Linux Docker: EFAULT "Bad address [write]" when writing .ffs_tmp files to SMB/CIFS share

[oxydus]

Hello,

I would like to report a reproducible write error with FreeFileSync on Linux inside a Docker container.

Environment:

- FreeFileSync version: 14.10
- Docker image: jlesage/freefilesync
- Docker image version: 26.07.1
- Platform: linux/amd64
- Host operating system: Unraid
- Destination: Synology NAS via an SMB/CIFS mount
- SMB protocol: SMB 3.1.1
- Synchronization type: Mirror
- Source: Unraid user share
- Destination path inside the container:
/remotes/VOJVODINA-NAS_Backup/Unraid/Media/

Problem:

During synchronization, FreeFileSync repeatedly stops with errors similar to:

Cannot write file
"/remotes/VOJVODINA-NAS_Backup/Unraid/Media/Anime/One Piece/S38/One Piece_S38E1112_Episode 1112-2b92.ffs_tmp"

EFAULT: Bad address [write]

The error occurs on different files and in different directories. It is not limited to one specific filename or media file.

The error appears while FreeFileSync is creating temporary ".ffs_tmp" files with "Fail-safe file copy" enabled.

Tests performed:

1. Writing directly to the same SMB mount from the Unraid host works:
- 1 GB test file completed successfully
- approximately 109 MB/s

2. Writing from inside the FreeFileSync Docker container also works:
- touch test completed successfully
- 1 GB dd test completed successfully
- 10 GB dd test completed successfully
- approximately 180–210 MB/s
- no EFAULT errors

3. The SMB mount remains available and writable after the FreeFileSync error.

4. No relevant CIFS, SMB, disk, or network errors appeared in the host kernel log at the time of the failure.

5. Docker logs contain only GTK warnings and no storage-related error:
Gtk-CRITICAL: gtk_widget_set_sensitive assertion failed

6. Disabling "Fail-safe file copy" appears to avoid creation of the .ffs_tmp files. I am currently testing whether this also prevents the EFAULT error.

Expected behavior:

FreeFileSync should be able to write temporary .ffs_tmp files to the SMB-mounted destination without returning EFAULT.

Actual behavior:

FreeFileSync intermittently fails with:

EFAULT: Bad address [write]

even though large writes to the exact same mounted directory succeed both from the host and from inside the container.

Please let me know if you need the FreeFileSync log file, Docker configuration, CIFS mount options, or additional reproduction tests.

Disabling "Fail-safe file copy" is currently being tested as a workaround. The synchronization is still running, so I cannot yet confirm whether it fully resolves the issue.

[oxydus]

Disabling "Fail-safe file copy" has not made it work properly