Description of problem:

When exporting multiple filesystems it is possible for rsync to incorrectly identify a file as having a hardlink to another entirely different file.

rsync is using the inode of a file as the only test to determine if two files are hardlinked.  However, it is possible for two files to have the same inode (because they are on different filesystems) but be exposed via a single rsync target.

Version-Release number of selected component (if applicable):rsync-3.0.9-17.el7


How reproducible:
Difficult to build reproduction case, but once created, works every time.


Steps to Reproduce (a):
1.Build two file systems (A and B), mount A on /srv/example/
2.Mount filesystem B on /srv/example/otherfs
3.Put a file in A with unique content and a known inode number
4.Put a different file in B with unique content the same inode number as the file in A
5.Have rsync server export /srv/example and all sub files and directories
6.Use the rsync server to sync down /srv/example with hardlink detection enabled (rsync -avH)
7.The files you've synced down should now be hardlinked, and thus the content of one file is incorrect.


Steps to Reproduce (b):
1.Build two file systems (A and B), mount A on /srv/example/
2.Mount filesystem B on /srv/example/otherfs
3.Put a file in A with unique content and a known inode number
4.Put a different file in B with unique content the same inode number as the file in A
5.Mount /srv/example via NFS3 on a system (NFS3 doesn't promise unique inodes) as /srv/nfsmounted
6.Have rsync server export /srv/nfsmounted and all sub files and directories
7.Use the rsync server to sync down /srv/nfsmounted with hardlink detection enabled (rsync -avH)
8.The files you've synced down should now be hardlinked, and thus the content of one file is incorrect.


Actual results:
Non identical files are treated as hardlinks and data is not transfered.

Expected results:
Only identical files treated as hardlinks.

Additional info:
If, in addition to checking the inode, rsync could check the file has the same number of bytes or has the same mtimestamp that should be sufficient to avoid the collision behavior.

Conversation on upstream email list:
https://www.mail-archive.com/rsync@lists.samba.org/msg29385.html