We ran into issues where mount(8) passed multiple lower layers as one
big string through fsconfig(). But the fsconfig() FSCONFIG_SET_STRING
option is limited to 256 bytes in strndup_user(). While this would be
fixable by extending the fsconfig() buffer I'd rather encourage users to
append layers via multiple fsconfig() calls as the interface allows
nicely for this. This has also been requested as a feature before.

With this port to the new mount api the following will be possible:

        fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir", "/lower1", 0);

        /* set upper layer */
        fsconfig(fs_fd, FSCONFIG_SET_STRING, "upperdir", "/upper", 0);

        /* append "/lower2", "/lower3", and "/lower4" */
        fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir", ":/lower2:/lower3:/lower4", 0);

        /* turn index feature on */
        fsconfig(fs_fd, FSCONFIG_SET_STRING, "index", "on", 0);

        /* append "/lower5" */
        fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir", ":/lower5", 0);

Specifying ':' would have been rejected so this isn't a regression. And
we can't simply use "lowerdir=/lower" to append on top of existing
layers as "lowerdir=/lower,lowerdir=/other-lower" would make
"/other-lower" the only lower layer so we'd break uapi if we changed
this. So the ':' prefix seems a good compromise.

Users can choose to specify multiple layers at once or individual
layers. A layer is appended if it starts with ":". This requires that
the user has already added at least one layer before. If lowerdir is
specified again without a leading ":" then all previous layers are
dropped and replaced with the new layers. If lowerdir is specified and
empty than all layers are simply dropped.

An additional change is that overlayfs will now parse and resolve layers
right when they are specified in fsconfig() instead of deferring until
super block creation. This allows users to receive early errors.

It also allows users to actually use up to 500 layers something which
was theoretically possible but ended up not working due to the mount
option string passed via mount(2) being too large.

This also allows a more privileged process to set config options for a
lesser privileged process as the creds for fsconfig() and the creds for
fsopen() can differ. We could restrict that they match by enforcing that
the creds of fsopen() and fsconfig() match but I don't see why that
needs to be the case and allows for a good delegation mechanism.

Plus, in the future it means we're able to extend overlayfs mount
options and allow users to specify layers via file descriptors instead
of paths:

        fsconfig(FSCONFIG_SET_PATH{_EMPTY}, "lowerdir", "lower1", dirfd);

        /* append */
        fsconfig(FSCONFIG_SET_PATH{_EMPTY}, "lowerdir", "lower2", dirfd);

        /* append */
        fsconfig(FSCONFIG_SET_PATH{_EMPTY}, "lowerdir", "lower3", dirfd);

        /* clear all layers specified until now */
        fsconfig(FSCONFIG_SET_STRING, "lowerdir", NULL, 0);

This would be especially nice if users create an overlayfs mount on top
of idmapped layers or just in general private mounts created via
open_tree(OPEN_TREE_CLONE). Those mounts would then never have to appear
anywhere in the filesystem. But for now just do the minimal thing.

We should probably aim to move more validation into ovl_fs_parse_param()
so users get errors before fsconfig(FSCONFIG_CMD_CREATE). But that can
be done in additional patches later.

This is now also rebased on top of the lazy lowerdata lookup which
allows the specificatin of data only layers using the new "::" syntax.

The rules are simple. A data only layers cannot be followed by any
regular layers and data layers must be preceeded by at least one regular
layer.

Parsing the lowerdir mount option must change because of this. The
original patchset used the old lowerdir parsing function to split a
lowerdir mount option string such as:

        lowerdir=/lower1:/lower2::/lower3::/lower4

simply replacing each non-escaped ":" by "\0". So sequences of
non-escaped ":" were counted as layers. For example, the previous
lowerdir mount option above would've counted 6 layers instead of 4 and a
lowerdir mount option such as:

        lowerdir="/lower1:/lower2::/lower3::/lower4:::::::::::::::::::::::::::"

would be counted as 33 layers. Other than being ugly this didn't matter
much because kern_path() would reject the first "\0" layer. However,
this overcounting of layers becomes problematic when we base allocations
on it where we very much only want to allocate space for 4 layers
instead of 33.

So the new parsing function rejects non-escaped sequences of colons
other than ":" and "::" immediately instead of relying on kern_path().

Link: https://github.com/util-linux/util-linux/issues/2287
Link: https://github.com/util-linux/util-linux/issues/1992
Link: https://bugs.archlinux.org/task/78702
Link: https://lore.kernel.org/linux-unionfs/20230530-klagen-zudem-32c0908c2108@braunerSigned-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>