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Subject: Re: [PATCH] OVL: add honoracl=off mount option.
To:     NeilBrown <neilb@suse.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Andreas Gruenbacher <agruenba@redhat.com>
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        Patrick Plagwitz <Patrick_Plagwitz@web.de>,
        "linux-unionfs@vger.kernel.org" <linux-unionfs@vger.kernel.org>,
        Linux NFS list <linux-nfs@vger.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <9ada42cb-8783-49bb-fd0d-31e7fb7dacf4@infradead.org>
Date:   Wed, 1 May 2019 22:08:19 -0700
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Hi Neil,

On 5/1/19 9:35 PM, NeilBrown wrote:
> 
> If the upper and lower layers use incompatible ACL formats, it is not
> possible to copy the ACL xttr from one to the other, so overlayfs

                           attr (?)

> cannot work with them.
> This happens particularly with NFSv4 which uses system.nfs4_acl, and
> ext4 which uses system.posix_acl_access.
> 
> If all ACLs actually make to Unix permissions, then there is no need

                       map (?)

> to copy up the ACLs, but overlayfs cannot determine this.
> 
> So allow the sysadmin it assert that ACLs are not needed with a mount
> option
>   honoracl=off
> This causes the ACLs to not be copied, so filesystems with different
> ACL formats can be overlaid together.
> 
> Signed-off-by: NeilBrown <neilb@suse.com>
> ---
>  Documentation/filesystems/overlayfs.txt | 24 ++++++++++++++++++++++++
>  fs/overlayfs/copy_up.c                  |  9 +++++++--
>  fs/overlayfs/dir.c                      |  2 +-
>  fs/overlayfs/overlayfs.h                |  2 +-
>  fs/overlayfs/ovl_entry.h                |  1 +
>  fs/overlayfs/super.c                    | 15 +++++++++++++++
>  6 files changed, 49 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index eef7d9d259e8..7ad675940c93 100644
> --- a/Documentation/filesystems/overlayfs.txt
> +++ b/Documentation/filesystems/overlayfs.txt
> @@ -245,6 +245,30 @@ filesystem - future operations on the file are barely noticed by the
>  overlay filesystem (though an operation on the name of the file such as
>  rename or unlink will of course be noticed and handled).
>  
> +ACL copy-up
> +-----------
> +
> +When a file that only exists on the lower layer is modified it needs
> +to be copied up to the upper layer.  This means copying the metadata
> +and (usually) the data (though see "Metadata only copy up" below).
> +One part of the metadata can be problematic: the ACLs.
> +
> +Now all filesystems support ACLs, and when they do they don't all use

   Not

> +the same format.  A significant conflict appears between POSIX acls

                                                                  ACLs

> +used on many local filesystems, and NFSv4 ACLs used with NFSv4.  There

                                                                    These (or the)

> +two formats are, in general, not inter-convertible.
> +
> +If a site only uses regular Unix permissions (Read, Write, eXecute by
> +User, Group and Other), then as these permissions are compatible with
> +all ACLs, there is no need to copy ACLs.  overlayfs cannot determine
> +if this is the case itself.
> +
> +For this reason, overlayfs supports a mount option "honoracl=off"
> +which causes ACLs, any "system." extended attribute, on the lower
> +layer to be ignored and, particularly, not copied to the upper later.
> +This allows NFSv4 to be overlaid with a local filesystem, but should
> +only be used if the only access controls used on the filesystem are
> +Unix permission bits.
>  
>  Multiple lower layers
>  ---------------------


-- 
~Randy