linux-ext4 - Re: [PATCH v3 08/16] xfs: Report case sensitivity in fileattr

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Message-ID: <CAEg-Je8LGZGGAQ3XLMQg8=XmJjvvJNShT3zkE-o2t2fv=VGeHw@mail.gmail.com>
Date: Tue, 13 Jan 2026 11:43:00 -0500
From: Neal Gompa <neal@...pa.dev>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Chuck Lever <cel@...nel.org>, vira@....codeaurora.org, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org, 
	linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org, 
	linux-cifs@...r.kernel.org, linux-nfs@...r.kernel.org, 
	linux-f2fs-devel@...ts.sourceforge.net, hirofumi@...l.parknet.co.jp, 
	linkinjeon@...nel.org, sj1557.seo@...sung.com, yuezhang.mo@...y.com, 
	almaz.alexandrovich@...agon-software.com, slava@...eyko.com, 
	glaubitz@...sik.fu-berlin.de, frank.li@...o.com, tytso@....edu, 
	adilger.kernel@...ger.ca, cem@...nel.org, sfrench@...ba.org, pc@...guebit.org, 
	ronniesahlberg@...il.com, sprasad@...rosoft.com, trondmy@...nel.org, 
	anna@...nel.org, jaegeuk@...nel.org, chao@...nel.org, hansg@...nel.org, 
	senozhatsky@...omium.org, Chuck Lever <chuck.lever@...cle.com>
Subject: Re: [PATCH v3 08/16] xfs: Report case sensitivity in fileattr_get

On Tue, Jan 13, 2026 at 11:02 AM Darrick J. Wong <djwong@...nel.org> wrote:
>
> On Mon, Jan 12, 2026 at 12:46:21PM -0500, Chuck Lever wrote:
> > From: Chuck Lever <chuck.lever@...cle.com>
> >
> > Upper layers such as NFSD need to query whether a filesystem is
> > case-sensitive. Populate the case_insensitive and case_preserving
> > fields in xfs_fileattr_get(). XFS always preserves case. XFS is
> > case-sensitive by default, but supports ASCII case-insensitive
> > lookups when formatted with the ASCIICI feature flag.
> >
> > Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
>
> Well as a pure binary statement of xfs' capabilities, this is correct so:
> Reviewed-by: "Darrick J. Wong" <djwong@...nel.org>
>
> [add ngompa]
>
> But the next obvious question I would have as a userspace programmer is
> "case insensitive how, exactly?", which was the topic of the previous
> revision.  Somewhere out there there's a program / emulation layer that
> will want to know the exact transformation when doing a non-memcmp
> lookup.  Probably Winderz casefolding has behaved differently every
> release since the start of NTFS, etc.
>

NTFS itself is case preserving and has a namespace for Win32k entries
(case-insensitive) and SFU/SUA/LXSS entries (case-sensitive). I'm not
entirely certain of the nature of *how* those entries are managed, but
I *believe* it's from the personalities themselves.

> I don't know how to solve that, other than the fs compiles its
> case-flattening code into a bpf program and exports that where someone
> can read() it and run/analyze/reverse engineer it.  But ugh, Linus is
> right that this area is a mess. :/
>

The biggie is that it has to be NLS aware. That's where it gets
complicated since there are different case rules for different
languages.



-- 
真実はいつも一つ！/ Always, there's only one truth!