On Mon, Aug 21, 2023 at 1:00 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>
> On Thu, 17 Aug 2023 at 13:05, Alexander Larsson <alexl@xxxxxxxxxx> wrote:
> >
> > This is needed to properly stack overlay filesystems, I.E, being able
> > to create a whiteout file on an overlay mount and then use that as
> > part of the lowerdir in another overlay mount.
> >
> > The way this works is that we create a regular whiteout, but set the
> > `overlay.nowhiteout` xattr on it. Whenever we check if a file is a
> > whiteout we check this xattr and don't treat it as a whiteout if it is
> > set. The xattr itself is then stripped and when viewed as part of the
> > overlayfs mount it looks like a regular whiteout.
> >
>
> I understand the motivation, but don't have good feelings about the
> implementation. Like the xattr escaping this should also have the
> property that when fed to an old kernel version, it shouldn't
> interpret this object as a whiteout. Whether it remains hidden like
> the escaped xattrs or if it shows up as something else is
> uninteresting.
>
> It could just be a zero sized regular file with "overlay.whiteout".
So, I started doing this, where a whiteout is just a regular file with
the xattr set. Initially I thought I only needed to check the xattr
during lookup and convert the inode mode from S_IFREG to S_IFCHR.
However, I also need to hook up readdir and convert DT_REG to DT_CHR,
otherwise readdir will report the wrong d_type. To make it worse,
overlayfs itself looks for DT_CHR to handle whiteouts when listing
files, so nesting is not working without that.
The only way I see to implement that conversion is to call getxattr()
on every DT_REG file during readdir(), and while a single getxattr()
on lookup is fine, I don't think that is.
Any other ideas?
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Alexander Larsson Red Hat, Inc
alexl@xxxxxxxxxx alexander.larsson@xxxxxxxxx
