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ovl: Fix nested backing file paths

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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 <johns@valvesoftware.com>
Signed-off-by: John Schoenick <johns@valvesoftware.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Fixes: def3ae83da ("fs: store real path instead of fake path in backing file f_path")
Cc: <stable@vger.kernel.org> # v6.7
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
André Almeida 2025-04-29 15:38:50 -03:00 • committed by Miklos Szeredi
commit 924577e4f6
1 changed files with 2 additions and 2 deletions
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