studioph / cifs-symlinks

Small reproducible example of issues with symlinks and CIFS

Symlinks

Reproducible example for this issue with os.walk not detecting symlinks correctly on mounted CIFS filesystems due to a bug in the kernel CIFS driver.

Running

1. Install samba (sudo apt install samba)
2. Fill out smb.conf with the needed values and place at /etc/samba/smb.conf
3. Restart samba to pick up new config (sudo systemctl restart smbd.service)
4. Mount the share somewhere (sudo mount -f cifs -o guest,vers=3.0,uid=1000,gid=1000,mfsymlinks //127.0.0.1/symlinks /mnt/cifs)
5. Run mklinks.sh to create some symbolic links from the client (mklinks.sh /mnt/cifs)
   • This will make some links in results that point to ../source
6. Compile the C program to show the discrepency between the system calls (gcc -o testdent testdent.c)
7. Run the C program and point it to the CIFS mount point results folder: ./testdent /mnt/cifs/results
   • The output should look like the following:

linked
D_TYPE=Regular file
LSTAT type=Link
STAT type=Directory

link.txt
D_TYPE=Regular file
LSTAT type=Link
STAT type=Regular file

Where the D_TYPE value is incorrectly reported for both symlinks

Notes

• os.walk uses scandir which uses readdir system call

  • Relies on d_type property of struct returned by readdir to determine if file, dir, link, etc

  • Done to avoid making extra stat system call for each item

  • If DT_UNKOWN, falls back to stat

  • d_type incorrectly reported as DT_REG with CIFS instead of DT_DIR/DT_LNK or DT_UNKNOWN

• pathlib uses os.path.is_ which uses stat system call

• CIFS does not natively support symlinks

  • Can emulate symlinks using mfsymlinks mount option in order to create symlinks from the client

  • Existing symlinks not created by client are presented as hardlinks and do not have the above issue with the wrong d_type value

Solutions

• Use NFS if possible (full POSIX compliance)

• Use alternative traversal methods (i.e. pathlib)

  • Less efficient (extra stat system call for each item to determine if file or folder)