Re: LInux NFSv4.1 client and server- case insensitive filesystems supported?

[Chuck Lever <chuck.lever@oracle.com>]

On 6/9/25 11:50 AM, Theodore Ts'o wrote:
> On Sun, Jun 08, 2025 at 05:52:36PM -0400, Chuck Lever wrote:
>> NFSD currently asserts that all exported file systems are case-sensitive
>> and case-preserving (either via NFSv3 or NFSv4). There is very likely
>> some additional work necessary.
> 
> If the underlying file system on the server side does do case
> insensitive lookups, how badly would it confuse the NFS client if a
> lookup of "MaDNeSS" and "maddness" both worked and returned the same
> file, even though readdir(2) only showed the existence of "MaDNeSS"
> --- despite the fact that the nfsd asserted that file system was
> case-sensitive?

IIRC the Linux NFS client used to cache READDIR results via the dcache.
It doesn't do that, these days. If the client mounted with
lookupcache=none, it might work OK?


>> Ted, do you happen to know if there are any fstests that exercise case-
>> insensitive lookups? I would not regard that simple test as "job done!
>> put pencil down!" :-)
> 
> There are.   See:
> 
> common/casefold
> tests/f2fs/012
> tests/generic/453
> tests/generic/454
> tests/generic/556
> 
> You'll need to make some adjustments in common/casefold for NFS,
> though.  The tests also assume that case insensivity can be adjusted
> on a per-directory basis, using chattr +F and chattr -F, and that
> probably isn't going to port over to NFS, so you might need to adjust
> the tests as well.

This is probably the more general concern:

- Both the NFSv3 and NFSv4 protocols mark a whole file system as either
  case sensitive or case insensitive.

- NFS protocols do not facilitate case-sensitivity to be be enabled or
  disabled from NFS clients.

It sounds like it would be difficult for NFS clients to make sense of
an exported extN file system that contained some case sensitive and some
case insensitive directories.


> Note that some of these tests also are checking Unicode case-folding
> rules, which is a bit different from the ASCII case-folding which FAT
> implemented.  It also might be interesting/amsuing to see what happens
> if you ran these tests where the NFS server was exporting, say, a
> case-folded file system from a MacOS server, or a Windows NFS server,
> and the client was running Linux NFS.  Or what might happen if you
> tried to do the same thing using, say, CIFS.   :-)

Re-exporting remote file systems is a massive jungle even on good days.

I think a narrower concern might be keeping lookup behavior consistent
between local accessors, NFS accessors, and accessors via SMB (Samba or
ksmbd). I admit I have no idea if that's possible.


-- 
Chuck Lever