Re: [RFC] DT_WHT

[Florian Weimer <fweimer at redhat dot com>]

On 03/05/2018 05:20 PM, Ben Hutchings wrote:
> On Mon, 2018-03-05 at 10:47 +0100, Florian Weimer wrote:
> > On 03/05/2018 02:50 AM, Ben Hutchings wrote:
> > > I have no opinion on that code, but it sounded like you proposed to
> > > remove the macro definition too.  I wanted to point out that the macro
> > > is meaningful and may be useful to applications.
> >
> > Useful in what sense?  What are its semantics?
>
> A white-out entry is a placeholder written to the upper layer of a
> union filesystem that hides an entry in the lower layer.
>
> You'll never see these directory entries when looking at the union
> filesystem, but can do if you look at the upper layer directly.

That's not documented anywhere.  The kernel never sets this flag.  The 
Coda file system simply copies it from the network, and I cannot find 
the current server code anywhere.

For all we know, DT_WHT might be visible on the union file system, and 
applications have to skip those entries manually if they want to avoid 
“file not found” errors.  People assumed that inode 0 had this property 
(but this wasn't really true).

Furthermore, for true whiteout entry support, I'd expect an S_IFWHT mode 
flag, but that's just not there in the upstream kernel.

Thanks,
Florian