Firefox can't save a file to the root of a MEMFS mount.

[dwmega]

Bug Report

Hey there!

I've been working on a WinFSP-based filesystem and found that Firefox was unable to save a file to the root of my mount. After a lot digging through logs, I found myself without any answers so I decided to perform the same test using the MEMFS example filesystem provided with WinFSP.

MEMFS seems to suffer from the same problem: When you try and download any file to the root of a filesystem, Firefox reports a "permissions error" although from the logs, I can't see exactly why.

If you save a file to a subdirectory of the root, everything works expected.

Interestingly, if you create a network mount that's mapped to some drive letter, everything works as expected, too.

I'm not really sure how to proceed with this issue so I figured I'm come to the source :)

Kind regards,
Dave Ward

How to Reproduce

You can reproduce the issue quite easily.

1. Create an MEMFS mount (make sure it isn't a network mount.)
2. Go to the WinFSP download page and try and save ChangeLog.md to the mount's root.
3. This will fail.

For comparison, try the above with a network mount (where the file will download without issues) or in a subdirectory of the mount (which will always work.)

Behaviors

The expected behavior is that we can download a file to the root of a mount.
Actual behavior is that Firefox reports permissions failures.

Environment

Windows 10.0.19045 Build 19045
WinFSP 2023

[billziss-gh]

I do not have Firefox, but I have the Tor Browser which I believe is based on Firefox. I just created a file system using memfs-x64.exe -m X: and I was able to save a file in its root directory using Tor.

Some questions:

• Can you post the full command line of how you launch MEMFS?
• Do you launch it from an non-elevated account or an elevated account (i.e. as Administrator)?
• Assuming that X: is your mount point what does the command cacls x:\ /s say?

[dwmega]

Hey there, @billziss-gh.

Thank you for quick response :)

Assuming memfs is in our path and we're currently in our home:
memfs-x64 -m root

Both Firefox and memfs are launched with standard privileges: No elevation.

The output of cacls is as follows:
C:\Users\k>cacls root /s
C:\Users\k\root "D:(A;OICI;FA;;;SY)(A;OICI;FA;;;BA)(A;OICI;FA;;;S-1-5-21-3900817571-2663857942-4197420431-1001)"

If I mount to a drive letter, the download succeeds.

Output of cacls for a drive-letter mount as follows:
C:\Users\k>cacls z: /s
Z:\ "D:P(A;;FA;;;SY)(A;;FA;;;BA)(A;;FA;;;WD)"

Kind regards,
Dave Ward

[dwmega]

Hey @billziss-gh,

If it helps, these are the i?cacls outputs for my filesystem.

In this example, I've created two mounts: One is a "directory-mount" and the other is a "drive-mount."

I can successfully download to the "drive-mount" but not to the "directory-mount"

C:\Users\k\Documents\Projects\sdk\srw-fuse-threaded-refactor\build64>cacls root /s
C:\Users\k\Documents\Projects\sdk\srw-fuse-threaded-refactor\build64\root "D:P(A;;FA;;;S-1-5-21-3900817571-2663857942-4197420431-1001)(A;;0x1200a9;;;WD)"


C:\Users\k\Documents\Projects\sdk\srw-fuse-threaded-refactor\build64>cacls Z: /s
Z:\ "D:P(A;;FA;;;S-1-5-21-3900817571-2663857942-4197420431-1001)(A;;0x1200a9;;;WD)"


C:\Users\k\Documents\Projects\sdk\srw-fuse-threaded-refactor\build64>icacls root
root LYSITHEA\k:(F)
     Everyone:(RX)

Successfully processed 1 files; Failed processing 0 files

C:\Users\k\Documents\Projects\sdk\srw-fuse-threaded-refactor\build64>icacls Z:
Z: LYSITHEA\k:(F)
   Everyone:(RX)

Successfully processed 1 files; Failed processing 0 files

C:\Users\k\Documents\Projects\sdk\srw-fuse-threaded-refactor\build64>

Kind regards,
Dave Ward