Compatibility with overlayfs

[Gao Xiang]

On Sat, Nov 30, 2019 at 10:15:25AM -0500, David Michael wrote:
> On Fri, Nov 29, 2019 at 9:13 PM Gao Xiang <hsiangkao at aol.com> wrote:
> > On Sat, Nov 30, 2019 at 09:29:04AM +0800, Gao Xiang via Linux-erofs wrote:
> > > Hi David,
> > >
> > > On Fri, Nov 29, 2019 at 03:22:15PM -0500, David Michael wrote:
> > > > Hi,
> > > >
> > > > I tried to test EROFS on Linux 5.4 as the root file system and mounted
> > > > a writable overlay (with upper layer on tmpfs) over /etc, but I get
> > > > ENODATA errors when attempting to modify files.  For example, adding a
> > > > user results in "Failed to take /etc/passwd lock: No data available".
> > > > Files can be modified after unlinking and restoring them so they're
> > > > created on the upper layer.  This is not necessary with other
> > > > read-only file systems (at least squashfs or ext4 with the read-only
> > > > feature).  I tried while forcing compact and extended inodes.
> > > >
> > > > Is EROFS intended to be usable as a lower layer with overlayfs?
> > >
> > > Yes, and overlayfs were used on our smartphones for development use
> > > only as well. I think it's weird, I will try it on the latest kernel
> > > now, and see if I can reproduce this issue soon...
> > >
> > > Thanks for your report!
> > >
> > > Thanks,
> > > Gao Xiang
> >
> > I have reproduced this issue -- That is due to erofs will return an
> > unexpected -ENODATA when calling listxattr without xattr and cause
> > copy_up fail:
> 
> Oh, sorry I forgot to mention I had xattrs disabled during my testing.
> I confirmed it works with xattrs, so I can use that as a workaround in
> binary distros until a fix is upstream.

Yes, selinux is enabled on Android, so every file has xattr...

I think it could be patched in advance if allowed (it's a trivial but
meaningful fix for noxattr overlayfs scenarios).

Since I've sent pull request for 5.5-rc1 days before. Considering that
it'd better to stay in linux-next for a while, I think it can be upstream
landed in 5.5-rc2 and backport to 5.4 then. (about 2 weeks later)

> 
> > since our products using xattr enabled EROFS with overlayfs,
> > so we didn't observe this issue before. So could you try
> > the following patch (If it can resolve the issue, I will
> > send it for 5.5-rc2 and backport to all stable version)?
> > Look forward to your feekback.
> 
> Yes, I applied the patch and everything works now.

Thanks for your confirmation. It's of great help to catch this. :-)
I'll resend this patch with proper commit message later.

Thanks,
Gao Xiang

> 
> Thanks.
> 
> David