Re: [PATCH] ovl: add ioctls to retrieve layer file descriptors

[Giuseppe Scrivano <gscrivan@redhat.com>]

Miklos Szeredi <miklos@szeredi.hu> writes:

> On Wed, 8 Jul 2026 at 12:00, Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>>
>> 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.

>
>> 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?

>> --- 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?

Regards,
Giuseppe