linux-kernel - Re: [PATCH] ovl: create UUIDs for file systems that do not set the superblock UUID

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Message-ID: <CAOQ4uxhT4pFzHjjKyoMOc3xVXXqyqc37zd=-pCx2+keA4e6NAg@mail.gmail.com>
Date:   Thu, 7 Nov 2019 09:08:44 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        kernel-janitors@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ovl: create UUIDs for file systems that do not set the
 superblock UUID

On Thu, Nov 7, 2019 at 1:43 AM Colin King <colin.king@...onical.com> wrote:
>
> From: Colin Ian King <colin.king@...onical.com>
>
> Some file systems such as squashfs do not set the UUID in the
> superblock resulting in a zero'd UUID.  In cases were two or more
> of these file systems are overlayed on the lower layer we can hit
> overlay corruption issues because identical zero'd overlayfs UUIDs
> are impossible to differentiate between.  This can be fixed by
> creating an overlayfs UUID based on the file system from the
> superblock s_magic and s_dev fields.  (This currently seems like
> enough information to be able create a UUID, but the could be
> scope to use other super block fields such as the pointer s_fs_info
> but may need some obfuscation).
>

The fix is incorrent. uuid stored in xattr needs to have persistent properties.
In the use case that you describe, the origin file handle should simply be
ignored.

Please test attached patch.

Thanks,
Amir.

View attachment "0001-ovl-fix-lookup-failure-on-multi-lower-squashfs.patch" of type "text/x-patch" (4073 bytes)