linux-kernel - Re: [PATCH v1 1/5] hostfs: Fix ephemeral inodes

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Date:   Sun, 21 May 2023 23:13:56 +0200 (CEST)
From:   Richard Weinberger <richard@....at>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     anton ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Christopher Obbard <chris.obbard@...labora.com>,
        Guenter Roeck <groeck@...omium.org>,
        Günther Noack <gnoack3000@...il.com>,
        kuba <kuba@...nel.org>, James Morris <jmorris@...ei.org>,
        Jeff Xu <jeffxu@...gle.com>, Kees Cook <keescook@...omium.org>,
        Paul Moore <paul@...l-moore.com>,
        Ritesh Raj Sarraf <ritesh@...labora.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Sjoerd Simons <sjoerd@...labora.com>,
        Willem de Bruijn <willemb@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v1 1/5] hostfs: Fix ephemeral inodes

----- Ursprüngliche Mail -----
> Von: "Mickaël Salaün" <mic@...ikod.net>
> hostfs creates a new inode for each opened or created file, which created
> useless inode allocations and forbade identifying a host file with a kernel
> inode.
> 
> Fix this uncommon filesystem behavior by tying kernel inodes to host
> file's inode and device IDs.  Even if the host filesystem inodes may be
> recycled, this cannot happen while a file referencing it is open, which
> is the case with hostfs.  It should be noted that hostfs inode IDs may
> not be unique for the same hostfs superblock because multiple host's
> (backed) superblocks may be used.
> 
> Delete inodes when dropping them to force backed host's file descriptors
> closing.
> 
> This enables to entirely remove ARCH_EPHEMERAL_INODES, and then makes
> Landlock fully supported by UML.  This is very useful for testing
> (ongoing and backported) changes.

Removing ARCH_EPHEMERAL_INODES should be a patch on its own, IMHO.

> These changes also factor out and simplify some helpers thanks to the
> new hostfs_inode_update() and the hostfs_iget() revamp: read_name(),
> hostfs_create(), hostfs_lookup(), hostfs_mknod(), and
> hostfs_fill_sb_common().
> 
> A following commit with new Landlock tests check this new hostfs inode
> consistency.
> 
> Cc: Anton Ivanov <anton.ivanov@...bridgegreys.com>
> Cc: Johannes Berg <johannes@...solutions.net>
> Cc: Richard Weinberger <richard@....at>
> Cc: <stable@...r.kernel.org> # 5.15.x: ce72750f04d6: hostfs: Fix writeback of
> dirty pages
> Cc: <stable@...r.kernel.org> # 5.15+

I'm not sure whether this patch qualifies as stable material.
While I fully agree that the current behavoir is odd, nothing user visible
is really broken so far.

> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> Link: https://lore.kernel.org/r/20230309165455.175131-2-mic@digikod.net

Other than that, patch looks good to me.

Thanks,
//richard