[Bug 244178] [FUSEFS]: underlying file modified leads to corrupted fuse data

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=244178

            Bug ID: 244178
           Summary: [FUSEFS]: underlying file modified leads to corrupted
                    fuse data
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: ben.rubson at gmx.com

Hi,

I still face, with 12.1-RELEASE-p2, the issue discussed here :
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230258#c35
Which continued there :
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235774

The issue is the following.

Let's assume raw file is the underlying file.
And fuse file the corresponding fuse file.

1. create raw file with some data
2. open fuse file
3. verify fuse size : correct
4. verify fuse data : correct
5. add some data to raw file
6. verify fuse size : correct (*)
7. verify fuse data : garbage
(*) as expected we have to use fuse options attr_timeout=0 entry_timeout=0.

So, if we keep the fuse file opened, while the underlying one is modified :
- we read the fuse file up to the expected size, perfect ;
- but we read garbage.

If we close and re-open the fuse file, then we correctly read it.

There are 2 possible workarounds :
- use FUSE direct_io mount option ;
- sysctl vfs.fusefs.data_cache_mode = 0.

I don't know whether or not the default behavior is correct.
But at least with Linux, we correctly read the data even keeping the fuse file
opened.
There may then still be a cache issue.

Many thanks for your feedback !

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