Re: [PATCH] OVL: add honoracl=off mount option.

From: Amir Goldstein
Date: Thu May 02 2019 - 07:46:58 EST

On Thu, May 2, 2019 at 12:35 AM NeilBrown <neilb@xxxxxxxx> 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
> 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
> 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@xxxxxxxx>
> ---
> 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
> +the same format. A significant conflict appears between POSIX acls
> +used on many local filesystems, and NFSv4 ACLs used with NFSv4. There
> +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.

I don't know. On the one hand "system." is not only ACLs.
On the other hand, "honoracl=off" is not the same as -o noacl,
but it sure sounds the same.

I'd be a lot more comfortable with "ignore_xattrs=system.nfs4_acl"
argument takes a comma separated list of xattr prefixes to ignore.

ovl_is_private_xattr() can be generalized to ovl_is_ignored_xattr(),
going over a blacklist of N>=1 which will also be called from
ovl_can_list(), because there is no point in listing the ACLs that
are ignored. right?