Fwd: overlayfs: cannot rename symlink if lower filesystem is FUSE/NFS

Fwd: overlayfs: cannot rename symlink if lower filesystem is FUSE/NFS

[Bagas Sanjaya]

Hi,

I notice a regression report on Bugzilla [1]. Quoting from it:

> Hi, 
> 
> We recently found a regression on linux kernel: rename(2) on a symlink through an overlayfs fails with ENXIO, when the lowerdir is FUSE.
> 
> *What happened*
> 
> When running `mv` command on a symlink file through overlayfs, and the overlayfs's lowdir on FUSE or NFS, the command fails with "No such device or address". This issue happens on kernel 5.15 and 6.1, but not on 5.10. 
> 
> *How to reproduce*
> Environment: Debian bookworm (kernel 6.1.0)
> 
> 1. To prepare the FUSE fs, create a file and a symlink under the VM's root dir:
> 
> ```
> ruiwen@instance-1:/tmp$ ls / -l | grep foo
> -rw-r--r--   1 root root     0 Aug 30 23:10 foo
> lrwxrwxrwx   1 root root     3 Aug 30 23:12 foolink -> foo
> ```
> and then run libfuse's passthrough (https://github.com/libfuse/libfuse/blob/master/example/passthrough.c), which mounts a FUSE filesystem by mirroring the root dir:
> 
> ```
> ruiwen@instance-1:~/fuse-3.16.1/build/example$ ./passthrough -o allow_other /tmp/fusemount
> ruiwen@instance-1:~/fuse-3.16.1/build/example$ ls /tmp/fusemount/ -l | grep foo
> -rw-r--r--   1 root root     0 Aug 30 23:10 foo
> lrwxrwxrwx   1 root root     3 Aug 30 23:12 foolink -> foo
> ```
> 
> 2. Create an overlayfs mount, with lower dir being the the mount point of FUSE filesystem.
> ```
> ruiwen@instance-1:/tmp$ mkdir -p fusemount upper work merged
> ruiwen@instance-1:/tmp$ sudo mount -t overlay overlay -o lowerdir=fusemount,upperdir=upper,workdir=work merged
> ruiwen@instance-1:/tmp$ ls -l merged/ | grep foo
> -rw-r--r--   1 root root     0 Aug 30 23:10 foo
> lrwxrwxrwx   1 root root     3 Aug 30 23:12 foolink -> foo
> ```
> 
> 3. Try to move the symlink and see the failure:
> ```
> ruiwen@instance-1:/tmp$ mv merged/foolink merged/foolink2
> mv: cannot move 'merged/foolink' to 'merged/foolink2': No such device or address
> ```
> 
> 
> *Some observations*
> 
> 1. Same bug has been reported at Debian Bug, where overlayfs is used with NFS: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1049885. This makes me think that the bug is more on overlayfs, but not on FUSE or NFS.
> 
> 2. This issue can be reproduced on kernel 5.15, 6.10, but CANNOT be reproduced on kernel 5.10. There is a noticeable change on 5.15 that is related to overlayfs: (https://github.com/torvalds/linux/commit/72db82115d2bdfbfba8b15a92d91872cfe1b40c6), which introduces copyup fileattr.
> 
> 3. When reproducing this bug, we found that the error ENXIO was actually from getting lower fileattr. In dmesg we see: "failed to retrieve lower fileattr (/link, err=-6)". So it seems that overlayfs for some reason fails to get the file attributes of the source file from the underlying filesystem.

See Bugzilla for the full thread.

Anyway, I'm adding it to regzbot:

#regzbot introduced: v5.10..v6.1 https://bugzilla.kernel.org/show_bug.cgi?id=217850
#regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=217850

Thanks.

[1]: https://bugzilla.kernel.org/show_bug.cgi?id=217850

-- 
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