linux-kernel - Re: [PATCH 0/6] overlayfs + casefolding

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Message-ID: <ztuodbbng5rgwft2wtmrbugwo3v5zgrseykhlv5w4aqysgnd6b@ef56vn7iwamn>
Date: Tue, 20 May 2025 11:21:30 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Amir Goldstein <amir73il@...il.com>
Cc: linux-fsdevel@...r.kernel.org, linux-bcachefs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-unionfs@...r.kernel.org, 
	Miklos Szeredi <miklos@...redi.hu>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 0/6] overlayfs + casefolding

On Tue, May 20, 2025 at 05:13:37PM +0200, Amir Goldstein wrote:
> On Tue, May 20, 2025 at 4:44 PM Kent Overstreet
> <kent.overstreet@...ux.dev> wrote:
> >
> > On Tue, May 20, 2025 at 04:33:14PM +0200, Amir Goldstein wrote:
> > > On Tue, May 20, 2025 at 4:12 PM Kent Overstreet
> > > > Amir, you've got two widely used filesystem features that conflict and
> > > > can't be used on the same filesystem.
> > > >
> > > > That's _broken_.
> > >
> > > Correct.
> > >
> > > I am saying that IMO a smaller impact (and less user friendly) fix is more
> > > appropriate way to deal with this problem.
> >
> > Less user friendly is an understatement.
> >
> > Obscure errors that only get reported via overloaded standard error
> > codes is a massive problem today, for _developers_ - have you never had
> > a day of swearing over trying to track down where in a massive subsystem
> > an -EINVAL is coming from?
> >
> > It's even worse for end users that don't know to check the dmesg log.
> >
> > And I support my code, so these would turn into bug reports coming
> > across my desk - no thanks; I already get enough weird shit from other
> > subsystems that I have to look at and at least triage.
> >
> > > > Users hate partitioning just for separate /boot and /home, having to
> > > > partition for different applications is horrible. And since overlay fs
> > > > is used under the hood by docker, and casefolding is used under the hood
> > > > for running Windows applications, this isn't something people can
> > > > predict in advance.
> > >
> > > Right, I am not expecting users to partition by application,
> > > but my question was this:
> > >
> > > When is overlayfs created over a subtree that is only partially case-folded?
> > >
> > > Obviously, docker would create overlayfs on parts of the fs
> > > and smbd/cygwin could create a case folder subtree on another
> > > part of the fs.
> > > I just don't see a common use case when these sections overlap.
> >
> > Today, you cannot user docker and casefolding on _different parts of_
> > the same filesystem.
> >
> > So yees, today users do have to partition by application, or only use
> > one feature or the other.
> >
> 
> Didn't say there was no problem.
> 
> Argued that your fix is a big gun and not worth the added complexity.
> 
> Let's see what Miklos thinks.
> 
> > This isn't about allowing casefolding and overlayfs to fix on the same
> > subtree, that would be a bigger project.
> >
> > > Perhaps I am wrong (please present real world use cases),
> > > but my claim is that this case is not common enough and therefore,
> > > a suboptimal EIO error from lookup is good enough to prevert crossing
> > > over into the case folded zone by mistake, just as EIO on lookup is
> > > enough to deal with the unsupported use case of modifying
> > > overlayfs underlying layers with overlay is mounted.
> > >
> > > BTW, it is not enough to claim that there is no case folding for the
> > > entire subtree to allow the mount.
> > > For overlayfs to allow d_hash()/d_compare() fs must claim that
> > > these implementations are the default implementation in all subtree
> > > or at least that all layers share the same implementation.
> >
> 
> Nevermind. Misread patch 6.

Since you were asking for use cases - docker & related are pretty widely
used for deploying things that are "unwieldy" within the normal packgae
manager ecosystem - and wine is case study #1 in that, where these days
people want to ship a specific version of wine with applications being
emulated (that's been tested with that application).

But wine wants casefolding, so - hapless user deploys docker image where
casefolding is enabled _but only on the subdir that holds Windows data_,
not the whole image.

Docker mounts the image, but then everything explodes when you try to
use it with what look to the user like impenetrable IO errors.

That's a bad day for someone, or more likely a lot of someones.