On 05/29/2018 07:45 AM, Miklos Szeredi wrote:
> From: Vivek Goyal <vgoyal@xxxxxxxxxx>
>
> By default metadata only copy up is disabled. Provide a mount option so
> that users can choose one way or other.
>
> Also provide a kernel config and module option to enable/disable metacopy
> feature.
>
> metacopy feature requires redirect_dir=on when upper is present.
> Otherwise, it requires redirect_dir=follow atleast.
>
> As of now, metacopy does not work with nfs_export=on. So if both
> metacopy=on and nfs_export=on then nfs_export is disabled.
>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
> Documentation/filesystems/overlayfs.txt | 30 ++++++++++++++++++++-
> fs/overlayfs/Kconfig | 19 ++++++++++++++
> fs/overlayfs/ovl_entry.h | 1 +
> fs/overlayfs/super.c | 46 ++++++++++++++++++++++++++++++---
> 4 files changed, 92 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
> index f087bc40c6a5..79be4a77ca08 100644
> --- a/Documentation/filesystems/overlayfs.txt
> +++ b/Documentation/filesystems/overlayfs.txt
> @@ -262,6 +262,30 @@ rightmost one and going left. In the above example lower1 will be the
> top, lower2 the middle and lower3 the bottom layer.
>
>
> +Metadata only copy up
> +--------------------
> +
> +When metadata only copy up feature is enabled, overlayfs will only copy
> +up metadata (as opposed to whole file), when a metadata specific operation
> +like chown/chmod is performed. Full file will be copied up later when
> +file is opened for WRITE operation.
> +
> +In other words, this is delayed data copy up operation and data is copied
> +up when there is a need to actually modify data.
> +
> +There are multiple ways to enable/disable this feature. A config option
> +CONFIG_OVERLAY_FS_METACOPY can be set/unset to enable/disable this feature
> +by default. Or one can enable/disable it at module load time with module
> +parameter metacopy=on/off. Lastly, there is also a per mount option
> +metacopy=on/off to enable/disable this feature per mount.
> +
> +Do not use metacopy=on with untrusted upper/lower directories. Otherwise
> +it is possible that an attacker can create an handcrafted file with
a handcrafted
> +appropriate REDIRECT and METACOPY xattrs, and gain access to file on lower
> +pointed by REDIRECT. This should not be possible on local system as setting
> +"trusted." xattrs will require CAP_SYS_ADMIN. But it should be possible
> +for untrusted layers like from a pen drive.
> +
> Sharing and copying layers
> --------------------------
>
> @@ -280,7 +304,7 @@ though it will not result in a crash or deadlock.
> Mounting an overlay using an upper layer path, where the upper layer path
> was previously used by another mounted overlay in combination with a
> different lower layer path, is allowed, unless the "inodes index" feature
> -is enabled.
> +or "metadata only copy up" feature is enabled.
>
> With the "inodes index" feature, on the first time mount, an NFS file
> handle of the lower layer root directory, along with the UUID of the lower
> @@ -293,6 +317,10 @@ lower root origin, mount will fail with ESTALE. An overlayfs mount with
> does not support NFS export, lower filesystem does not have a valid UUID or
> if the upper filesystem does not support extended attributes.
>
> +For "metadata only copy up" feature there is no verification mechanism at
> +mount time. So if same upper is mouted with different set of lower, mount
mounted
> +probably will succeed but expect the unexpected later on. So don't do it.
> +
> It is quite a common practice to copy overlay layers to a different
> directory tree on the same or different underlying filesystem, and even
> to a different machine. With the "inodes index" feature, trying to mount
> diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
> index 5d1d40d745c5..e0a090eca65e 100644
> --- a/fs/overlayfs/Kconfig
> +++ b/fs/overlayfs/Kconfig
> @@ -64,6 +64,7 @@ config OVERLAY_FS_NFS_EXPORT
> bool "Overlayfs: turn on NFS export feature by default"
> depends on OVERLAY_FS
> depends on OVERLAY_FS_INDEX
> + depends on !OVERLAY_FS_METACOPY
> help
> If this config option is enabled then overlay filesystems will use
> the inodes index dir to decode overlay NFS file handles by default.
Is that ... dir ^^^ == directory ? Please spell it out.
> @@ -124,3 +125,21 @@ config OVERLAY_FS_COPY_UP_SHARED
> To get a maximally backward compatible kernel, disable this option.
>
> If unsure, say N.
> +
> +config OVERLAY_FS_METACOPY
> + bool "Overlayfs: turn on metadata only copy up feature by default"
> + depends on OVERLAY_FS
> + select OVERLAY_FS_REDIRECT_DIR
> + help
> + If this config option is enabled then overlay filesystems will
> + copy up only metadata where appropriate and data copy up will
> + happen when a file is opended for WRITE operation. It is still
opened
> + possible to turn off this feature globally with the "metacopy=off"
> + module option or on a filesystem instance basis with the
> + "metacopy=off" mount option.
> +
> + Note, that this feature is not backward compatible. That is,
> + mounting an overlay which has metacopy only inodes on a kernel
> + that doesn't support this feature will have unexpected results.
> +
> + If unsure, say N.
--
~Randy
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