Amir Goldstein [off-list ref] writes:

On Wed, Jul 8, 2026 at 2:31 PM Giuseppe Scrivano [off-list ref] wrote:

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Miklos Szeredi [off-list ref] writes:

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On Wed, 8 Jul 2026 at 12:00, Giuseppe Scrivano [off-list ref] wrote:

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Add two ioctls to overlay filesystem to allow userspace to retrieve
information about the overlay layers:

OVL_IOC_OPEN_LAYER:     return an O_PATH fd to the root of a layer.
                        arg == 0 returns the upper layer (-ENOENT if
                        no upper is configured), arg >= 1 returns
                        lower layers (-ENOENT if index is out of
                        range).

We could do this with a plain open() call.  Something like the magic
symlinks we have under /proc/PID/fd/.   Question is where could these
live...

is there any existing user of such a mechanism?  I don't see any mount
specific info under /proc or /sys.

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OVL_IOC_GET_LAYERS_INFO: copy a struct ovl_layers_info to userspace
                        with numlower, numlowerdata, and has_upper.

Isn't this info obtainable via statmount(2) already?  If not, it
should be there, instead of a specialized ioctl.

no that is not exposed by statmount and I don't see any way to export
file system specific data through it.  Do you've anything in mind?

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--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h

@@ -35,6 +35,8 @@ struct ovl_layer {
        struct vfsmount *mnt;
        /* Trap in ovl inode cache */
        struct inode *trap;
+       /* Keeps the original fsmount file alive for OVL_IOC_OPEN_LAYER */
+       struct file *origin;

Don't need to keep the file open: the only info missing is the
original vfsmount, everything else is already there to reconstruct the
file.

I didn't manage to get that to work.  As soon as the userspace process
closes the mount fd that was passed to fsconfig, the anonymous mount
namespace is destroyed and dissolve_on_fput sets mnt->mnt_ns to NULL.

So whenever I try to use this mount again from userspace, it is not
usable because the mount namespace is empty, causing check_mnt() to
fail.

Do you have any suggestions on how to solve this problem?

The suggestion was to store origin->f_path->mnt instead of storing origin file,
because you only end up using the origin vfsmount.

the reason I am keeping the file and not just the vfsmount is that the
file is what keeps the mount namespace alive (preventing
dissolve_on_fput from fire).

Should we export open_detached_copy from fs/namespace.c?  I've not
tested it, but it might work.  Are there other ways to solve it that I
am not seeing?

Thanks,
Giuseppe