Porting FUSE filesystems

[Dave Vasilevsky]

Thanks for everyone's work on Minix!

I recently ported my FUSE filesystem, squashfuse[1], to Minix 3.2.1. If anybody wants to test squashfuse, please use branch_0.2, and follow the instructions for Minix in the PLATFORMS file: https://github.com/vasi/squashfuse/tree/branch_0.2

I'd like to detail what changes were necessary to squashfuse, so the next person to try a FUSE port has an easier time, and so the idiosyncrasies in Minix's FUSE implementation can be fixed. Please let me know if I got anything wrong.

1. Missing callbacks:

It looks like the FUSE callbacks 'opendir', 'releasedir', 'open' and 'release' aren't being called on Minix.

For squashfuse, I had to force 'readdir' and 'read' to lookup the path on each call, instead of relying on data cached in the above callbacks.

2. Missing context variables:

There's a void* argument to fuse_main() that is supposed to be passed to the 'init' callback. Then the return value of 'init' is supposed to be avaiable in fuse_get_context()->private_data during other callbacks. On Minix, neither of these happen.

Similarly, fuse_opt_parse() takes a void* that should get passed to the fuse_opt_proc callback, but this doesn't happen on Minix.

For squashfuse, I used global variables to substitute for the missing contexts.

3. st_ino in getattr callback:

The 'getattr' callback is documented by upstream FUSE to ignore the st_ino field of struct stat, but on Minix st_ino is required. Thankfully, squashfuse has an obvious way to fill in st_ino. Other filesystems may have to generate and cache a synthetic inode number.

4. Parsing of unmatched arguments:

Unmatched '-o option' arguments are supposed to be passed by fuse_opt_parse() to the option callback, but that doesn't seem to happen. Filesystems may have to roll their own argument parser, as I did.

5. Strange build flags:

The fuse headers require that _POSIX_C_SOURCE be undefined. This is unusual, though not out of spec.

Linking squashfuse with the usual flags '-lfuse' or '-lrefuse -lpuffs' fails on Minix, it also needs '-lminixfs -lsys -lminlib'.

Static linking appears necessary, and the '-Wl,-wrap,main' argument too.

It would be great if all this this was documented somewhere.

6. Mounting is unusual:

On most OSes, you mount an archive with squashfuse by running:

  $ squashfuse path/to/archive.squashfs path/to/mountpoint

But on Minix, FUSE filesystems must be invoked like so:

  $ mount [OPTIONS] -t myfilesystem /dev/block_device path/to/mountpoint

This doesn't work well for filesystems like squashfuse that don't require a block device. My workaround is the require the user to pass a dummy block device, and then add a '-o image=/full/path/to/archive.squashfs' option. Other filesystems may have to do something similar.

(Maybe vnconfig in future versions will help with this?)

7. Setup is complex:

A stanza for squashfuse must be added to system.conf.d. It seems like a reboot is necessary after that, though maybe there's another way.

The squashfuse executable must be copied to both /usr/bin/pkg/squashfuse and /usr/bin/pkg/mount.squashfuse . It doesn't seem like /usr/bin or /usr/local/bin works, for some strange reason.

Again, some documentation of this would be great.

Good luck with your next FUSE port!

Cheers,

Dave