linux-ext4 - Re: which lower filesystems are actually supported?

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Date:   Mon, 03 Jun 2019 01:41:33 -0400
From:   Gabriel Krisman Bertazi <krisman@...labora.com>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     "Theodore Ts'o" <tytso@....edu>,
        Marco Nelissen <marco.nelissen@...il.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: which lower filesystems are actually supported?

Amir Goldstein <amir73il@...il.com> writes:

> On Sun, Jun 2, 2019 at 9:01 PM Theodore Ts'o <tytso@....edu> wrote:
>>
>> On Sun, Jun 02, 2019 at 09:42:54AM +0300, Amir Goldstein wrote:
>> > [+cc ext4] Heads up on bug reports "Overlayfs fails to mount with ext4"
>> >
>> > On Sat, Jun 1, 2019 at 11:02 PM Marco Nelissen <marco.nelissen@...il.com> wrote:
>> > >
>> > > According to the documentation, "The lower filesystem can be any filesystem
>> > > supported by Linux", however this appears to not actually be the case, since
>> > > using a vfat filesystem results in the mount command printing "mount:
>> > > wrong fs type, bad option, bad superblock on overlay, missing codepage or
>> > > helper program, or other error", with dmesg saying "overlayfs: filesystem on
>> > > '/boot' not supported".
>> > > (that's from ovl_mount_dir_noesc(), when ovl_dentry_weird() returns nonzero)
>> >
>> > Specifically for vfat it is weird because of
>> > dentry->d_flags & (DCACHE_OP_HASH | DCACHE_OP_COMPARE)
>> > because it is case insensitive.
>>
> [...]
>> >
>> > I am guessing when people start using case insensitive enabled ext4,
>> > this problem
>> > is going to surface, because the same ext4 (e.g. root fs) could be
>> > used for samba
>> > export (case insensitive) and docker storage (overlayfs).
>>
> [...]
>>
>> We *might* be able to only set the dentry functions on directory
>> entries belonging to directories which have the casefold flag set,
>> instead of simply setting it on all ext4 dentry entries.  But still
>> won't change the fact that overlayfs is going to have case
>> insensitivity support if we want the combination of overlayfs &&
>> casefold to be supported.
>>
>
> My intention was not that overlayfs should support casefold, just that
> an isolated casefold subdir in an ext4 fs shouldn't make the entire fs
> not usable with overlayfs.

That is a reasonable request.  I discussed a bit with Ted about how to
not set dentry functions filesystem wide,  because that gets in the way
of fscrypt.  I don't have a definite answer on how to do it, but it is
something that I will try to fix to enable fscrypt+casefold support.

> Incidentally, we already ran into a similar issue with ext4 encryption.
> Issue was reported by OpenWRT developers and fixed by:
> d456a33f041a fscrypt: only set dentry_operations on ciphertext dentries
>
> I recon casefold is taking a similar direction to the fs/crypto library, so
> solution should be similar as well.
>
> BTW, is casefold feature mutually exclusive with encryption feature?
> Because if it isn't, d_set_d_op() in __fscrypt_prepare_lookup() is
> going to WARN_ON dentry already has ext4_dentry_ops.

Not yet, and that is part of the reason. Right now, these two features
cannot be enable simultaneously, but it is on my todo list to support
that case.

> Thanks,
> Amir.

-- 
Gabriel Krisman Bertazi