ovl: Fix nested backing file paths

When the lowerdir of an overlayfs is a merged directory of another
overlayfs, ovl_open_realfile() will fail to open the real file and point
to a lower dentry copy, without the proper parent path. After this,
d_path() will then display the path incorrectly as if the file is placed
in the root directory.

This bug can be triggered with the following setup:

 mkdir -p ovl-A/lower ovl-A/upper ovl-A/merge ovl-A/work
 mkdir -p ovl-B/upper ovl-B/merge ovl-B/work

 cp /bin/cat ovl-A/lower/

 mount -t overlay overlay -o \
 lowerdir=ovl-A/lower,upperdir=ovl-A/upper,workdir=ovl-A/work \
 ovl-A/merge

 mount -t overlay overlay -o \
 lowerdir=ovl-A/merge,upperdir=ovl-B/upper,workdir=ovl-B/work \
 ovl-B/merge

 ovl-A/merge/cat /proc/self/maps | grep --color cat
 ovl-B/merge/cat /proc/self/maps | grep --color cat

The first cat will correctly show `/ovl-A/merge/cat`, while the second
one shows just `/cat`.

To fix that, uses file_user_path() inside of backing_file_open() to get
the correct file path for the dentry.

Co-developed-by: John Schoenick <[email protected]>
Signed-off-by: John Schoenick <[email protected]>
Signed-off-by: André Almeida <[email protected]>
Fixes: def3ae83da02 ("fs: store real path instead of fake path in backing file f_path")
Cc: <[email protected]> # v6.7
Signed-off-by: Miklos Szeredi <[email protected]>

0 files changed, 0 insertions, 0 deletions