Re: [PATCH] ovl: consistent behavior for immutable/append-only inodes

[Amir Goldstein <amir73il@gmail.com>]

On Tue, Jun 8, 2021 at 4:52 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Sun, 6 Jun 2021 at 16:46, Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > When a lower file has immutable/append-only attributes, the behavior of
> > overlayfs post copy up is inconsistent.
> >
> > Immediattely after copy up, ovl inode still has the S_IMMUTABLE/S_APPEND
> > inode flags copied from lower inode, so vfs code still treats the ovl
> > inode as immutable/append-only.  After ovl inode evict or mount cycle,
> > the ovl inode does not have these inode flags anymore.
> >
> > We cannot copy up the immutable and append-only fileattr flags, because
> > immutable/append-only inodes cannot be linked and because overlayfs will
> > not be able to set overlay.* xattr on the upper inodes.
>
> Ugh.
>
> > Instead, if any of the fileattr flags of interest exist on the lower
> > inode, we set an xattr overlay.xflags on the upper inode as an indication
> > to merge the origin inode fileattr flags on lookup.
> >
> > This gives consistent behavior post copy up regardless of inode eviction
> > from cache.
> >
> > When user sets new fileattr flags, we break the connection with the
> > origin fileattr by removing the overlay.xflags xattr.
> >
> > Note that having the S_IMMUTABLE/S_APPEND on the ovl inode does not
> > provide the same level of protection as setting those flags on the real
> > upper inode, because some filesystem check those flags internally in
> > addition or instead of the vfs checks (e.g. btrfs_may_delete()), but
> > that is the way it has always been for overlayfs.
>
> That's fine, underlying filesystem is just a backing store.
>

Immutability of underlying files was not my concern.
My concern was that vfs does not provide full protection and that some
protection is provided in fs level, because I saw IS_APPEND/IS_IMMUTABLE
sprinkled all over the place in fs (e.g. ext4_setattr()), but I guess those are
just leftovers and I was over concerned.

> > As can be seen in the comment above ovl_check_origin_xflags(), the
> > "xflags merge" feature is designed to solve other non-standard behavior
> > issues related to immutable directories and hardlinks in the future, but
> > this commit does not bother to fix those cases because those are corner
> > cases that are probably not so important to fix.
> >
> > A word about the design decision to merge the origin and upper xflags -
> > Because we do not copy up fileattr and because fileattr_set breaks the
> > link to origin xflags, the only cases where origin and upper inodes both
> > have xflags is if upper inode was modified not via overlayfs or if the
> > system crashed during ovl_fileattr_set() before removing the
> > overlay.xflags xattr.  In both cases, modifiying the upper inode is not
> > going to be permitted, so it is better to reflect this in the overlay
> > inode flags.
>
> So why not implement the non-merge (#3) behavior unconditionally?
> That would solve all issues related to fileattr, right?
>

I suppose so. Note that #3 fileattr_get is still a merge between upper fileattr
and the 4 overlay stored flags, but for inode flags it will not be a merge.

I can give this a shot.

While you are here, do you think that will be sufficient for the on-disk format
of overlay.xflags?

struct ovl_xflags {
        __le32 xflags;
        __le32 xflags_mask;
}

Thanks,
Amir.