln -s /tmp/rsync_bug/mnt src/dir
     rsync -aHx --delete --numeric-ids  src/  dst1
     rm src/dir
     mkdir src/dir
     cp -a mnt/test src/dir/
     rsync -aHx --delete --numeric-ids --link-dest /tmp/rsync_bug/dst1  src/ 
dst2 || echo RSYNC FAIL

Output for me:

     buczek at theinternet:~$ sudo ./rsync-bug.sh
     + '[' -d /tmp/rsync_bug ']'
     + umount /tmp/rsync_bug/mnt
     + rm -rf /tmp/rsync_bug
     + mkdir /tmp/rsync_bug
     + cd /tmp/rsync_bug
     + mkdir mnt
     + mount -t tmpfs tmpfs mnt
     + ln -s BLABLA mnt/test
     + mkdir src
     + ln -s /tmp/rsync_bug/mnt src/dir
     + rsync -aHx --delete --numeric-ids src/ dst1
     + rm src/dir
     + mkdir src/dir
     + cp -a mnt/test src/dir/
     + rsync -aHx --delete --numeric-ids --link-dest /tmp/rsync_bug/dst1 src/
dst2
     rsync: failed to hard-link /tmp/rsync_bug/dst1/dir/test with dir/test:
Invalid cross-device link (18)
     rsync error: some files/attrs were not transferred (see previous errors)
(code 23) at main.c(1210) [sender=3.1.4dev]
     + echo RSYNC FAIL
     RSYNC FAIL
     buczek at theinternet:~$

The second rsync fails, because src/dir is a link to another filesystem in the
--link-dest tree but a directory in the new src and src/dir/test is a
non-regular file.

Happens with 3.0.9 and with the current git master version.

We've seen our rsync based backup triggering automounts on the backup server
or fail from time to time, because of the above problem. Users sometimes move
directories somewhere else and create a symlink to the new destination in the
old place or visa versa. So with incremental backups you do have real files one
day and a symlink to the same tree another day.


Donald
-- 
Donald Buczek
buczek at molgen.mpg.de
Tel: +49 30 8413 1433