[Bug 276876] [fusefs] sysutils/fusefs-exfat FUSE protocol violation (direct_io) and illegal UTF-8 sequence

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=276876

            Bug ID: 276876
           Summary: [fusefs] sysutils/fusefs-exfat FUSE protocol violation
                    (direct_io) and illegal UTF-8 sequence
           Product: Ports & Packages
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Individual Port(s)
          Assignee: samm@FreeBSD.org
          Reporter: jakub_lach@mailplus.pl
             Flags: maintainer-feedback?(samm@FreeBSD.org)
          Assignee: samm@FreeBSD.org

I have installed sysutils/exfat-utils and sysutils/fusefs-exfat, attempting to
backup MBR fat32 (/mnt/) card to larger one (exFAT GPT /media/ to future proof
it).

I have used following procedure -

# sudo gpart destroy -F /dev/da0
# sudo gpart create -s gpt /dev/da0
# sudo gpart add -t ms-basic-data -b 1m -a 1m da0
# sudo mkexfatfs /dev/da0p1

# kldload fusefs
# sudo mount.exfat /dev/da0p1 /media/
# sudo mount_msdosfs /dev/da1s1 /mnt/

# sudo cp -r -p /mnt/ /media

After approximately 30GB and 1h, the following message appeared -

Feb  7 18:50:54 Thinkpad kernel: WARNING: FUSE protocol violation for server
mounted at /media: cache incoherent!  To prevent data corruption, disable the
data cache by mounting with -o direct_io, or as directed otherwise by your FUSE
server's documentation.  This warning will not be repeated.

I presume it may be mitigated by -

# sudo mount.exfat -o direct_io /dev/da0p1 /media/

That was first letdown though, as the default "should work". 

Another thing that surprised me was that writing to /media/ was allowed without
sudo. It appears it is a generic property with fuse, as it was the case also
with NTFS (using mkntfs -vf).

Unfortunately, about 3/4 in the following appeared -

Feb  7 20:14:17 Thinkpad mount.exfat[43704]: illegal UTF-8 sequence
Feb  7 20:14:17 Thinkpad syslogd: last message repeated 1 times
Feb  7 20:16:10 Thinkpad syslogd: last message repeated 6 times
Feb  7 20:17:46 Thinkpad syslogd: last message repeated 18 times

This I find the most strange, as the files which were copied resided at /mnt/
where fat32 MBR was mounted. cp printed the problematic files, which sure
enough contained some ü or similar. But somehow fat32 didn't have problem with
them?

After the cp exited, I've ran the following at /mnt/ at /media/ -

# find . -type f | sort > ~/list

It appeared the files with problematic names were missing. Moreover, the
overall size of /mnt/ and /media/ (reported by du) is different (not so sure if
this is a matter of corruption or just missing files). 

The second error would explain the difference (but is quite unexplainable as
/mnt/ was fat32), but the first one, speaking of corruption - is the most
worrying. 

Overall I'm not confident that exFAT can be confidently used for backup. I have
previously successfully tested the same procedure for fat32 GPT backup (same
hardware)

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