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

[Giuseppe Scrivano <gscrivan@redhat.com>]

Amir Goldstein <amir73il@gmail.com> writes:

> On Wed, Jul 8, 2026 at 2:31 PM Giuseppe Scrivano <gscrivan@redhat.com> wrote:
>>
>> 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?
>
> 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