Re: [PATCH v3 2/2] ovl: introduce new "uuid=off" option for inodes index feature

[Amir Goldstein]

On Thu, Sep 24, 2020 at 7:38 PM Pavel Tikhomirov
<ptikhomirov@xxxxxxxxxxxxx> wrote:
>
> This replaces uuid with null in overelayfs file handles and thus relaxes
> uuid checks for overlay index feature. It is only possible in case there
> is only one filesystem for all the work/upper/lower directories and bare
> file handles from this backing filesystem are uniq. In other case when
> we have multiple filesystems lets just fallback to "uuid=on" which is
> and equivalent of how it worked before with all uuid checks.
>
> This is needed when overlayfs is/was mounted in a container with index
> enabled (e.g.: to be able to resolve inotify watch file handles on it to
> paths in CRIU), and this container is copied and started alongside with
> the original one. This way the "copy" container can't have the same uuid
> on the superblock and mounting the overlayfs from it later would fail.
>
> Note: In our (Virtuozzo) use case users inside a container can create
> "regular" overlayfs mounts without any "index=" option, but we still
> want to migrate this containers with CRIU so we set "index=on" as kernel
> default so that all the container overlayfs mounts get support of file
> handles automatically. With "uuid=off" we want the same thing (to be
> able to "copy" container with uuid change) - we would set kernel default
> so that all the container overlayfs mounts get "uuid=off" automatically.
>
> That is an example of the problem on top of loop+ext4:
>
> dd if=/dev/zero of=loopbackfile.img bs=100M count=10
> losetup -fP loopbackfile.img
> losetup -a
>   #/dev/loop0: [64768]:35 (/loop-test/loopbackfile.img)
> mkfs.ext4 loopbackfile.img
> mkdir loop-mp
> mount -o loop /dev/loop0 loop-mp
> mkdir loop-mp/{lower,upper,work,merged}
> mount -t overlay overlay -oindex=on,lowerdir=loop-mp/lower,\
> upperdir=loop-mp/upper,workdir=loop-mp/work loop-mp/merged
> umount loop-mp/merged
> umount loop-mp
> e2fsck -f /dev/loop0
> tune2fs -U random /dev/loop0
>
> mount -o loop /dev/loop0 loop-mp
> mount -t overlay overlay -oindex=on,lowerdir=loop-mp/lower,\
> upperdir=loop-mp/upper,workdir=loop-mp/work loop-mp/merged
>   #mount: /loop-test/loop-mp/merged:
>   #mount(2) system call failed: Stale file handle.
>
> If you just change the uuid of the backing filesystem, overlay is not
> mounting any more. In Virtuozzo we copy container disks (ploops) when
> crate the copy of container and we require fs uuid to be uniq for a new
> container.
>
> v2: in v1 I missed actual uuid check skip
> v3: rebase to overlayfs-next, replace uuid with null in file handles,
> split ovl_fs propagation to function arguments to separate patch, add
> separate bool "uuid=on/off" option, move numfs check up, add doc note.

change log does not belong in commit message.
Move after --- please.

>
> CC: Amir Goldstein <amir73il@xxxxxxxxx>
> CC: Vivek Goyal <vgoyal@xxxxxxxxxx>
> CC: Miklos Szeredi <miklos@xxxxxxxxxx>
> CC: linux-unionfs@xxxxxxxxxxxxxxx
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
> ---
>  Documentation/filesystems/overlayfs.rst |  6 ++++++
>  fs/overlayfs/Kconfig                    | 17 +++++++++++++++++
>  fs/overlayfs/copy_up.c                  |  3 ++-
>  fs/overlayfs/namei.c                    |  5 ++++-
>  fs/overlayfs/ovl_entry.h                |  1 +
>  fs/overlayfs/super.c                    | 25 +++++++++++++++++++++++++
>  6 files changed, 55 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
> index 580ab9a0fe31..4f9cc20f255c 100644
> --- a/Documentation/filesystems/overlayfs.rst
> +++ b/Documentation/filesystems/overlayfs.rst
> @@ -563,6 +563,12 @@ This verification may cause significant overhead in some cases.
>  Note: the mount options index=off,nfs_export=on are conflicting for a
>  read-write mount and will result in an error.
>
> +Note: the mount option uuid=off (or corresponding module param, or kernel
> +config) can be used to replace UUID of the underlying filesystem in file
> +handles with null, and effectively disable UUID checks. This can be useful in
> +case the underlying disk is copied and the UUID of this copy is changed. This
> +is only applicable if all lower/upper/work directories are on the same
> +filesystem, otherwise it will fallback to normal behaviour.
>
>  Volatile mount
>  --------------
> diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
> index dd188c7996b3..888c6e5e71ee 100644
> --- a/fs/overlayfs/Kconfig
> +++ b/fs/overlayfs/Kconfig
> @@ -61,6 +61,23 @@ config OVERLAY_FS_INDEX
>
>           If unsure, say N.
>
> +config OVERLAY_FS_INDEX_UUID_OFF

Please get rid of all the double negatives.
config/param uuid_off should be uuid and default to Y.
Otherwise this looks fine to me.

> +       bool "Overlayfs: export null uuid in file handles"
> +       depends on OVERLAY_FS
> +       help
> +         If this config option is enabled then overlay will replace uuid with
> +         null in overlayfs file handles, effectively disabling uuid checks for
> +         them. This affects overlayfs mounted with "index=on". This only can be
> +         done if all upper and lower directories are on the same filesystem
> +         where basic fhandles are uniq. In case the latter is not true
> +         overlayfs would fallback to normal uuid checking mode.
> +
> +         It is needed to overcome possible change of uuid on superblock of the
> +         backing filesystem, e.g. when you copied the virtual disk and mount
> +         both the copy of the disk and the original one at the same time.
> +
> +         If unsure, say N.
> +
>  config OVERLAY_FS_NFS_EXPORT
>         bool "Overlayfs: turn on NFS export feature by default"
>         depends on OVERLAY_FS
> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
> index 3380039036d6..0b7e7a90a435 100644
> --- a/fs/overlayfs/copy_up.c
> +++ b/fs/overlayfs/copy_up.c
> @@ -320,7 +320,8 @@ struct ovl_fh *ovl_encode_real_fh(struct ovl_fs *ofs, struct dentry *real,
>         if (is_upper)
>                 fh->fb.flags |= OVL_FH_FLAG_PATH_UPPER;
>         fh->fb.len = sizeof(fh->fb) + buflen;
> -       fh->fb.uuid = *uuid;
> +       if (ofs->config.uuid)
> +               fh->fb.uuid = *uuid;
>
>         return fh;
>
> diff --git a/fs/overlayfs/namei.c b/fs/overlayfs/namei.c
> index f058bf8e8b87..0262c39886d0 100644
> --- a/fs/overlayfs/namei.c
> +++ b/fs/overlayfs/namei.c
> @@ -159,8 +159,11 @@ struct dentry *ovl_decode_real_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
>         /*
>          * Make sure that the stored uuid matches the uuid of the lower
>          * layer where file handle will be decoded.
> +        * In case of index=nouuid option just make sure that stored

leftover index=nouuid

Thanks,
Amir.