On Fri, Sep 2, 2016 at 1:36 PM, Raphael Hertzog <hertzog@xxxxxxxxxx> wrote:
> Hi,
>
> On Wed, 31 Aug 2016, Felipe Sateler wrote:
>> overlayfs does not support renaming directories when the directories
>> live in the lower filesystem:
> [...]
>> Unfortunately this means that dpkg fails at least in the case where a
>> directory is converted into a file: apt 1.3~rc2 moves
>> /usr/share/bug/apt/script to /usr/share/bug/apt . This causes dpkg to
>
> That's the change at the package level. The operation that fails
> at the dpkg level is rename('/usr/share/bug/apt', '/usr/share/bug/apt.dpkg-tmp').
>
>> fail when running in an overlayfs with the following error:
> [...]
>> unable to move aside './usr/share/bug/apt' to install new version: Invalid cross-device link
>
> I got the same failure here but I also saw similar reports in a Kali live
> system where overlayfs is used for persistence (and with the upgrade
> of gedit). So that error is affecting many Kali users. It's not a
> very rare error (ex: https://bugs.kali.org/view.php?id=3473,
> https://bugs.kali.org/view.php?id=3476,
> https://bugs.kali.org/view.php?id=3361,
> https://bugs.kali.org/view.php?id=3365).
>
> I'm putting in copy the overlayfs kernel maintainer... Hello Miklos,
> that restriction above (cf
> https://github.com/torvalds/linux/blob/v4.8-rc2/fs/overlayfs/copy_up.c#L318-L322)
> is very problematic for us.
>
> Do you have plans to get rid of it?
You are the first to report that this quirk actually causes problems
in real life.
I have plans to fix this by adding a "redirector" attribute so that
copied up directories (and perhaps files) can refer to lower directory
that is found at a different location relative to the root of the
overlay from the location of the upper directory.
Maybe an example can make this more clear:
lower/foo/bar/baz
upper/foo/bar <- whiteout
upper/moved/here <- redirects to "foo/bar"
will result in the tree:
overlay/moved/here/baz
> It does not seem very correct to put the burden on user-space to be aware
> of overlayfs restrictions such as this one. Renaming a directory is
> not something that happens often in practice in the uses cases where
> we use overlayfs but it's still frequent enough to deserve better than
> a EXDEV error IMO and dpkg trying to rename "foo/" into "foo.dpkg-tmp/"
> is in its right to assume that this rename will not cross any device
> boundary.
The EXDEV trick just works for mv(1), hence this didn't seem to be a
big issue in practice.
Thanks,
Miklos
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